LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye SOILS AND SOIL SCIENCES Willy Verheye National Science Foundation, Flanders/Belgium, and Geography Department, University of Gent, Belgium Keywords: Bulk density, cation exchange capacity, color, mapping unit, organic material, permeability, pH, regolith, soil profile, soil survey, structure, texture, water retention, weathering Contents N E SA SC M O PL -E O E LS C H S AP TE R S 1. Introduction 2. Soils and Soil Science 3. Soil Formation and Soil Forming Processes 4. The Soil Profile 5. Soil Composition and Soil Properties 6. Soil Survey and Classification Glossary Bibliography Biographical Sketch Summary Soil science is a relatively new discipline which has mainly developed since the 1880s. It uses terms, methods and processes borrowed from other basic disciplines like climatology, geology, chemistry, physics and biology, but with a direct application to soils. At present, it is difficult to speak about one single science but as soil sciences, as they cover several fields including: pedology (or pedogenesis), soil survey (or mapping), classification and applied soil sciences like soil fertility, soil conservation, land evaluation or soil and land management. U In this chapter an overview is given of the concepts of modern soil sciences. The process of weathering and gradual evolution of a regolith towards a mature soil profile is described. Soil composition and main soil properties, as well as their inter-relations with other characteristics and impact on land use are discussed. Finally, a summary is given of the basic principles of soil survey and soil classification. As an overhead chapter on the topic of soil sciences in this Encyclopedia, this article provides a synthetic overview, with direct references to the more detailed information included in the 17 chapters which make up this topic. 1. Introduction The term “soil”, derived from L. solum, has many definitions. Geologists and road engineers consider the soil primarily as an inert unconsolidated weathering product of the underlying rock, a nuisance that must be quarried and removed before reaching the material of their interest, i.e. the basement for construction. Alternatively, soil (or dirt) can also be used for filling excavations or providing foundations. For many other users, ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye however, including farmers and earth science specialists, the soil is primarily a medium for plant growth or crop production and water storage, and a major source of living. Hence, these users and farmers in particular, pay automatically more attention to the inherent characteristics of soils and their management, because to them soil is more than useful, it is indispensable. N E SA SC M O PL -E O E LS C H S AP TE R S Throughout history farmers have learned, through trial and error, to observe differences in soils and to improve, wherever possible, their properties. Long before our era the Greeks were aware of the beneficial effects of applying manure or using ash or sulfur as soil amendments. Since the Roman Empire traditional land use practices have been passed from generation to generation, but always in a conception that soil is a rather inert material to which fertilizers and water has to be added for producing crops. This concept dominated people’s minds until as recent as the late nineteenth and early twentieth centuries, and it is only since von Liebig in 1840 discovered the role of nutrients in crop production and Dokouchaiev in 1880 made the link between soil properties and bioclimatic zones that the soil is considered a dynamic body with variable properties and potential depending on variations in climate, vegetation and parent material. In the same line Jenny in 1941 defined the 5 soil forming factors which, still today, guide pedogenetic thinking and research (see: A Brief History of Soil Science). In the present-day concept the soil is considered a product of evolution and changes over time, with an own morphology and properties. The morphology of each soil, as expressed by a vertical section of different layers or horizons, is a direct reflection of the effects of the five genetic factors responsible for its development. This dynamic and evolutionary nature is embodied in the universal definition of soil as: U a natural body, located at the interface between the atmosphere, lithosphere and biosphere, consisting of layers of unconsolidated mineral and/or organic constituents of variable thickness which have been subjected to and influenced by genetic and environmental factors of: parent material, climate (including moisture and temperature effects), macro- and microorganisms, and topography, all acting over a period of time and producing a product-soil that differs from the material from which it is derived in many physical, chemical, and biological properties and characteristics. The upper limit of the soil is air or shallow water. Its lower limit coincides with the lower limit of biologic activity, as reflected by the rooting depth of native perennial plants. This active soil section in between corresponds with what is commonly defined as the solum. As soils differ in their properties both in the vertical and horizontal sense, their study and characterization should involve both the vertical succession of overlying horizons and spatial variations. The first aspect requires the observation and study of a soil profile pit, approximately 1m x 1m x 1m in size and being considered representative for the soil around. This small basic entity, from which one can observe variations in properties and extract samples for analytical investigations, is called a pedon. It is the smallest volume that can be called a soil, but large enough to exhibit a full set of horizons. ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye The combination of various pedons with minor differences within a larger landform is called a polypedon. Such minor differences may relate to the nature and arrangement of horizons, or to the degree of expression of one or more horizons below the depth of normal plowing. In fact, a polypedon corresponds usually with what is often described as a soil series. Soils of the same series have a similar horizon sequence and nearly identical properties of the horizons. 2. Soils and Soil Science N E SA SC M O PL -E O E LS C H S AP TE R S Soil science is the study concerned with observing and describing, collecting, establishing and systematizing facts, principles and methods in order to acquire an indepth knowledge of the soils, their properties and potential for production and conservation. Soil science uses an integrated multidisciplinary approach in the sense that it borrows concepts, techniques and processes from other sciences, but with a focus on soils. Soil science relies on 7 major supporting sciences or sub-disciplines (Figure 1): U Climatology which affects the pedo-climate in terms of moisture and temperature conditions in the solum, and thus influences physical and chemical soil processes and plant and animal life; Geology which determines the nature and constitution (mineralogy) of the parent material from which the soil profile develops; Geomorphology (landform evolution) and hydrology which have a major impact on runoff, erosion and sedimentation processes, and differential warming up of soils; Physics, the basic laws of which determine the nature, intensity and interrelationships between the solid, liquid and gaseous soil components; Chemistry, concerned with the chemical constitution, chemical properties and chemical reactions in the soil, and their direct effect on soil fertility and nutrient supply to plants; and Soil (micro) biology dealing with the soil fauna, the vegetation above and below the soil surface, as well as the microscopic soil population, and their role in various transformations and the liberation of nutrients. Soil science as a discipline is relatively young compared to other sciences like mathematics or astronomy, the origin of which dates back for more than 2 000 years. The first who started to systematically study soils was the Russian geographer Dokouchaiev in the late 19th century, but his work became only known at the international floor after it had been published in German and disseminated by Marbut and Jenny in the USA (see: A Brief History of Soil Science). The disastrous Dust Bowl in the Mid West in the 1930s gave an additional impetus to the study of soils and was the start of the Soil Conservation Service in the US. Elsewhere in the world the growing interest in soils work received a major push after World War II when in a number of European countries a national soil survey institute was established (in 1947 in Belgium, 1952 in France, 1966 in The Netherlands, etc.) while in the tropics the rapid development and increased demand of plantation crops asked for an better knowledge and understanding of soil-plant relations in these areas. ©Encyclopedia of Life Support Systems (EOLSS) N E SA SC M O PL -E O E LS C H S AP TE R S LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye Figure 1: Supporting basic sciences and current sub-disciplines of soil science At present, there are four major disciplines within soil science focusing on different applications and users (Figure 1): U Pedology, focusing on the formation (pedogenesis) and development of soils as recognized in the characteristics of the soil profile. It deals with fundamental and academic research aspects of soils, and includes the description and analytical characterization of the soil; Soil survey which describes the soil properties (making use of field and laboratory observations as referred to above) and delineates the geographical distribution of the different soils; Soil classification which organizes the soils and their particular properties on the basis of a hierarchical system of pre-defined criteria and classes. Though these criteria may vary as a function of the objectives, most international classification systems have a pedogenetic background; and Applied soil sciences which interpret the soil properties in function of their ranking and potential for a specific objective. This includes applications (1) for housing and construction, focusing mainly on physical and much less on chemical properties; (2) for agronomic and crop production objectives, including soil fertility, suitability for drainage and irrigation, etc.; (3) for soil conservation and the protection of soils against physical loss by erosion or by chemical deterioration; and (4) land evaluation which assesses the production and use potential of the soil, including the development and monitoring of land use practices. The latter domain has since the 1980s gradually become a key issue ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye for land resource management and land use planning. The various aspects of soil sciences described above are discussed at large in different separate chapters in this section: Soil Physics, Soil Chemistry and Soil Fertility, Soil Biology and Microbiology, Soil Biochemistry, Soil Mineralogy. 3. Soil Formation and Soil Forming Processes N E SA SC M O PL -E O E LS C H S AP TE R S Soil formation takes place in two consecutives stages, starting with a simple weathering (disintegration and decomposition) of rocks and minerals giving rise to an unconsolidated regolith (from Gr. rhegos, covering, and lithos, stone), and followed by a soil profile development, whereby the regolith material is gradually modified and a horizon sequence develops under the combined action of climate, vegetation, topography and time. 3.1. Weathering and Regolith Formation Weathering is basically a combination of destruction and synthesis. It breaks up rocks, modifies or destroys their physical and chemical characteristics, and carries away the soluble products and some of the solids. These changes are accompanied by a continuous decrease in particle size and by the release of soluble constituents, which are subject to loss in drainage waters or recombination into new (secondary) minerals. U There are three major forms of weathering: physical, chemical and biological weathering. Physical or mechanical weathering takes places under conditions where water is no active agent to enhance chemical reactions. It is particularly active in deserts or in polar areas where temperature changes create internal pressures in the rock and produce cracks (see: Dry Lands and Desertification, and Soils of Arid and Semi-Arid Areas). Chemical weathering is mainly related to the concerted action of water, oxygen and organic chemicals released by higher plants and microorganisms. While physical weathering results generally in a broad breakdown of soil and rock components, chemical weathering affects much more intensively the composition of soil material. The three major weathering processes related to water are hydrolysis (the dissociation from H + and OH − ions from H 2 O ), hydration (addition of a water molecule to the mineral) and dissolution (the solubility of a compound and its elimination from the environment). Biological weathering processes are activated by living agents (animals, higher plants, microorganisms) and are mainly responsible for both the decomposition and disintegration of rocks and minerals. The processes related to these weathering forms have been discussed at large in: Dry Lands and Desertification. 3.2. Soil Profile Formation and Horizon Development Soil profile development is basically a re-arrangement of soil particles into soil horizons, each of them with specific properties. Soil formation can proceed rather fast in aggressive humid tropical climates, but is much slower in cold or dry climates; when the surface layers are eroded, the (active) root zone comes nearer to the regolith and soils ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye are rejuvenated. Conditions that hasten the rate of soil development are: (1) a warm, humid climate, (2) forest vegetation, (3) permeable unconsolidated material low in lime content, and (4) flat or lowland topography with good drainage. Factors that tend to retard development are: (a) a cold or dry climate, (b) grass vegetation, (c) the presence of impermeable consolidated material high in lime, and (d) a steeply sloping topography. N E SA SC M O PL -E O E LS C H S AP TE R S Weathering and soil formation can be studied by changes of color, structure and texture in the field, by laboratory analyses and by microscopic observations and techniques (see: Soil Mineralogy, and Soil Microscopy and Micromorphology). The processes involved in soil profile formation and horizon development are: (a) gains or additions of water, organic and mineral material, (b) losses of such material from the soil, (c) transformation of mineral substances within the soil, and (d) translocation or movement of soil material from one point to another, involving movement in solution (leaching) or in suspension (eluviation) of clay, organic matter or hydrous oxides. Conditions that retard or offset horizon differentiation are due to: (a) mixing of material by burrowing animals, (b) removal of surface soil by water or wind, (c) creep, and (d) accretion of sediments in floodplains. There are 9 fundamental processes that affect profile differentiation: Humification - The process of transformation, i.e. decomposition of raw organic material into humus under the influence of soil microorganisms. During this process, the soluble organic substances regroup themselves into large molecules and become poorly soluble. In the strict sense, the term focuses in particular on the phase which follows the decomposition of the organic debris and which consists mainly of processes of synthesis and building up of new molecules through microbial or physicochemical pathways. U Eluviation and illuviation - The process of removal of soil constituents in suspension or in solution by percolating water from the upper to the lower layers. It encompasses mobilization and translocation of mobile constituents (mainly clay) resulting in a textural differentiation, or leaching of soluble elements like salts. Calcification and decalcification- The movement of soluble calcium carbonate in the soil, involving their leaching, movement, precipitation and accumulation in various soil layers. The general reaction which controls the movement of carbonate is: CaCO3 + H 2 O + CO 2 → Ca(HCO3 ) 2 If CO 2 and H 2 O are present, i.e. under an active biological activity, the reaction proceeds in the right direction, with the formation of soluble bicarbonate. When CO 2 and water are not active, i.e. in the dry season when the biological activity is largely reduced, the reaction proceeds to the left and insoluble calcium carbonate precipitates. This is, for example, what happens in Mediterranean soils which develop under an alternate humid winter and dry summer period (see: Mediterranean Soils). ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye Podzolization - The process of extreme leaching of bases in an acid environment, relatively poor in weatherable minerals, characteristic of regions with a (very) humid boreal or tropical climate. It involves the eluviation of acid and complex-forming humus that becomes mobile and gets leached from the upper part of the profile, and their subsequent deposition in the lower horizons. The process is the most active under pine tree forests (see: Forest, Range and Wildland Soils). N E SA SC M O PL -E O E LS C H S AP TE R S Lateritization (currently replaced by the connotations allitization and ferrallitization) The process that removes silica and soluble bases from the upper layers of the soil, creating a relative accumulation and concentration of sesquioxides (Fe and Al-oxides) in the soil. As the alkaline bases are removed from the seat of their formation, the residual soil is acidic in reaction. Though considerable eluviation takes place, there is no marked horizonation as the eluviated materials are not re-deposited in the lower layers. Depending on the intensity of the weathering process the residual soils are dominated by Fe and Al compounds (ferrallitization) or by Al-(hydr)oxides only (allitization). These processes act most intensively in warm and humid tropical climates with 20002500 mm annual rainfall and high temperatures (>22°C) throughout the year (see: Soils of the Humid and Sub-Humid Tropics). Gleization - A process of soil formation under an anaerobic environment and leading to the development of a gley horizon with green-blue colors, related to the reduction of soluble ferrous iron under water-logged conditions. Where the groundwater fluctuates considerably with the season, the gley shows distinct mottling of yellow and rusty brown colors caused by alternate oxidation and reduction phenomena. Salinization - The process of accumulation of salts, such as sulfates or chlorides in the form of a salty horizon. It is active under conditions of highly saline or brackish groundwater, and evaporation being higher than precipitation, so that salts move up by capillary action from the groundwater. Desalinization is the removal, by leaching of excess soluble salts from horizons that contain enough soluble salts to impair plant growth (see: Salinity and Alkalinity Status of Arid and Semi-arid Lands). U Alkalinization - A process involving the accumulation of sodium ions on the exchange complex of clays, resulting in the formation of a sodic soil (Solonetz). At this moment a high soil pH (>8.5) develops, soil colloids are dispersed and a very poor soil structure develops. The organic matter dissolved under alkaline conditions forms black organoclay coatings on the ped surfaces giving the soil a dark-colored appearance (Black Alkali Soils). Solodization or de-alkalinization - The removal of Na + from the exchange sites and the dispersion of clay, promoted by the addition of Ca 2+ to the formerly alkaline soil, often under the form of easily soluble gypsum (see: Dry Lands and Desertification). Pedoturbation - The process of mixing the soil due to faunal activity (ants, earthworms, moles, termites, etc.), plant roots, natural swell-shrink processes or by man-made land management practices. It is very active in boreal areas covered by long-time and usually undisturbed forest vegetation (see: Forest, Range and Wildland Soils). ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye The rate of soil development varies with the intensity of the processes involved, and with the age of the soil. In dry climates there is almost no water for either chemical processes or organic material production. Hence, soil formation is mainly limited to an incomplete physical breakdown of soil components. In a humid, warm climate hydrolysis, dissolution and leaching are much more intensive, and soil properties rapidly change into a material that is composed of stable mineral components. Individual processes vary also in intensity over time. Under ideal conditions a recognizable soil profile may develop within a couple of centuries. But under less favorable environments, as is the case in deserts, the time taken for soil development may extend over several thousand years. 4. The Soil Profile N E SA SC M O PL -E O E LS C H S AP TE R S An examination of a vertical section of a soil in the field reveals the presence of more or less distinct horizontal layers. Such a section is called a profile, and the individual layers are regarded as horizons. The horizons above the parent material are collectively referred to as the solum (from Gr. Solum, soil, land or a parcel of land). The topsoil corresponds with the upper part of the solum which is enriched in humus and is the major zone of root development. When the soil is cultivated this topsoil may correspond with the plow layer. The underlying layers between the topsoil and the regolith or parent material are referred to as the subsoil. Figure 2 is an example of a deep profile developed over aeolian loam. The slightly darker topsoil extends over the upper 20-25 cm. The subsoil includes a B-horizon with a well-developed structure (25-60 cm) overlying a less well-structured and rather compact C horizon (60- 110cm+) Profile descriptions in the field make a distinction between master horizons named with capital letters like O, A, E, B, C, R (Table 1). Subordinate distinctions within these master horizons are indicated by lower case letters used as suffixes to designate specific kinds of characteristics (Table 2). U Surface or near-surface horizons relatively high in organic matter (OM) are designated O or A horizons, the difference between the two being determined by the amount of organic matter present. The surface horizon is considered organic if its OM content is >30% and the mineral fraction has >50% clay, or if the OM content is >20% and the mineral fraction has no clay. Wetness generally favors the increasing thickness of O horizons and may qualify as peat when it is > 40cm deep and its OM is >50%. The mineral horizons on the other hand, are low in OM and may overlie either a B or C horizon, or even directly the rock mass. Beneath the O or A horizon, in some environments, there is a light-colored horizon relatively leached of iron compounds called the E horizon; this is identical to the A2 horizon of some (older) classifications. The B horizon commonly is beneath the surface horizon(s), unless the latter has been eroded and the B horizon appears on top of the new soil. It encompasses a multitude of soil characteristics relatively to those of the assumed parent material. Among the B horizon characteristics are: clay accumulation, a (yellowish) red color, the accumulation of iron compounds with or without organic matter, and the residual concentration of resistant materials following the removal of ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye U N E SA SC M O PL -E O E LS C H S AP TE R S more soluble constituents under conditions of intensive weathering and leaching. Figure 2: A typical soil profile developed over aeolian loam, Belgium The slightly weathered C horizon is beneath the B horizon, and forms the transition with the partly weathered saprolite (see: Soils of the Humid and Sub-Humid Tropics) or unaltered bedrock, the R horizon. In desert environments, carbonate buildup plays an important role in soil morphology and genesis, and horizons high in carbonate are designated K (see: Soils of Arid and Semi-arid Areas). Soil horizons can be further subdivided by adding a number to the master horizon designation. A3 and B1 are used for horizons transitional from the A to the B, with A3 having properties closer to A, and B1 having properties closer to B. Transitional ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye horizons can also be designated AB or AC if a more detailed subdivision is not possible nor warranted. B2 is used for that part of the B horizon that displays the maximum expression of the properties upon which the B horizon is defined (e.g. B2t for the maximum expression of an argillic B horizon). A B3 horizon still retains many properties of the B2 horizon but is transitional to the underlying horizon. A second number can be added for even further subdivision (B21t, B22t and B31), based on subtle changes in such properties as color or texture. Within the C horizon, numbers are used to denote a vertical sequence of layers (C1, C2, etc.). N E SA SC M O PL -E O E LS C H S AP TE R S Since 1960 the Soil Conservation Service of the USDA has introduced a new set of names for soil horizons because the more traditional terms, such as an A or B horizon, were neither precisely defined nor used in the same sense by all workers. This new terminology has now largely been taken over by other worldwide classification systems, the World Reference Base for Soil Resources for example. The new terms, such as a mollic or epipedon or an argillic subhorizon, are precisely defined, so much that at times recognition of such horizons may require laboratory analysis. The boundaries between these units and those of the A, B and C horizons in a profile do not necessarily coincide. At present, both systems are in use: the traditional A, B, C nomenclature is mainly used for profile descriptions and field-related investigations, whilst the modern terminology is mainly applied for soil classification purposes. Table 1 summarizes the major properties of the different master horizons. As the correspondence with the Soil Taxonomy nomenclature is not always evident (because criteria partly overlap), the correlations are only approximate. Table 2 defines the subordinate distinctions. Symbol U H O A Layer/horizon description and corresponding horizons in Soil Taxonomy (*) Layer dominated by organic material, formed from the accumulation of undecomposed or partially decomposed organic material at the soil surface. Saturated with water for prolonged periods or once saturated, but now artificially drained. *Histic epipedon: Surface horizon saturated for 30 or more consecutive days in some time of most years, with 75% or more sphagnum, and low bulk density. Surface accumulation of organic material, consisting of un-decomposed or partially decomposed litter ( leaves, needles, twigs, moss, lichens, etc.) overlying generally a mineral soil. Not saturated with water for a prolonged period. The mineral fraction is only a small percentage (generally < 50%) of the volume of the material. *See mollic or umbric horizons below. Mineral surface horizon in which all or much of the original rock structure has been obliterated, having one of the following: (a) organic matter accumulation intimately mixed with mineral soil, (b) properties resulting from cultivation or similar human disturbance, (c) a morphology different from underlying B or C horizons. *Mollic epipedon: Dark colored (chroma < 4.0, value < 3.5 moist), with > 1% organic material, base saturation >50%. Often grass vegetation. *Umbric epipedon: Similar to mollic epipedon except that base saturation is < 50% . Often associated with (tropical) forest vegetation. ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye E N E SA SC M O PL -E O E LS C H S AP TE R S B *Ochric epipedon: Too light in color and too low in organic matter than mollic or umbric epipedons. Often associated with young soils and/or semiarid vegetation. Or A2 horizon. Mineral horizon below O or A and above B horizon, characterized by loss of clay, Fe, Al or a combination of these, leaving a concentration of sand and silt particles. Light colors are mainly due to the color of primary mineral grains. *Albic horizon: Surface or lower horizon with light color determined by the color of the sand or silt particles: value, moist, is 4 or more, or value, dry, is 5 or more. If value, dry, is 7 or more, or value, moist, is 6 or more, chroma is 3 or less. If value, dry, is 5 or 6, or value, moist is 4 or 5, chroma is close to 2. Subsurface horizon under O, A or E horizons, showing little or no evidence of original rock structure and having one or a combination of the following: evidence of illuviation, removal of carbonates, residual concentration of sesquioxides, coatings, well developed structure, brittleness. *Argillic B horizon: Subsurface horizon with more silicate clay than in the A or E: 3% or more if overlying horizon has <15% clay; 1.2 times the amount of clay if overlying horizon has 15-40% clay; 8% or more if overlying horizon has >40% clay. Clay in B horizon is translocated (illuviation) from overlying horizons or has formed in place, or both. Clay translocations are recognized in the field by oriented clay films on mineral grains, small channels or ped surfaces *Natric B horizon: Like argillic horizon, but with columnar or prismatic structure and ESP > 15 in some sub-horizon. *Spodic B horizon: Occurs generally below an E horizon, holds a concentration of organic matter and/ or sesquioxides translocated from E horizon. *Oxic B horizon: Highly weathered subsurface horizon with low CE and dominated by hydrated oxides of iron and aluminum and 1:1 clays. *Cambic B horizon: Subsurface horizon enough altered to eradicate most rock structure, form some stable soil structure and remove or redistribute primary carbonate. Has higher chroma or redder hue than underlying horizons. Subsurface horizon, excluding bedrock, holding material from which the soil has developed. Lacks properties of A and B horizons, but includes weathering as shown by mineral oxidation, accumulation of silica, carbonates, or more soluble salts, and gleying. Consolidated bedrock underlying soil. Sufficiently coherent when moist to make hand digging with a spade impractical. U C R * More complete and updated definitions can be found in USDA Soil Taxonomy (1975) and related website. Table 1: Soil horizon nomenclature and brief description of master horizons (adapted from FAO Guidelines for Profile Description, 1990 and Soil Taxonomy, 1975) Symbol Description b Buried soil horizon. c Concretions or nodules that have accumulated in a significant amount. ca Accumulation of calcium carbonate in amount greater than the parent material is presumed to have had; can occur in A, B, C and R horizons. ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye cs f g h ir j k m N E SA SC M O PL -E O E LS C H S AP TE R S n o Accumulation of gypsum in amount greater than the parent material is presumed to have had; can occur in A, B, C or R horizons. Frozen soil, with layers that contain permanent ice or are perennially colder than 0°C Strong gleying or reduction of iron, so that colors approach neutral, with or without mottles; can occur in A, B and C horizons. Accumulation of organic material (humus), appearing as coatings on grains or as silt-size pellets; common in podzols. Illuvial concentration of iron, appearing as coatings on grains or as siltsize pellets; common in podzols. Jarosite mottles. Accumulation of carbonates, commonly calcium carbonate. Strong irreversible cementation (>90% cemented) or induration, for example by accumulation of iron, calcium carbonate or silica. Accumulation of exchangeable sodium. Residual accumulation of sesquioxides, different from s (below) which indicates illuvial accumulation of organic matter + sesquioxides. Plowing or other tillage disturbance by Man. Accumulation of secondary silica (quartz). Strong reduction. Illuvial accumulation of sesquioxides in combination with amorphous dispersible organic matter. Cementation by silica, as nodules or as a continuous medium; if cementation is continuous the horizon is called a duripan or silcrete. Accumulation of translocated clay, like in argillic B horizon. Occurrence of plinthite, iron-rich and humus-poor material, firm when moist, and hardening irreversibly when exposed to air. Development of color or structure, in particular in B horizons. Characteristics of a fragipan, related to firmness, brittleness or high bulk density. Accumulation of gypsum. Accumulation of salts more soluble than gypsum. p q r s si t v w x U y z Table 2: Subordinate soil properties (adapted from FAO Guidelines for Profile Description, 1990; Soil Taxonomy, 1975, and recent updates) 5. Soil Composition and Soil Properties 5.1. Soil Composition An average soil is composed of unconsolidated mineral (inorganic) compounds and decayed plant (organic) material. Depending on their organic matter content soils can therefore be differentiated into two major types. Soils with less than 20% organic matter (or 30% if the clay content is high) are called mineral soils; those that have more organic material are termed organic soils. The latter are by no means as extensive as are mineral soils, yet their total area is more than 300 million ha, worldwide. ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye 5.1.1. Organic Soils Organic soils occur when high amounts of organic matter accumulate in poorly drained areas. After plant decay water replaces air in pores and voids, thus preventing rapid oxidation. Peat deposits are mainly found in cold climates (Russia, Canada, Finland) though they can also be extensive in the tropics (12 million ha in Indonesia and Malaysia, 1.2 million ha in Louisiana, US). The composition of the peat varies depending on its origin and content: sedimentary peat, fibrous peat, woody peat. N E SA SC M O PL -E O E LS C H S AP TE R S Peat is often used, either mixed with mineral soil for potted plants and for home flower and vegetable gardens, or as a substratum for gardens and lawns. Its use as domestic fuel is declining, though it is still prevalent in remote areas of the Soviet Republic, Ireland, Finland and Scotland (where it is an important component for giving a specific flavor in the production of whiskey). Finally, it provides also an excellent substratum for field vegetable production. The term muck is used to describe peat that is markedly decomposed whereby the original plant parts can (almost) no more be identified. In Soil Taxonomy (USDA, 1975) peat soils are called Histosols, and they include at suborder level Fibrists (where the fibrous organic material can still easily be recognized), Saprists (where the original plant fibers have mostly disappeared) and two intermediate types: Hemists and Folists. The most important characteristics of organic soils are: U a dark brown to intense black color; a high content of at least partly un-decomposed organic matter; a low bulk density, of the order of 0.20-0.30 g/cm3; a high water holding capacity on a weight basis, i.e. 2 to 4 times their dry weight (though part of this water might not be available to plants); a loose structure and physical condition in general; most peat soils are porous, and easy to cultivate, which makes them very desirable for vegetable production; a high surface area and corresponding high CE, 2 to 10 times higher than mineral soils, on a weight basis; this advantage disappears however on a volume basis; and a relatively low pH 5.1.2. Mineral Soils All other soils are called mineral soils. These are composed of a solid phase and of pores which might be filled either by water or by air. The solid phase is made up of a dominant mineral fraction and a less important organic fraction. A good or high quality soil holds 48% of mineral fraction, 2% organic fraction, 25% water and 25% air. The relative proportion of pores filled with water or air varies. The mineral fraction contains a more or less inert part of coarse elements, sand and loam, which provides a foothold for roots and forms the soil skeleton. Besides, it holds an active fraction (clay and fine silt) with specific surface properties; the physicochemical properties of these are at the basis of electrical charges and ion ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye absorption, and the formation of structural elements which influence the pore volume and the air and water conditions of the profile. The organic fraction is mainly concentrated in the surface layers and originates from the decay of organic products of fauna and flora. It affects the sorption capacity of the topsoil and the soil structure of the profile as a whole, and thus also the moisture status of the root zone. The relative importance of the water and gas phases in the soil is determined by soil porosity. It varies between 25 and 50% and is influenced by soil texture (particle size), soil structure and soil compaction. These properties are discussed in more detail in the following sections. 5.2. Soil Texture N E SA SC M O PL -E O E LS C H S AP TE R S Soil texture refers to the relative proportions of various size particles in a given soil. Particle size analysis enables to fix adequately the percentages of the various constituents of the soil. This soil characteristic has an important impact on the soil moisture status and aeration, as well as on other qualities like workability, root penetration and anchorage, cation retention, etc. Sandy soils are considered as “light”, clayey soils as “heavy” since they are either easy or more difficult in tilling and cultivation. In soil sciences, the soil texture is quantified in terms of particle size composition of the fine earth fraction, in particular in their relative proportion of mineral particles of less than 2 mm in size. Particles with a diameter > 2mm are considered as belonging to the so-called coarse fraction and are not part of the “fine soil fraction”. Coarse fragments are categorized as gravel (particle size diameter 2mm- 7.5cm), stones (diameter 7.5-25 cm) and cobbles (diameter >25cm). For engineering and construction applications, equal attention is given to both the fine and the coarse fractions, and a somewhat different classification of texture is taken into consideration (see: Soil Physics). U The fine soil fraction is classified, after destroying the aggregates, according to size categories corresponding to an international scale: clay : < 2 μm; fine silt : 2-20 μm; coarse silt (sometimes also called very fine sand): 20-50 μm; fine sand: 50-200 μm; medium sand: 200 μm - 1 mm; and coarse sand: 1-2 mm. It is recalled that some national classification systems (Germany, Russia, many Anglo-Saxon countries) still use slightly different size limits, but there is a general tendency to rely to one uniform system as described above. Measurement - Soil texture is primarily estimated through a (rapid and cheap) field test followed by a more detailed (more time-consuming and more costly) laboratory analysis of a selected number of samples. The field test is conducted through a finger test, i.e. by rubbing a moist or moistened sample between thumb and fingers. Experienced field soil scientists are able to estimate the relative proportions of sand, silt and clay at a <5% error. Clay feels as a smeary paste, silt is smooth and soft like talc powder, and sand feels abrasive with individual ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye grains being observed with a 10-power hand-lens. Laboratory tests do generally confirm well these field estimations, though there are situations where this is not so. A good example of this is the highly weathered tropical soils (Oxisols) where individual clay particles are cemented by iron to form pseudo-silt and pseudo-sands. The field texture of these soils is therefore rather coarse and the soil profile behaves as such in terms of moisture retention and water permeability characteristics. The laboratory tests which apply a preliminary destruction of the iron bounds as a standard procedure, give generally a much finer texture. Both estimations have their value as the field texture is well correlated with the soil hydraulic properties (water retention, permeability) while the laboratory data are more in line with the physicochemical characteristics of the soil. N E SA SC M O PL -E O E LS C H S AP TE R S The commonly used laboratory methods for particle size analysis are the ones designed by Bouyoucos and Robinson; for engineering purposes the focus is more on sedimentation methods (see: Soil Physics). The Bouyoucos hydrometer method is relatively accurate (though somewhat underestimating the clay content) and fast. It determines the 3 textural fractions clay, silt and sand without separating them. The sample is first dispersed with a sodium pyrophosphate solution, treated with a highspeed soil mixer, and then poured in a cylinder to which distilled water is added to bring the contents up to volume. With the help of a stirrer the soil suspension is thoroughly mixed and the time noted. The rate of fall of suspended particles is related to size: sand settling faster than silt and silt settling faster than clay, based on Stoke’s law: V = 2 gr 2 ( d1 - d 2 ) / 9η ′ where: V = velocity of fall (cm/sec); g = acceleration of gravity (cm/sec2, say 981); r = equivalent radius of particle (cm); d1 = density of particle (gram/cm3, say 2.65 for soil); d 2 = density of medium (gram/cm3, say 1 for water) and η ′ = viscosity of medium (dyne sec/cm2). U Two hydrometer readings are taken of the soil suspension using a special soil hydrometer. A reading taken after 40 seconds determines the weight of silt and clay remaining in suspension, since the sand has settled to the bottom. Subtraction of the 40 seconds reading from the sample weight gives the grams of sand. Another reading after 2 hours gives the weight of clay in the sample. The silt is calculated by difference: add the percentage of sand and the percent of clay and subtract from 100. The Robinson method is based on the same principles. A given weight of oven-dry soil is sieved through a series of different sand-size sieves, from where the sand fractions are determined, and the supernatant liquid is put into suspension. An aliquot of the suspended material is taken at different times and at corresponding depths in the suspension, based on the Stoke’s equation. The air-dried fractions are expressed on a percentage basis. Textural diagram - The relative importance of the different particle size fractions, generally regrouped into 3 main classes: clay (0-2 μm), silt (2-50 μm) and sand (50 μm- ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye 2 mm) is represented on a triangular diagram, whereby the proportion of each of the 3 fractions is marked and the characteristic point of a soil or a given horizon is the intersection of 3 lines parallel to the sides obtained by plotting on each side the values in percentage of clay, silt and sand. There exist a number of different textural diagrams but the most currently and nowadays almost exclusively used diagram is the so-called USDA or FAO triangle composed of 12 classes (Figure 3). For small-scale identification and mapping at national or continental scales these classes may be regrouped. In the FAO-UNESCO Soil Map of the World or the WRB Soil Classification only 3 major classes are withheld: coarse texture : containing < 15% clay and > 70% sand; medium texture: containing <35% clay and < 70% sand; and fine texture: containing > 35% clay U N E SA SC M O PL -E O E LS C H S AP TE R S Figure 3: Soil textural triangle, composed of 12 classes Impact on classification and land use – Clay translocation through the profile is an expression of soil profile evolution. The presence of an argillic horizon in the profile ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye and of clay coatings on ped surfaces are important diagnostic criteria for the classification of soils. Texture has also a major impact on the hydraulic properties of soils, both in terms of water infiltration (permeability) and water retention capacity, as well as on consistence and tillage properties, porosity and aeration, etc. It determines also management practices, as for example the type of irrigation to be applied (flood irrigation is not economical in too permeable soils). Table 3 gives a good correlation between some of these parameters. It shows that loamy sand can hold only half of plant-available water and almost three times less than a loam and clay soil, respectively. These correlations can be used to certain extent (because it disregards the impact of structure) as a rule of thumb in projects where no analytical facilities are available and rapid decisions need to be taken. Bulk density (g/cm3) 1.65 (1.55-1.80) 1.40 (1.35-1.50) 1.35 (1.30-1.40) 1.30 (1.25- 1.40) 1.25 (1.20-1.30) Field capacity (%) 9 (6-12) 22 (18-26) 27 (23-31) 31 (27-35) 35 (31-39) Permanent wilting point (%) 4 (2-6) 10 (8-12) 13 (11-15) 15 (13-17) 17 (15-19) Water content mm/m soil 8 (**) (7-10) 17 (14-19) 19 (17-22) 21 (15-23) 23 (20-25) N E SA SC M O PL -E O E LS C H S AP TE R S Soil Texture Permeability Total (cm/h) pore space (%) Loamy sand 5 38 (2.5-25) (32-42) Loam 1.3 47 (0.8-2.0) (43-49) Clay loam 0.8 49 (0.25-1.5) (47-51) Silty clay 0.25 51 (0.03-0.5) (49-53) Clay 0.05 53 (0.01-0.1) (51-55) * Average figures and range (between brackets). ** The moisture content on a volume basis (mm water per meter of soil) is obtained by subtracting weight percentages at field capacity and permanent wilting point, multiplied by bulk density. U Table 3: Correlation between texture and a number of other soil properties: bulk density, permeability, water retention capacity (*) The presence of a clay accumulation zone in the profile can sometimes affect the internal moisture and nutrient uptake status for plants and crops. Up to a certain point, a clay increase in the subsoil is desirable as it can increase the amount of water and nutrients stored in that zone. By slightly reducing the rate of water movement through the soil, it will reduce the rate of nutrient loss through leaching. However, if the accumulation of clay is too drastic– so as to form a clay-pan, for example– it will severely restrict the movement of water and air, as well as the penetration of roots in the Bt horizon. It will also tend to increase the amount of water from rainfall or irrigation that will occur as runoff on sloping land. Finally, there is also a close correlation between clay content and cation exchange capacity (CE) through the specific surface area and charge properties. Hence, 100% clay soils should have a CE of 10-15 me/ 100 g soil when the dominant clay type is kaolinte, 15-40 me when it is illite-vermiculite, 70-75 me when it is montmorillonite and >100 ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye me when it is allophane (see: Soil Mineralogy). 5.3. Soil Structure While texture concerns primary the size of soil particles, soil structure refers to their arrangement and coagulation. Structure is, therefore, the combination or arrangement of single soil particles (sand, silt, clay) into secondary compounds or peds. These peds are formed as a result of the coagulation of primary minerals bound by a cement like microbial gum, iron oxide, organic carbon compounds (in particular polysaccharides) or clay. Peds are separated from adjoining peds by surfaces of weakness. U N E SA SC M O PL -E O E LS C H S AP TE R S Structure is a descriptive term and cannot be adequately quantified. Though there is no formal agreement between researchers on the mechanisms that affect structural development, it is believed that the following factors play a role: (1) wetting and drying; (2) freezing and thawing; (3) physical activity of roots and soil animals; (4) the influence of decaying organic matter and rest products of microorganisms; (5) modifying effects of absorbed cations (examples are Ca and Na); and (6) soil tillage. Figure 4: Types of structure: A: prismatic; B: columnar; C: angular blocky; D: subangular blocky; E: platy; F: granular (USDA Soil Survey Manual, 1951) Measurement – Structure is directly defined in the field by observing the nature and form of the soil peds in the profile. It is characterized by three criteria: type, class, and grade or degree of development. The type corresponds to the macroscopic appearance, shape and arrangement of the peds. Class indicates ped size, i.e. the width or thickness of the structural aggregate. The grade is the degree of aggregation or structure development; it expresses the differential between cohesion within peds and adhesion ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye between peds. Grade of structure varies with soil moisture contents and tends to be stronger as the soil dries; it is qualified as: structureless, weak, moderate and strong. There are 4 principal types of soil structure: spheroidal, including granular and crumb structures; platy-like; prism-like, including prismatic and columnar structures; and block-like, including subangular blocky and cube-like structures (Figure 4). Impact on other soil properties – Structure modifies the influence of texture with regard to moisture, heat and air relationships in the profile. The macroscopic size of most peds results in the existence of inter-ped spaces much larger than those that can exist between adjacent sand, silt and clay particles within peds. It is this structural effect on the pore space relationships that makes structure so important (see: Soil Physics). N E SA SC M O PL -E O E LS C H S AP TE R S Structure is important for the movement of water through the soil and for surface erosion. The structure of the surface layers, though it varies from soil to soil, tends to produce larger pores than would be the case if the soil had no structure. These pores allow the soil to take up large amounts of rain water, and thus to minimize runoff and surface erosion. The fact that many structural aggregates are water stable is important because it means that the percolating waters are fairly free of clay particles. However, such aggregates may become unstable in water and lead to a compact soil, as is the case in soils saturated with Na. The presence of free CaCO3 in the soil has an opposite effect: it improves structural stability. 5.4. Soil Consistence Soil consistence is a term used to describe the resistance of a soil to mechanical stresses or manipulations at various moisture contents. It is a composite expression of those cohesive and adhesive forces that determine the ease with which a soil can be reshaped. Whereas structure deals mainly with the shape, size and distinctness of natural soil aggregates, consistence refers to the strength and nature of the faces between particles. This physical soil property affects mainly traffic and tillage conditions of the land. U Like structure it can not be properly quantified and is therefore commonly estimated by feeling and manipulating the soil by hand, or by pulling a tillage instrument through it. The consistence of soils is generally described at 3 moisture levels: dry, moist and wet. If the soil is dry, the terms soft or hard are used; when moist, the terms loose, friable or firm are used; if wet the terms sticky or plastic are used. 5.5. Soil Color Soil color is easily determined in the field in a semi-quantitative way by comparing the field color to a standard Munsell Color chart. This consists of 175 color chips arranged systematically on seven charts according to hue, value and chroma, e.g. the three variables that combine to give the colors. Figure 5 is an example of chart 7.5 YR. The hue (on the top right of the chart) refers to the dominant wave-length or color of light between red, yellow and blue. Value (on the ordinate) or brilliance refers to the total quantity of light, obtained by adding a black or white fraction; it increases from dark to light colors. Chroma (on the abscissa) is the relative purity or intensity of the dominant ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye N E SA SC M O PL -E O E LS C H S AP TE R S wavelength of light; it increases with decreasing proportions of white light. Figure 5: Example of a Munsell Color Chart (Munsell Color, 1975) U The Munsell notation of color is a systematic numerical and letter designation of each of the three variable color properties described above left side of Figure 5). These properties are always given in the order: hue, value and chroma. Hence, a color is marked as, for example, 7.5 YR 6/4, whereby 7.5 YR stands for the hue, 6 for the value and 4 for the chroma. This corresponds to a light brown color, which is directly visualized in the Munsell Color book (right side of Figure 5). In most uniform soils there is normally only one color which defines the soil matrix. This is normally expressed by indicating the exact correspondence with the Munsell color (example 7.5 YR 6/4) or by referring to the nearest-by color (example: 7.5 YR 6/5 indicates that the soil color is intermediate between 7.5 YR 6/4 and 6/6). In a less uniform horizon or in layers with a spotted pattern (as in gley soils subject to a fluctuating water table), the color of both the dominant matrix and of all the mottles is marked. Impact on other soil properties – Soil color is an indirect expression of a number of important soil qualities which are otherwise not easily quantified. A black color is ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye usually an expression of organic matter accumulation at the surface, though the intensity of the black color is not always linked to the amount of carbon in the soil. This is the case, for example, in tropical savanna soils under grass vegetation, or in most Oxisols where the red matrix color masks to a certain extent the darker organic matter. In some soils, like in Podzols, the color difference in the profile marks the position of the leaching zone (pale white albic horizon) and the organic material accumulation in depth (humus B-horizon). In peat soils well decomposed organic matter such as humus is black or nearly so, while raw peat is usually brown. N E SA SC M O PL -E O E LS C H S AP TE R S Under well drained conditions the matrix color is an expression of the nature of the parent material and of the pedogenetic development stage of the profile over different rocks. Worldwide, soils developed over limestone remain white to yellowish white (10 YR), but with an increased leaching of CaCO3 the color turns yellow (7.5 YR) or yellowish red (5 YR). Deep red to purple colors (10 R–2.5 YR) are typical for basaltic substrata; yellowish red (5 YR) is the dominant hue for granitic soils; and shales produce definitely yellowish soils (7.5 YR). Likewise are salt crusts characterized by a shiny white color, while calcium carbonate accumulation layers have a yellowish-white hue. In combination with the position in the profile, the color is also an indirect measure of the drainage. While a uniform deep red matrix color in tropical and subtropical soils is always an expression of a good internal drainage, the presence of a grayish-red mottling in depth indicates a slower internal drainage and a fluctuating water table, dominated by alternating reduction and oxidation conditions. 5.6. Bulk Density and Soil Porosity U Bulk density is a measure of the weight of soil per unit volume (g/cm3), usually given on an oven-dry basis. It is an important parameter to estimate soil porosity. Bulk density is mainly influenced by texture and organic matter content, and is therefore often extrapolated from these two parameters (Table 3). For clay soils it is in the order of 1.0 to 1.3 g/cm3; for sandy soils it varies between 1.3 and 1.8. Organic soils have lower bulk densities, with figures in the order of 0.3- 0.8. g/cm3. Completely packed mineral soils, i.e. without any pore space between the particles have a (particle) density of 2.62.65 g/cm3. The latter figure corresponds to the density of the most common minerals in soils, quartz, feldspars and colloidal silicates. When an unusual amount of heavy minerals (magnetite, epidote, zircon) is present in the soil, its density may attend 2.75 g/cm3 or even higher. Bulk density is rarely measured. It can be quantified in the field using Burger cylinders or a membrane densitometer. In the laboratory, the weight of a sample coated with paraffin in air is compared with that of a sample immersed in mineral oil (method based on hydrostatic pressure). Bulk density is nevertheless an important parameter in quantitative soil studies and in particular to convert laboratory data(which are mostly given on a soil-weight basis) of other soil parameters into volume-based information Such information is necessary, for example, in irrigation and drainage studies (for calculating the amount of water that has to be supplied to a crop or that has to be drained from the soil) or for estimating the rates of clay formation and carbonate ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye accumulation in the profile. To convert per cent soil into weight per unit volume, the per cent figure is multiplied by bulk density. The bulk density of a soil, or of the different horizons in the profile, permits to assess the total volume and weight of soil over a given area and depth and, subsequently, of the volume occupied by mineral soil and pores (see: Soil Physics). The weight of a 20 cm thick surface horizon with a bulk density of 1.3 g/cm3 is approximately 10,000 m2 (surface) x 1.3 g/cm3 (bulk density) = 2,600,000 kg/ha. N E SA SC M O PL -E O E LS C H S AP TE R S Soil porosity or total pore space can be assessed directly from bulk density. The calculation is based on the following reasoning: if the entire soil volume would be occupied by soil solids the pore space volume would be zero and the particle or bulk density would be 2.60 – 2.65. If this density decreases, pores space increases, and the volume occupied by solids decreases in inverse proportion to the bulk density. Hence, for a bulk density of 1.3 g/cm3, the volume of solids is: 1.3 g/cm3 ×100 = 50% 2.6 g/cm3 Inversely, the pore space is 100% - 50% = 50%. A proper interpretation of the pore space in view of soil aeration and water infiltration measurements is only possible if also the size of the pores is known, and this is mainly determined by soil texture: sandy soils have mainly macro-pores and therefore allow water to infiltrate easily, while clayey soils are dominated by micro-pores; the latter have a low infiltration potential but a high water storage capacity. 5.7. Water Retention and Infiltration U Soil pores can be filled with water or air. In waterlogged soils, i.e. where the groundwater level is at the surface, all pores are filled with water. Because most plants need water and air both, this complete soil saturation can not persist for a long period before plants start to suffer and, therefore, pores must be alternately filled with water and air. In non-saturated soils water can freely move both in the vertical (infiltration, capillary rise) and horizontal (lateral) direction insofar that alternate oxidation and reduction conditions occur. Part of the soil water is, however, more or less fixed to the soil particles due to the so-called matric forces. These are forces which resist gravitation and, thus, retain water in the soil. The infiltration rate and permeability of a soil depend mainly on the texture, which determines the size of the pores. The measurement of the soil permeability can be operated in the field by the double ring infiltrometer test. This measurement takes several hours because after an initial rapid water infiltration the permeability tends towards an equilibrium stage, and it is this stabilized rate which is the correct value. The standardized infiltration rate of a soil is mainly determined by texture, but other ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye factors like structure, the presence of coarse fragments, mole and earthworm holes, and clay mineral composition might affect the final figure as well. For this reason, infiltration tests are always carried out in triplicate, and if the values still differ consistently additional measurements are needed. Approximate infiltration rates which can be used as a rule of thumb are given in Table 3. In soils where horizons have a different textural composition the water infiltration rates will differ accordingly. Under these conditions, the layer with the lowest infiltration will determine the overall permeability of the profile. The effect of soil permeability can be summarized as follows: N E SA SC M O PL -E O E LS C H S AP TE R S In sandy, very permeable soils the soluble components will rapidly be leached and will concentrate in the groundwater; in fine-textured soils this process is much slower; On clayey soils the rainwater of heavy storms will stagnate on the surface and temporarily obstruct soil aeration on flat surfaces or enhance erosion on slopes. Water retention and water holding capacity of a soil are primarily affected by soil porosity. In large pores water is mainly under the influence of gravitational forces; in smaller pores water is held by matric forces. Water is held within the soil pores with varying degrees of tenacity depending on the amount of water present and on the size of pores. Experimental research has shown that water which is held in the soil with a force of less the 0.3 atm. (sometimes less than 0.1 atm. depending on texture) can not be retained - and is thus lost for plant uptake - but is evacuated as drainage water; the critical pressure at which this process starts is called field capacity. U On the other hand, the water that is retained by the matric forces at a strength of more than 15 atm is no more available for most plants (some xerophytes are an exception) because they are not able to overcome this suction strength; this critical point is called the wilting point. Obviously, a considerable amount of water in soils, e.g. the one found in the smallest micro-pores and around individual soil particles, is thus not available to higher plants. The water that is held by the soil with a strength between 0.3 and 15 atm. is considered as useful for the vegetation. The principles which are at the basis of these hydraulic properties of the soils are discussed at large in Soil Physics. In conclusion, the total amount of water in the soil consists of: Gravitation water, e.g. the water that is held in the macro-pores up to field capacity; all excess water from a rain storm is immediately drained from the profile; Absorbable capillary water: this is the water that is retained in the pores < 8 microns and which is held with a suction force between 0.3 and 15 atm. Non-absorbable capillary water stored in the finer pores, and retained with a force > 15 atm which is too high for most plants; Hygroscopic water; it is the water that is mainly absorbed from air humidity and is held by very strong matric forces in the very fine pores; Crystal water is contained in the crystalline lattices of clay minerals and therefore is not available to plants as well. ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye These different forms of water are gradually released by heating the soil progressively up to 1000 °C. 5.8. Soil Air and Aeration Soil pores can be filled with water or air. Except in completely saturated soils the latter fraction occupies 15 to 30% of all pores. In a well aerated soil, i.e. in a soil where there is a free exchange of soil air and atmospheric air, the composition of the soil air is almost similar in oxygen and nitrogen concentrations. The CO 2 concentration is, however, higher in the soil due the mineralization of humus and the respiration of roots (see: Soil Biochemistry). In local pockets and soil parts where there is no free exchange the concentrations of O2, N and CO 2 can change rather drastically. N E SA SC M O PL -E O E LS C H S AP TE R S Soil air concentrates mainly in the pores not occupied by water. After a rain, large pores are the first to be vacated by the soil water, followed by the medium-sized pores as water is removed by evaporation and plant uptake. Thus, the soil air first occupies the large pores and then, as the soil dries out, also those intermediate in size. This explains why clayey soils (with a high proportion of tiny pores), are generally poorly aerated. In such soils, water dominates and the rate of diffusion of air in and out the soil is small. As a result, levels of CO 2 are high while those of O2 are low, i.e. conditions unsatisfactory for good plant growth. In a well aerated soil the difference in composition between the atmosphere and the soil air at 15 cm depth should be approximately 20.6% O2 ; 79.2% N and 0.25% CO 2 in the soil, as compared to 20.97% O2 ; 79.0% N and 0.03% CO 2 in the atmosphere. U The CO 2 production in soil is estimated at 5 g/m2/day, but this figure can change as a function of root activity and season of the year. It depends on temperature, moisture and the nature and amount of humus. The level of the CO 2 content in the soil is thus a good parameter for the soil biological activity and for soil fertility in general. The interchange of gases between the soil and the free atmosphere above depends primarily on two factors: (a) the rate of biochemical reactions influencing the soil gases, and (b) the actual rate at which each gas is moving into or out of the soil; the latter is also determined by the porosity, and thus by the bulk density, texture and structure. Obviously, the more rapid O2 is taken up by the plants and CO 2 is correspondingly released, the greater will be the necessity for the exchange of gases. Because the total pore space as well as the average size of the pores is much less in the deeper horizons, the subsoil is usually more deficient in oxygen than is the topsoil. Likewise seasonal differences in soil-air composition will occur because of the uneven wetting of the soil and of biological activity. 5.9. Organic Matter Organic matter is present in varying amounts in mineral soils and is then mainly concentrated near the surface. The major source of soil organic matter is plant tissues. ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye Even in cropped areas approximately 10% of the crop is left in the soil (tops and roots) and subsequently digested by microorganisms; animals are usually considered secondary sources of organic material. Raw organic tissues incorporated into the soil are attacked by a host of different soil organisms. The easily decomposed compounds quickly succumb, first yielding intermediate substances and finally the simple soluble products. Hence, two major kinds of organic compounds tend to be stabilized in the soil: the microbial resistant compounds of higher plant origin, and the new compounds such as polysaccharides and polyuronides which are synthesized by microorganisms and held as part of their tissue. Both compounds provide the basic framework for humus, which makes up the bulk of all soil organic matter (see: Soil Biology and Microbiology and Soil Biochemistry). N E SA SC M O PL -E O E LS C H S AP TE R S Along this humus forming process, a number of side reactions occur that bind proteins and other organic nitrogen compounds as integral parts of the humus complex, thereby protecting the N from degradation to simple inorganic forms. Regardless of the mechanism by which nitrogen is bound, the important fact is that the resultant product, i.e. the newly formed humus, is quite resistant to further microbial attack. The chemistry of humus, as well as the processes involved in its formation is exceedingly complex. Large amounts of CO 2 are involved during its formation, with carbon being the dominant component. The composition of humus is, however, relatively uniform. On a weight basis the dry matter is mostly carbon and oxygen, with <10% of hydrogen and inorganic elements (ash). On an elemental basis more than 90% of dry matter is C, N and O; with N, S, P, K and Ca as minor components. Its major characteristics can be summarized as follows: U Colloidal humus particles are composed of C, N, H and O. They contain 36% nitrogen and 58% carbon. The organic matter content can be estimated by multiplying the percentage of carbon by 1.724 Its surface area is very high, generally exceeding that of silicate clays. The colloidal surface of humus is negatively charged, but is pH dependent. At high pH the CE of humus far exceeds that of silicate clays (average 260 cmol/kg) Its water holding capacity on a weight basis is 4-5 times that of silicate clays, in other words humus will absorb from a saturated atmosphere water equivalent to 80-90% of its weight. Humus has a low plasticity and cohesion, which helps in a good aggregate formation and stability. Humus is an important reservoir of P and S. The C/N ratio is an indicator of the decomposition of the humus: it is >20 in peat soils and ranges around 10-12 in well aerated soils. This ratio varies moreover with the nature of the humus, the stage of its decomposition, the nature and depth of the soil, and the climatic and other environmental conditions under which it is formed Impact on other soil properties and land use – Soil organic matter is important to many other soil properties, especially to soil structure, water retention, permeability and ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye cation exchange capacity. In tropical soils dominated by low activity clays and in sandy soils poor in clay, humus is the most important soil component for nutrient fixation and water retention. Organic matter acts as a binding agent between individual particles and, thus, encourages the formation of clumps or aggregates. Organic matter influences the physical and chemical properties of soils far out of proportion to the small quantities present. Furthermore, it supplies energy and body building constituents for most of the microorganisms, and is a direct supplier of nutrients for plants and crops. 5.10. Soil pH N E SA SC M O PL -E O E LS C H S AP TE R S The soil pH (or soil acidity) is an expression of the ion concentration in the soil and in the soil solution, and as such it is a good parameter to characterize the exchangeable complex of the cations adsorbed to colloid surfaces. Ions commonly present are Ca 2+ , Mg 2+ , K + , Na + (the four of them often called the alkaline earths), H + , Cl − , NO3− , SO 4 2− , HCO3− , CO32− and OH − . The relative proportion of these ions determines the soil pH. The chemical concept of pH stands for the logarithm of the reciprocal of the concentration of H+ ions in a solution. It can vary between 0 and 14 in chemical solutions, but in the soil its range is limited between 2.5 and 9.5. The neutral value is 7.0. Below and above that figure the soil is considered acid or alkaline (basic). In acid sulfate soils the pH can drop to a figure as low as 2.5-3. In podzols, peat soils and highly weathered tropical soils it is often less than 5; in degraded leached soils between 4 and 5; in brown forest soils between 5 and 6.5; in rendzina and calcareous soils between 7.5 and 8.5; in sodic soils between 8.2 and 9.5. Table 4 defines the pH ranges as they are commonly used in soil science literature. U Hydrogen ions are derived from rainfall and from organic and inorganic acids produced within the soil. The pH of rain varies between 3.0 and 9.8. Pure rainwater, in equilibrium with atmospheric CO 2 at 25°C has an average pH of 5.7. Values lower than this are thought to be due partly to atmospheric pollution, whereas higher values are attributed to salts derived both from ground water or wind-blown components. Carbonic acid is formed within the soil by a combination of CO 2 (the content of which varies with respiration by plant roots and microorganisms) and water. The pH of a given soil is not uniform throughout. There might be slight variations (less than one unit) from place to place due to variations in CO 2 or organic acid concentrations, the content and composition of the exchangeable bases, or the presence of nearby roots, since these commonly contain adsorbed H + . Such slight fluctuations are observed when soils dry up, especially above average field temperatures (pH increases), or in spring and summer when biological activity increases and more acid exudates are released (pH decreases). More important variations in soil pH (more than one unit difference) take place under ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye conditions whereby: N E SA SC M O PL -E O E LS C H S AP TE R S The amount of adsorbed hydrogen and aluminum increases (a) in the case of organic matter decomposition and carbonic acid ( H 2 CO3 ) formation which results from the reaction of CO 2 with water; (b) in the case of transformation of inorganic acids like H 2SO 4 and HNO3 , both formed from organic decay and microbial action on certain fertilizer materials such as sulfur and aluminum sulfate, (c) in the case of acid rainwater (pH 2-4.5), where the acidity in the atmosphere is generated from oxides of nitrogen and sulfur coming mostly from the combustion of coal, gasoline and other fossil fuels. The amount of adsorbed bases increases due to: (a) weathering which releases exchangeable cations from minerals and makes them available for adsorption; (b) soil management techniques are applied like liming or irrigation with water that contains various kinds of salts; (c) avoidance of leaching, for example through natural conditions in arid soils. The resistance to a change in the pH of a soil solution is called soil buffering. It can mainly be related to the tendency to achieve equilibrium between the hydrogen ion concentration of the soil solution (active acidity) and the changing amount of hydrogen and aluminum ions adsorbed on the soil colloids. Obviously, the higher the exchange capacity of a soil, the greater will be its buffering capacity. Determination of soil pH- There are several ways to determine the soil pH depending on the accuracy required. The most accurate method is by using a pH meter; in this electrometric method the hydrogen concentration of the soil solution is balanced against a standard hydrogen electrode or a similarly functioning electrode. U A more simple and easy but somewhat less accurate method consists in the use of a soil color indicator and its comparison with a standard color scale. By using a number of dyes, either separately or mixed, a pH range of 3-8 is easily covered. In making such a pH determination the sample is saturated with the dye, and after standing in contact a few minutes a drop of the liquid is run out and its color observed. This method is accurate within about 0.2 unit. Impact on land use- The soil pH determines to a large extent the availability and potential uptake of plant nutrients from the soil (see: Soil Chemistry and Fertility). A pH range of 6-7 seems to promote the most readily availability of plant nutrients. Under lower pH values nutrients are either absent or become less available. From pH < 4.5 – 5.0 an appreciable amount of Al, Fe, Mn are soluble and become toxic to certain plants or, as is the case for Fe, bind P in the soil so as to make it unavailable for root uptake. However, if in such acid soils the pH is rapidly increased, precipitation can take place and the amount of these ions in solution decreases until at neutrality or thereabout certain plants may suffer from a lack of available manganese and iron. This is especially true if a decidedly acid sandy soil is suddenly brought to a neutral or alkaline condition by an excessive application of lime. If the reaction is held within a range of 6.0-7.0 the deficiency of Fe and Mn may be avoided. Copper and zinc are ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye affected in the same way by a rise in pH, the critical point being between pH = 6 and 7; above 7 their availability definitely declines. Under moderately alkaline conditions (pH between 7.0 and 8.5) Ca 2+ is the dominant cation. In soils with a pH between 7.3 and 8.5 there is free lime in the root zone and this may negatively affect the growth and production of some plants and crops. At pH > 8.5 an excess of Na + both affects nutrient uptake of most plants and soil structure and aeration (Table 4). Definition Extremely acid Very strongly acid Strongly acid Medium acid Slightly acid Neutral Mildly alkaline Moderately alkaline Strongly alkaline Very strongly alkaline Impact on Base Saturation and Cation Activity BS < 15%; much exchangeable Al which is toxic BS 15-35%; moderate to few exchangeable Al BS 35-50%; H dominant; no more exchangeable Al BS approx. 50%; H dominant BS 50-80%; few H; alkaline earths dominate BS 80-100%; exchangeable Ca and Mg dominate BS 100%; exchangeable Ca dominates; free Ca BS 100%; Ca dominates; few exchangeable Na BS 100%; much exchangeable Na that is toxic BS 100%; exchangeable Na dominates, toxic N E SA SC M O PL -E O E LS C H S AP TE R S pH range < 4.5 4.5 – 5.0 5.1 – 5.5 5.6 – 6.0 6.1 – 6.5 6.6 – 7.3 7.4 – 7.8 7.9 – 8.4 8.5 – 9.0 > 9.1 Table 4: Impact of different pH ranges on base saturation and ion activity Soil organisms are influenced by the pH fluctuations of the soil solution. Bacteria, except those that oxidize sulfur to sulfuric acid, and actinomycetes generally function better in mineral soils at intermediate and higher pH values; their activity is sharply curtailed when the pH drops below 5.5. Fungi however are particularly versatile, flourishing satisfactorily at a wide range of pH. In normal soils, therefore, fungi predominate at the lower pH values, but at the intermediate and higher ranges they meet strong competition from the bacteria and actinomycetes. U All in all, a soil in the intermediate pH range presents the most satisfactory biological regime. Nutrient conditions are favorable without being extreme, and phosphorus availability is at a maximum. For this reason, soil pH manipulation is a current management technique in farming. A rise of the pH is sometimes necessary under conditions that the soil is naturally acid or because of the acidifying effect of certain fertilizers; this is achieved by adding a Ca-compound, either under the form of lime ( CaCO3 ) which works slowly, or gypsum ( Ca 2SO 4 ) which is much faster. A reduction of the soil pH is sometimes desirable to favor such plants as rhododendrons or azaleas, or to discourage certain diseases, especially actinomycetes which produce potato scab. In gardens and over small surfaces this may be done by mixing acid forming organic matter (leaf-mold, pine needles, tan-bark, sawdust and moss peat) with the soil. Otherwise, or on larger surfaces, a similar result may be achieved by using acidifying chemicals such as ferrous sulfate, sulfur or sulfuric acid. ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye 5.11. Cation Exchange Complex Most soil colloids, both inorganic and organic, have a net electronegative surface charge, and cations liberated from natural weathering are attracted to these charged surfaces. The strength of cation attraction varies with the colloid and the particular cation. The cation exchange capacity (CE) of a soil stands for the total quantity of cations it can absorb or exchange under well defined pH conditions. N E SA SC M O PL -E O E LS C H S AP TE R S The CE of a soil is expressed in moles of positive charge per unit mass: cmol/kg or in milliequivalents per 100 g of soil (me/100g soil). Cations are adsorbed and exchanged on a chemically equivalent basis. This means that one mole of charge is provided by one mole of H + , K + or any other monovalent cation, by ½ mole of Ca 2+ , Mg 2+ or other divalent cation, or by 1/3 mole of Al3+ or other trivalent cation. In older literature CE has been expressed as milliequivalent per 100g of soil. As in the International System of Units (SI) 1 milliequivalent per 100g of soil is equal to 1 centimole of positive or negative charge per kg of soil, it is easy to compare soil data using either of these methods. The CE of a soil is determined by its clay and humus content, the type of the clay and the pH. The sand and silt fractions have exposed unsatisfied negative bonds, but due to a low specific surface, they contribute little to the CE of most soils. Soils high in clay and humus have a high CE. Because of the various surface characteristics of different clay types (see: Soil Mineralogy) also the type of clay is important. Kaolinite has a CE of 15 me/100 g soil, illite 25-40 me/100 g soil, and montmorillonite 70-100 me/100 g soil. Well-humified organic matter can attain between 180 and 250 me/100 g soil. On weight basis, humus particles have thus a greater nutrient retention capacity than clay. However, clay is generally present in larger amounts and, therefore, its total contribution to the chemical and physical properties are usually equal or even higher than that of humus. U The CE can also vary with the soil pH. At very low pH only the permanent charge of the clay (linked to isomorphous substitutions in the silicate clay crystals) and a small portion of the charges of organic colloids hold ions that can be replaced by cation exchange. As the pH is raised the H + held by organic colloids and silicate clays such as kaolinite becomes ionized and is replaceable. Also the aluminum hydroxyl ions are removed releasing thereby additional exchange sites on the mineral colloids. The variable charges are pH dependent. The net result is an increase in the negative charge on the colloids and, in turn, an increase in the CE. In most cases the CE is determined at pH = 7 or above. This means that it includes most of these charges dependent on pH as well as the permanent charges. Cation Exchange Capacity, Exchangeable Bases and Base Saturation- There are two groups of natural cations that can occupy the exchange sites of clay and humus: Ca 2+ , Mg 2+ , K + and Na + on the one hand and H + and Al3+ on the other hand. The two groups of cations have opposing effects on soil acidity and alkalinity. Hydrogen and aluminum tend to dominate very acid soils. Most of the other cations, called ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye exchangeable bases, neutralize soil acidity and dominate in neutral and alkaline soils. The proportion of CE occupied by these bases is called the percentage base saturation. This parameter is a good expression of the amount of nutrients available for plant uptake. As this parameter is directly related to the pH concentration, there exists a good correlation between the percentage base saturation of a given soil and its pH (Table 4). The greater the supply of a given cation from weathering, the greater the likelihood that it will be adsorbed according to the law of mass action, and the higher will be the base saturation. The amounts and kinds of cations actually adsorbed, however, are significantly affected by cation valence and hydrated radius. N E SA SC M O PL -E O E LS C H S AP TE R S Cations with a greater valence are adsorbed more efficiently than cations of a lower valence (example Ca 2+ versus K + ). Moreover, for a given valence the cation with the smallest hydrated radius will move the closest to the negatively-charged soil particle and be more strongly adsorbed. The energy of adsorption decreases as the square of the distance. Considering some of the most common exchangeable cations in soils, the replace-ability series is usually Al>Ca>Mg>K>Na . Exchangeable hydrogen is difficult to include in the series because of the uncertainty of its hydration properties. Calcium is adsorbed more strongly than sodium because it has both a greater valence and a smaller hydrated radius. As a result, sodium is readily leached from soils of humid regions, while calcium is preferentially adsorbed, also because it is frequently the most abundant exchangeable cation. A few soil colloids have a positive charge; this is the case for iron and aluminum hydroxides (sesquioxides) and amorphous materials such as allophane and can attract negatively charged anions like Cl− or NO 3− or H 2 PO 4 − . This process affects mainly the uptake of mineral fertilizers provided in such a form. U Measuring Cation Exchange Capacity and Exchangeable Bases - The CE is determined routinely by saturating all exchange adsorption sites with a single index cation, such as ammonium NH 4 OAc method) or barium ( BaCl2 -triethanolamine method) at pH = 7, followed by the displacement of the NH 4 with 1N KCl, and determination of NH 4 in the percolate. The individual bases are directly determined in the percolate by flame photometry or atomic absorption spectrometer. 6. Soil Survey and Classification Soil surveying is the process of identifying and describing soils and to locate their extension on a map. The Soil Survey Manual (USDA, 1951) defines a soil map as a cartographic document designed to show the distribution of soil types or other soil mapping units in relation to other prominent physical and cultural features of the earth’s surface. The units may be shown separately or as soil associations named and defined in terms of taxonomic units. This definition is intended to exclude maps showing single soil characteristics like texture, slope, depth, color, or arbitrary combinations of two or more of these; maps showing soil qualities like fertility, drainage or erosion risk; or maps showing individual soil genetic factors or combinations of them. ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye The process of mapping or surveying consists of walking over the land at regular intervals and taking notes of soil differences at the surface and in the nature of superposed horizons in the profile. Therefore, it needs to use auger and profile pit descriptions. It also incorporates the study of all related surface features, such as slope gradients, evidence of erosion, land use, vegetative cover, and cultural features. Boundaries are drawn directly on topographic or cadastral maps, upon availability, or on aerial photographs representing, in most places, changes from one soil to another. 6.1. Types of Soil Surveys N E SA SC M O PL -E O E LS C H S AP TE R S There are four main types of soil surveys, generally determined by the scale and density of observations: detailed, semi-detailed, reconnaissance and schematic surveys. Detailed surveys are at scale 1/20,000 or larger. Most of the survey work is done in the field, based on observations at a density of 1 per cm2 of map. The soil unit is the type or series, and these are generally homogeneous in terms of direct agricultural or other uses at farm or parcel levels. Detailed soil surveys allow making recommendations on the type of crops to be grown, including a reasonable estimation of yield levels. Semi-detailed surveys are at scales between 1/20,000 and 1/50,000. The density of field observations varies between 1 and 4 per cm2, but the latter is only acceptable if good aerial photos are available and after the surveyor has been able to make a good correlation between inherent soil properties and surface features directly observed on the photos. The mapping unit is generally not homogeneous and is either a complex or an association of soils. Semi-detailed surveys are useful for regional planning and rural development over small areas, with recommendations focusing on groups of crops (cereals, tree crops) and general yield estimations. U Reconnaissance surveys refer to mapping scales between 1/50,000 and 1/200,000; the map unit is invariably a soil association. This is a grouping of soils related to each other in a catena or landscape relationship. In other words, the soil association indicates the presence of 3 or more soils, qualified as dominant (covering more than 50% of surface), associated (covering 15-50% of the surface) or inclusions (covering less than 15% of the surface). This level of mapping is useful in the study of extensive land use problems or for regional agricultural or ecological planning. It does not allow making reasonable yield estimations. The density of observations varies between 1 to 50 and 250 ha and is a function of the use that can be made from remote sensing techniques (Landsat and other satellite images); they are made to identify the main properties of the individual units within the association. Therefore, reconnaissance surveys can be made quickly and at reasonably low cost. Schematic surveys are operated at scales smaller than 1/200,000 and are in fact more geomorphologic studies than real soil investigations. Most of the work is focused on the delimitation of major landscape units and of their subsequent characterization in terms of soils. They require good satellite images and are only carried out as a first step in exploring almost virgin land where little or no land data are available. The main purpose of this type of mapping is to localize areas where more in-depth studies might be useful in a later stage, and/or to exclude areas where such investigations are not economical for the given purpose. ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye 6.2. Soil Survey Procedures Soil surveys are based on the description of soil properties at representative locations in the landscape (point observations), and the subsequent grouping of these observations in more or less homogeneous soil units (aerial units). Three types of procedures can be applied: grid mapping, free mapping and the ethno-pedological approach. N E SA SC M O PL -E O E LS C H S AP TE R S In the grid mapping approach individual field observations are carried out at fixed distances from each other, following a close grid which covers the land independently of topographical or hydrographical variations. The advantage of this system is that it describes all parts of the land equally without any subjective interpretation from the surveyor. It is often conducted by un-experienced soil surveyors; in detailed surveys where all soils are to be included, however small their extension may be, they must be mapped. Its disadvantage is that it is time-consuming and often includes observations which were not really necessary for a good mapping of the area concerned. The principle of free surveying is based on the concept that the field observations should only be carried out to accurately describe and delineate the different types of soils, making use of modern techniques of mapping like remote sensing applications. In other words, free surveying can only be applied after a preliminary interpretation of the aerial photo cover has allowed a good delineation of homogeneous landforms which coincide with characteristic soil units. It requires good survey experience and the ability to understand the relationship between simple landforms and soil cover and, therefore, it can only be carried out if a good photo cover is available. The advantage of this method is that it is faster and more economical than grid mapping. It is often used in semidetailed and reconnaissance surveys, where it forms a first step in the survey, followed by the identification of zones which need more detailed investigations. In schematic surveys it is the only applicable procedure. U As field mapping is a time-consuming and costly procedure it often depends on the availability of funds, and in recent years such funds are often lacking. To overcome this problem there is now a tendency to integrate local untapped knowledge in the system by asking local people to identify the different types of soils they know in their area and to show in the field where these soils are located, viz. where their properties change. In a subsequent step the soil surveyor needs to describe the soils identified and to control the field limits through a few additional field observations. The advantage of this system is that it is relatively fast and cheap, while the end product is reasonably accurate. The correct application of this procedure requires nevertheless that the soil surveyor should be able to decide for himself to what extent the ethno-pedological information meets the requirements of the study, and to what extent additional data should be collected. 6.3. Purpose and Use of Soil Maps The greatest objective of drafting a soil map is to know and understand the nature and properties of the soils and their extension, slope and degree of erosion, etc. for a subsequent optimal land use and natural resource management. To meet these objectives the soil map should not only be accurate, but it should also be accompanied by a proper report describing the soil properties registered (generally supported by analytical data) ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye and providing the interpretation of such data and recommendations for the proper objectives of the study. In the past many soil surveys have failed to meet the objectives of the users by remaining too descriptive and academic, without due attention to the needs of those who have commanded the study. This point has been a major matter of concern at the 18th World Soil Congress in 2006 in Philadelphia, USA, when the future research directions and opportunities of soil science were taken into consideration. N E SA SC M O PL -E O E LS C H S AP TE R S There are quite a number of institutions which are keen to make use of soil survey investigations. This refers in the first place to regional planning agencies concerned with land capability, land use and rural development strategies. Irrigation and drainage needs are exclusively based on soil properties and their changes. Large-scale fertilizer applications, in particular under conditions where over-consumption should be avoided for environmental reasons, are linked to soil properties and plant needs. Soil conservation planning and environmental protection require a good knowledge of the soils concerned and their relative extension in the landscape. Banks and other money-lending agencies use soil surveys in determining security for loans. Real estate companies and individuals interested in buying or selling land make extensive use of soil surveys to assess the market value of the land (see: The Value and Price of Land). Taxes are based on soil types in some countries. Soil information is also used in highway and drainage works, by various kinds of manufacturers in selecting suitable locations for factories. 6.4. Soil Classification U Objects are classified with a view of arranging them into classes or groups, based on a hierarchical system of criteria or factors. Soil classifications use diagnostic soil properties like the presence of specific horizons, texture, depth, etc. which can either be recognized or measured in the field or in the laboratory and are an expression of soil formation (pedogenesis) and profile development. Because criteria might differ or their relative weight in the hierarchical system might be unequal, there exist different soil classification systems. The principles of soil classification in general and of the soils in different agro-climatic zones in particular have been discussed at large in: Soil Geography and Classification, Soils of Arid and Semi-Arid areas, Mediterranean Soils, and Soils of the Humid and Sub-humid Tropics, respectively. A first genetic classification was suggested about 1880 by the Russian scientist Dokoutchaiev. The principle has been further developed by European and American researchers in the beginning of the last century, and can largely be found back in the French CPCS classification. In this system, now largely abandoned, soils are classified on the basis of their obvious or presumed pedogenetical history, even if the profile properties that should reflect this evolution are not always clearly observed. More recent classifications, like US-Soil Taxonomy and the World Reference Base for Soil Resources (WRB) use mainly more quantified criteria. Though very often the two systems use similar diagnostic criteria they also differ in that the first includes pedoclimate as an important differentiating factor, while for the latter this criterion is not taken into consideration. ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye The particularity of modern systems is that they identify classes of soils on the basis of properties as found today and that can be measured quantitatively either in the field or in the laboratory. Moreover, the measurements so obtained can be verified by others. This lessens the likelihood of controversy over the place of a given soil in the classification system – which is common when scientists deal with systems where genesis or presumed genesis is the basis for the classification. The advantage of a quantitative system over the one based primarily on (presumed) soil genesis are: (1) it permits classification of soils rather than soil forming processes; (2) it focuses on the soil rather than on related sciences such as geology and climatology; (3) it permits the classification of soils of unknown genesis as only the knowledge of their properties is needed; (4) it permits greater uniformity of classification as applied by a large number of soil scientists. Differences in interpretation of how a soil was formed do not influence its classification in this scheme. N E SA SC M O PL -E O E LS C H S AP TE R S The second significant feature of a system like Soil Taxonomy is the nomenclature employed, especially for the broader classification categories. The names give a different connotation of the major characteristics of the soils in question – a connotation easily understood in many languages since Latin or Greek root words are the basis for the names. Tables 5 and 6 give an overview of the formative syllables, derivation and meaning for the Soil Orders and Suborders, the highest hierarchical categories in Soil Taxonomy (USDA, 1975 and updates). The most recent versions of the World Reference Base for Soil Resources (FAO, 2006) follow this trend by using clearly defined prefix and suffix qualifiers. Order U 1. Entisol 2. Vertisol 3. Inceptisol 4. Aridisol 5. Mollisol 6. Spodosol 7. Alfisol 8. Ultisol 9. Oxisol 10. Histosol 11. Andisol 12. Gelisol Formative syllable Ent Ert Ept Id Oll Od Alf Ult Ox Ist And El Derivation Meaning Meaningless syllable L. verto, turn L. inceptum, beginning L. aridus, dry L. mollis, soft Gr. spodos, wood ash Meaningless syllable L. ultimus, last F. oxide, oxide Gr. histos, tissue Modified from ando L. gelare, to freeze Recent soil Inverted soil Inception or young soil Dry soil Soft soil Ashy (Podzol) soil Pedalfer (Al-Fe) soil Ultimately leached soil Soils with Fe and Al oxides Organic soils, tissues visible Having andic properties Frozen soils Table 5: Formative elements, derivations and meanings of Orders in Soil Taxonomy (USDA, 1975 and http://soils.usda.gov/technical/classification/ taxonomy) Formative element Alb Anthr Aqu Derivation of formative element L. albus, white Gr. anthropos, human L. aqua, water ©Encyclopedia of Life Support Systems (EOLSS) Connotation of formative element Presence of an albic (bleached) horizon Modified by humans Characteristics associated with wetness LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye L. arare, to plow L. argilla, clay Gr. boreas, northern L. calcis, lime L. cambrare, to exchange Gr. kryos, ice cold L. durus, hard L. fibra, fiber L. fluvius, river L. folia, leaves L. gypsum, gypsum L. hemi, half Gr. histos, tissue L. humus, earth Gr. orthos, true L. per, throughout (time) Gr. psammos, sand Modified from rendzina L. base of sal, salt Gr. saprose, rotten L. torridus, hot and dry L. turbidus, disturbed L. udus, humid L. ustus, burnt L. vitrium, glass Gr. xeros, dry Mixed horizon Presence of an (illuvial) argillic horizon Cool soil Presence of a calcic horizon Presence of a cambic horizon Cold Presence of a duripan Least decomposed stage Floodplain Mass of leaves Presence of gypsic horizon Intermediate stage of decomposition Presence of organic (peaty) material Presence of organic matter The common ones Periodic moisture regime Sand texture High carbonate content Presence of a salic horizon Most decomposed stage Torric moisture regime Presence of cryoturbation Udic moisture regime Ustic moisture regime Presence of volcanic glass Xeric moisture regime N E SA SC M O PL -E O E LS C H S AP TE R S Ar Arg Bor Calc Camb Cry Dur Fibr Fluv Fol Gyps Hem Hist Hum Orth Per Psamm Rend Sal Sapr Torr Turb Ud Ust Vitr Xer Table 6: Formative element in the names of Suborders in Soil Taxonomy (USDA, 1975 and http://soils.usda.gov/technical/classification/ taxonomy) U Glossary Adsorption: Aeration (soil): Available water: Base saturation percentage: The attraction of ions or compounds to the surface of a solid. Colloids adsorb large amounts of ions and water; different from absorption which refers to the movement of ions and water into the plant root as a result of a diffusion process. The process by which soil pores are filled by air from the atmosphere. In a well-aerated soil, the soil air is similar to the atmosphere above. Less well aerated soils contain more carbon dioxide and less oxygen than the atmosphere above the soil. The portion of water in a soil that can be readily absorbed by plant roots, i.e. the water that is held between field capacity (pF 1) and wilting point (pF 4.2). The extent to which the adsorption complex of a soil is saturated with exchangeable cations other than hydrogen and aluminum. It is expressed as a percentage of the total cation exchange capacity. ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye Bulk density: Cation exchange capacity (CE): Consistence: Eluviation: Epipedon (in Soil Taxonomy): N E SA SC M O PL -E O E LS C H S AP TE R S Gley soil: The mass of dry soil (after drying at 105°C) per unit bulk volume, generally expressed in grams/cm3. The sum total of exchangeable cations a soil can adsorb, generally expressed in me/ 100 grams soil or cmol/kg soil. The combination of properties of soil material that determines its resistance to crushing and its ability to be molded or changed in shape. Such terms as loose, friable, soft, plastic, and sticky describe soil consistence. The movement of soil material in suspension (or in solution) from one or more soil layers. A diagnostic surface horizon that includes the upper part of the soil darkened by organic matter, or the upper eluvial horizons, or both. Soil developed under conditions of poor drainage resulting in reduction of iron and other elements and in gray colors and mottles. The process of deposition and accumulation of soil material removed from one horizon, usually from an upper to another lower horizon in the soil. The process of removing materials in solution from the soil by percolating waters A soil consisting predominantly of, and having its properties determined predominantly by mineral compounds. Usually contains < 20% organic matter, but may contain an organic surface layer up to 30 cm thick. Highly decomposed organic material in which the original plant parts are no more recognizable. Contains more mineral matter and is usually darker in color than peat. A soil that contains a high percentage (>20%) organic matter throughout the solum. The unconsolidated and more or less chemically weathered mineral (or organic matter) from which the soil is developed by pedogenic processes. Unconsolidated organic soil material consisting largely of slightly or non-decomposed organic matter accumulated under conditions of excessive moisture. A unit of soil structure such as an aggregate, crumb, prism, block, or granule, formed by natural processes, in contrast with a soil clod which is formed artificially. A three-dimensional body of soil with lateral dimensions large enough to permit the study of horizon shapes and relations. Its area ranges from 1 to 10 m2. The logarithm of the soil moisture tension expressed in centimeters height of a column of water. Is an expression of the available water in the soil. Critical pF levels are: pF = 1 (field capacity) and pF = 4.2 (wilting point). The plant available moisture is the water retained between these two tensions. The negative logarithm of the hydrogen-ion activity of a soil. Illuviation: Leaching: Mineral soil: Muck: Organic soil: U Parent material: Peat: Ped: Pedon: pF: pH (soil): ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye Profile: Regolith: Soil association: N E SA SC M O PL -E O E LS C H S AP TE R S Soil series: The degree of acidity (or alkalinity) of a soil as determined by means of a glass or other suitable electrode (or other indicator) for a specified soil-water ratio (mostly 1: 1 or 1: 2.5). A vertical section of a soil through all its horizons, extending into the parent material. It is usually composed of an A-B-C horizon sequence, or a fraction thereof. The unconsolidated mantle of weathered rock and soil material on the earth’s surface; loose earth materials above solid rock without any obvious sign of soil formation. A group of defined and named taxonomic soil units occurring together in an individual and characteristic pattern over a geographic region comparable to plant associations in many ways. A lower unit of soil classification, e.g. a subdivision of a family, consisting of soils that are similar in all major profile characteristics. The combination or arrangement of primary soil particles into secondary particles, units or peds. These secondary units may be, but usually are not, arranged in the profile in such a manner as to give a distinctive characteristic pattern. The secondary units are characterized and classified on the basis of size, shape and degree of distinctness into classes, types and grades, respectively. The systematic examination, description, classification and mapping of soils in an area. Soils are usually classified in the field according to obvious differences in profile characteristics. The relative proportions of the various soil particles. Soil structure: Soil survey: U Soil texture (or particle size distribution): Soil type: Solum: Waterlogged: Weathering: The lowest unit in the natural system of soil classification: a subdivision of a soil series and consisting of or describing soils that are alike in all characteristics including the texture of the A horizon. The term is also used in a more general context as a kind of soil with specific characteristics. The upper and most weathered part of the soil profile, containing the A and B horizons. Saturated with water. The process of physical and chemical changes produced in rocks, at or near the earth’s surface, by atmospheric agents. Bibliography Baize, D. (2004). Petit Lexique de Pédologie. INRA Editions, Paris, 271p. [An extended explicative dictionary on pedological terms in French, with an English-French index]. Bigham, J.M. and Ciolkosz, J. , eds. (1993). Soil Color. Soil Science Society of America Publication 31, Madison, WI-USA, 159p. [A compilation of 9 papers dealing with the relation between soil color and other soil properties]. Birkeland, P.W. (1979). Pedology, Weathering and Geomorphological Research. Oxford University Press, London/New York, 285p. [A standard work on mineral weathering processes and soil formation]. ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye Brady, N.C. (1984). The Nature and Properties of Soils. Ninth Edition. MacMillan Publishing Company New York, 750p. [An in-depth overview of soil properties with a focus on their impact in field conditions and agricultural applications; with an extended glossary on soil science terms]. Brown, J.R. (1987). Soil Testing: Sampling, Correlation, Calibration and Interpretation. Soil Science Society of America Special Publication 21, Madison, WI-USA [A compilation of 12 articles, including test procedures, interpretations and adapted methods for special soil types]. Duchaufour, Ph. (1998). Handbook of Pedology: Soils – Vegetation – Environment (translated by V.A.K. Sharma). A.A. Balkema, Rotterdam, The Netherlands and Brookfield, US, 264p. [An overview of soil science in terms of properties and processes, with an extensive reference list focused on French and German authors]. FAO (1990). Guidelines for Soil Description. Third Revised Edition. Soil Resources, Management and Conservation Service, Land and Water Development Div., FAO Rome,70p. [A manual with all technical terms used in the field description of soils]. N E SA SC M O PL -E O E LS C H S AP TE R S FAO (2006); World Reference Base for Soil Resources 2006: A Framework for International Classification, Correlation and Communication; FAO World Soil Resources Report 103, FAO, Rome, 128p [The most recent version explaining in full details the concept and application of this new framework for worldwide soil classification]. Foth, H.D. (1990). Fundamentals of Soil Science. Eight Edition. John Wiley and Sons, New York, 360p. [A standard work on properties, soil forming processes and the classification of soils, with due attention to plant nutrition and soil conservation; with an excellent glossary of soil science terms]. Hellin, J. (2006). Better Land Husbandry: From Soil Conservation to Holistic Land Management. Science Publishers, Enfield, New Hampshire, US, 309p. [A book focusing on the ecological aspects of soils and their management in modern times, mainly concentrated on situations from Latin and South America]. Lozet, J. and Mathieu, C. (1991). Dictionary of Soil Science. Second, Revised and Enlarged Edition. A.A. Balkema, Rotterdam, The Netherlands, 348p. [An excellent publication translated from French, explaining the most common soil science terms; a revised French edition was published in 2002]. Munsell Color (1975). Munsell Soil Color Charts. Macbeth Div. of Kollmorgen Corp., Baltimore, USA [The color scale universally accepted as the standard for soil color determination]. Pansu, M., Gautheyrou, J. and Loyer, J-Y. (2001). Soil Analysis: Sampling, Instrumentation, Quality Control. Translated by V.A.K. Sharma. A.A. Balkema, Lisse, The Netherlands, 489p. [A detailed technical manual on the field sampling, laboratory analysis and equipment for the study of soil properties]. U Reybold, W.U. and Petersen, G.W., eds. (1987). Soil Survey Techniques. Soil Science Society of America Special Publication 20, Madison, WI-USA, 98p. [A compilation of 8 articles dealing with different aspects of soil survey techniques, including the use of remote sensing and microcomputer techniques]. Soil Survey Staff (1951). Soil Survey Manual. US Dept. Agriculture Handbook 18, US Government Printing Office, Washington, DC., 503p [A timeless reference book and the basis for soil survey techniques and related terminology worldwide]. USDA (1975). Soil Taxonomy: A Basic System of Soil Classification for Making and Interpreting Soil Surveys. US Department of Agriculture, Soil Conservation Service, Agricultural Handbook 436, Washington, DC, 754p [Background document explaining the concept and structure of Soil Taxonomy; the system has undergone several updates and modifications, see: http://soils.usda.gov/technical/classification/ taxonomy]. WRB (2006). World Reference Base for Soil Resources 2006: A Framework for International Classification, Correlation and Communication. IUSS/ISRIC/FAO, FAO World Soil Resources Report 103, FAO, Rome, 128p. [The most recent version explaining in full detail the concept and application of this new framework for worldwide soil classification]. ©Encyclopedia of Life Support Systems (EOLSS) LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI – Soils and Soil Sciences - Willy Verheye Biographical Sketch U N E SA SC M O PL -E O E LS C H S AP TE R S Willy Verheye is an Emeritus Research Director at the National Science Foundation, Flanders, and a former Professor in the Geography Department, University of Ghent, Belgium. He holds an MSc. in Physical Geography (1961), a PhD. in soil science (1970) and a Post-Doctoral Degree in soil science and land use planning (1980). He has been active for more than thirty-five years, both in the academic world, as a professor/ research director in soil science, land evaluation, and land use planning, and as a technical and scientific advisor for rural development projects, especially in developing countries. His research has mainly focused on the field characterization of soils and soil potentials and on the integration of socio-economic and environmental aspects in rural land use planning. He was a technical and scientific advisor in more than 100 development projects for international (UNDP, FAO, World Bank, African and Asian Development Banks, etc.) and national agencies, as well as for development companies and NGOs active in intertropical regions. ©Encyclopedia of Life Support Systems (EOLSS)
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