CONTENTS
Soil
1 Introduction ............................................................................................................................................................................................................................................... 4.1
2 What is soil? ............................................................................................................................................................................................................................................. 4.1
3 Classification ........................................................................................................................................................................................................................................... 4.2
4 Physical characteristics ................................................................................................................................................................................................................ 4.2
4.1 Texture ......................................................................................................................................................................................................................................... 4.3
4.1.1 Size classification of soil particles ................................................................................................................................................. 4.3
4.1.2 Texture classes ...................................................................................................................................................................................................... 4.4
4.2 Soil structure ......................................................................................................................................................................................................................... 4.7
4.3 Soil consistency ................................................................................................................................................................................................................. 4.7
4.4 Bulk density ......................................................................................................................................................................................................................... 4.8
4.5 Porosity and pore size................................................................................................................................................................................................. 4.8
4.6 Infiltration ................................................................................................................................................................................................................................ 4.9
4.7 Permeability ........................................................................................................................................................................................................................... 4.9
4.8 Water holding capacity ......................................................................................................................................................................................... 4.10
4.8.1 Principles of capillarity ........................................................................................................................................................................... 4.10
4.8.2 Differences in the water holding capacity of soil ..................................................................................................... 4.11
4.9 Quantification of soil water ................................................................................................................................................................................ 4.12
4.9.1 Saturation point ................................................................................................................................................................................................ 4.13
4.9.2 Field capacity .................................................................................................................................................................................................... 4.13
4.9.4 Permanent wilting point......................................................................................................................................................................... 4.13
4.9.4 Total available soil water ..................................................................................................................................................................... 4.13
4.9.5 Readily available water .......................................................................................................................................................................... 4.14
4.9.6 Hygroscopic water ....................................................................................................................................................................................... 4.15
4.10 Soil depth ............................................................................................................................................................................................................................. 4.15
4.11 Calculation examples ............................................................................................................................................................................................. 4.15
5 Chemical properties of soil.................................................................................................................................................................................................. 4.18
5.1 pH ................................................................................................................................................................................................................................................. 4.18
5.2 Sodium adsorption ratio ..................................................................................................................................................................................... 4.18
5.3 Saline quality .................................................................................................................................................................................................................... 4.18
6 Organic properties ......................................................................................................................................................................................................................... 4.20
7 Factors influencing the irrigation of soil ............................................................................................................................................................. 4.22
7.1 Colour of the soil ......................................................................................................................................................................................................... 4.22
7.2 Effective soil depth .................................................................................................................................................................................................... 4.22
7.3 Texture of the soil ....................................................................................................................................................................................................... 4.22
7.4 Clay content ....................................................................................................................................................................................................................... 4.22
7.5 Soil structure ..................................................................................................................................................................................................................... 4.22
7.6 Soil pH ..................................................................................................................................................................................................................................... 4.23
7.7 Quality of irrigation water ................................................................................................................................................................................. 4.23
8 Soil samples .......................................................................................................................................................................................................................................... 4.23
9 A typical soil analysis report ............................................................................................................................................................................................. 4.24
10 Soil conservation ......................................................................................................................................................................................................................... 4.26
11 Drainage................................................................................................................................................................................................................................................. 4.26
11.1 Types of drainage ....................................................................................................................................................................................................... 4.26
12 Leaching requirements.......................................................................................................................................................................................................... 4.29
13 References ........................................................................................................................................................................................................................................... 4.31
Appendix A ................................................................................................................................................................................................................................ 4.32
All rights reserved
Copyright  2004 ARC-Institute for Agricultural Engineering (ARC-ILI)
ISBN 1-919849-24-6
Soil
4.1
1 Introduction
The evaluation of soil for irrigation purposes requires the co-operation of a number of specialists in their
own fields, and the execution of a number of specialised investigations. A thorough knowledge of soil
types in a potential irrigation project is absolutely essential for both economic and technical reasons. Risk
analysis, advantages regarding the planned irrigation development and the layout of the planned irrigation
scheme, per se, are all dependent on a thorough knowledge of the site and the nature of the soils in the
proposed irrigation scheme.
An irrigation scheme must be adapted to the soil and the general agricultural system, and not vice versa.
When incorrect irrigation practices are applied to certain soils, it can have adverse financial implications.
This also applies to cases where soil, which is unsuitable for irrigation, is irrigated. Such practices can also
result in the deterioration of soil - a limited resource. Deterioration may be caused by the soil becoming
brackish, erosion, water logging, compaction, or by drainage problems. The use of low quality water can
also have a negative effect on the soil. This chapter gives an overview of the background and basic
principles of soils and information, which is important for irrigation. It is important that designers of
irrigation systems understand them and apply them in practice. The idea is not to turn designers into soil
experts, but to bring home the importance of soil with regard to irrigation.
2 What is soil?
In soil science (pedology), the term “soil” broadly refers to the unconsolidated inorganic and organic
material on the immediate surface of the earth, which serves as a natural medium for the growth of land
plants. It comprises a mixture of solid inorganic particles mixed with water, air and organic material. Soil
is an integral part of the landscape, and its properties, appearance and distribution are determined by the
soil formation factors, namely climate, material of origin (bed rock), topography, flora, fauna and time.
Under the influence of these factors, the upper portion of the earth’s crust undergoes changes in a vertical
direction. These changes with depth occur together with the formation of horizontal layers, or horizons.
Soil never occurs as a continuous, homogenous entity, as it varies from place to place. It is essential
therefore, to dig more than one profile hole covering various places within the specific area that must be
irrigated, in order to determine where the differences lie. A profile hole may be described as a vertical
hole, dug down to the underlying material, in which the horizons may be seen.
• Soil profiles
All soils may be characterised by the continuation of certain horizontal layers, or horizons. This
occurrence is known as the soil profile. The best known, and most important part of the soil, is the top
layer or A-horizon. An investigation of this zone will reveal many of the processes that occur in the
layer. From an irrigation point of view, the B-horizon plays an important role regarding internal
drainage, saturation and possible salt accumulation.
The characteristics of soil as an entity shall determine that the soil be investigated to a greater depth.
o
Peripheral genetically related areas or horizons differ regarding the physical, chemical and
biological characteristics such as colour, structure, texture, types and numbers of organisms that
are present.
4.2
Irrigation User’s Manual
Figure 4.1: Schematic representation of a hypothetical soil profile with its underlying bedrock
3 Classification
The primary objective of the soil classification system is the identification and labelling of soils according
to an ordered system of defined classes. The underlying relationship, between soil properties and the
communication thereof, is thus made possible.
As no suitable international soil classification system existed for the detailed identification and mapping of
South African soils, a local system was developed. It is known as the “Binomic system” and was first
published in 1977.
In 1991, a revised system was published as “Soil classification: a Taxonomic system for South Africa”.
The system comprised two categories or levels of classes, namely a higher or general level which included
soil types, and a lower, more specific level of soil families. For the purposes of irrigation, the physical and
chemical characteristics, as well as the crop-water relationship, is of importance and, if more information
is desired, the above-mentioned publication may be consulted.
4 Physical characteristics
Material can occur in three forms or “phases”, namely solids, liquids and gas phases. The physical
condition of soil refers to the mutual relation between the three phases of material that occurs in soil, i.e.
how much solid material (soil particles and stone), liquid material (water) and air are in soil. The physical
condition of soil has a huge influence on the utilisation potential thereof, either for production of
agricultural crops or for use as construction material (such as for building roads or dams). Two of the most
important physical characteristics of soil are the texture and structure thereof. The physical characteristics
of soil are those properties that may be measured by physical means and may be expressed in physical
terms, such as: structure, colour, density, porosity, hydraulic conductivity, texture and depth.
Soil
4.3
4.1 Texture
Texture is defined as the relation between the amount of sand, silt and clay occurring in the soil.
Structure refers to the way in which the individual sand, silt and clay particles bond to form larger soil
units or aggregates. Texture and structure combined determine how many pores there are in the soil
and therefore also the soil ability to conduct air and water. Texture and structure also determine the
ease with which soil can be tilled and what its erodibility is. The amount of water in soil has a large
influence on soil aeration and soil temperature.
Before soil texture and structure can be considered in greater detail, however, it is necessary to discuss
soil particle size classification first.
4.1.1 Size classification of soil particles
The different particles which occur in soil, are described in general terms as sand, silt and clay. The
terms sand, silt and clay however refer more correctly to the different size fractions occurring in
soil. The physical dimensions used to describe (and to confine) the size of a specific soil particle,
e.g. silt is completely arbitrary and will differ depending on which classification system is used.
Different size limits therefore exist for the various soil fractions. “Silt” is defined by the USDA
(United States Department of Agriculture) as particles of sizes between 0,05 and 0,002 mm, while
according to the ISSS-classification (International Soil Science Society) of sizes between 0,02 and
0,002 mm. In general, it can be said that in all classification systems the size sequence is:
Sand > silt > clay
The size classification used in South Africa is based on the USDA-system and is set out in
Table 4.1. The information in Table 4.1 shows the size confinements of soil particles as a diameter.
Since the geometric shape of soil particles is irregular, a single figure cannot be used to indicate the
actual size. For this reason, the size of particles is referred to in terms of the equivalent spherical
diameter (e.s.d). The e.s.d. of a particle is defined as a diameter of a sphere that consists of the same
material as the soil particle (e.g. quarts) and which will sink at the same speed in a liquid (e.g.
water). For example, the e.s.d. of a quarts particle with an irregular shape (e.g. angular), is set equal
to the diameter of a perfectly round quarts particle which will sink at the same speed in water.
It is important to note that the term “soil” (sometimes referred to as fine soil) only refers to
particles smaller than 2,00 mm. Many soils, however, also contain fragments larger than 2,00 mm.
Depending on the size thereof, it is referred to as stones, gravel or cobble stones.
Some of the physical characteristics, which are distinctive of the different size classes, are
illustrated in Table 4.2.
4.4
Irrigation User’s Manual
Table 4.1: Size confinements of soil particles according to the SA-system
Size class
Diameter (mm)
Stones
> 250,0
Cobbles
250,0 – 75,00
Gravel
75,00 – 2,00
Very coarse sand
2,00 – 1,00
Coarse sand
1,00 – 0,50
Medium sand
0,50 – 0,25
Fine sand
0,25 – 0,10
Very fine sand
0,10 – 0,05
*Silt
0,05 – 0,002
Clay
< 0,002
*Silt is sometimes subdivided into coarse silt (0,05 – 0,02 mm) and fine silt (0,02 – 0,002 mm)
Table 4.2: Physical characteristics of different size classes
Characteristics
Water holding capacity
Capillary rising ability
Infiltration rate
Cohesion and plasticity
Heat transfer
Aeration/gas exchange
Cation adsorption ability
Coarse sand
Very low
Limited
Very fast
Limited
Fast
Very good
Limited
Texture class
Fine sand
Silt
Low
Moderately low
Moderately high High
Fast
Slow
Very low
Moderately high
Moderately fast Slow
Good
Moderately good
Extremely low
Low
Clay
Very high
Very high
Extremely slow
Very high
Very slow
Very poor
Very high
4.1.2 Texture classes
The use of soil texture as a measure with which the general characteristics of soil can be evaluated
and predicted, is complicated by the fact that in nature, an infinitive number of sand:silt:clay
relations are possible. It is therefore convenient to use a smaller number of texture classes. Each
class is distinguished by well-defined confinement values of the amount of sand, silt and clay,
which has to be present in that class. Thirteen texture classes, identical to those of the USDA, are
currently used in South Africa. A diagrammatical exposition of the classes and their confines, are
shown in Figure 4.1. This figure, which is known as a texture triangle, is used to determine the
texture class of soil, of which the particle size compound is known. Each side of the triangle
represents a size fraction (sand, silt and clay) and runs clockwise from 0% to 100%. If soil has
30% sand, 60% silt and 10% clay, the various percentages can be marked on the three axes
respectively and relevant class determined from the point where the three readings intercept each
other.
With the exception of loam, the class name indicates the size fraction (or fractions) which are
dominant. In this way, it can be deduced that in the “sandclay” texture class, sand and clay are the
two dominant size fractions. In the texture class “loam”, there is more or less an equal amount of
sand, silt and clay present in the soil. These types of soils are also accepted to have favourable soil
physical characteristics in almost all respects, such as being readily tillable, good air and water
permeability and reasonably high water-holding abilities. Where texture is concerned, loamy soils
contain slightly less clay than sand and silt as can be deduced from Figure 4.2, the maximum clay
content of loamy soil is 27%.
The name of texture classes in which the word “sand” occurs, can be adapted to indicate the
dominant sand sub-fraction. A sand-grading card is used for this purpose. Examples are: “fine
sand loam”, “coarse sand” and “loam fine sand”. The same principle also applies when the soil
contains quite a lot of gravel and/or larger stones. Examples are “stony loam” and “gravelly sandclay”.
Soil
4.5
Figure 4.2: Soil texture chart
The classes, namely sand, loamy sand, sand-loam, and sand-clay-loam, are further subdivided according to
the percentage of the sand fraction consisting of coarse, medium or fine sand. Refer to Figure 4.3.
Figure 4.3: Sand grading chart
4.6
Irrigation User’s Manual
The following thirteen texture classes are used in the SA-system (in sequence of progressive amounts
of silt plus clay):
1.
Pure sand
2.
sand
3.
Loamy sand
4.
sandy loam
5.
loam
6.
silt loam
7.
silt
8.
sandy clay loam
9
clay loam
10. silt-like clay loam
11. sandy clay
12. silt-like clay
13. clay
It is sometimes convenient to divide the thirteen texture classes into three broad categories, namely coarse,
medium and finely textured. The categories are then as follows:
Table 4.4: Broad category division of texture classes
Coarse
Classes (1-4)
Sands
Loam sands
Sand loams
Medium
Classes (5-7)
Loams
Silt loams
Silt
Fine
Classes (8-13)
All texture classes
with the name clay therein
Two further terms used generally in practice are “light” and “heavy”. When the sand fraction is dominant
in the soil, we speak of a light soil, which then indicates that it is readily tillable. On the other hand, there
are soils with a fine texture where silt and/or clay is the dominant fraction. Such soils are known as heavy
soils, because it is plastic and tough when wet and hard and difficult to break when dry – it is therefore
difficult to till. The words light and heavy refer to arability of soil as determined by the texture class
thereof. A general “hand-feel” method, whereby a “sausage” can be rolled to determine the texture, is
shown in Figure 4.4.
Hand-feel method
Figure 4.4: Hand-feel method whereby the texture class of soil can be determined
Soil
4.7
4.2 Soil structure
The term structure refers to the aggregation or composition of various primary soil particles in groups or
secondary particles, which are separated from bordering aggregates by planes of weakness. The visible
macro-structure is undoubtedly dependent on the nature of the arrangement of primary particles, or
texture, which cannot be differentiated clearly with the naked eye. This influences the rate at which
water and air penetrate the soil and move through the profile. Root penetration and production ability
are also influenced. The structure of the soil can be changed by plant action (formation of structure) and
cultivation (destruction of structure). Irrigation water containing large quantities of sodium destroys
structure through the dispersion or extinction of clay particles. Soil structure types are a classification of
soil structures based upon the size of the aggregates or peds, and their arrangement in the profile. In
general, reference is made to the soil structure types as plate, prismatic, columnar, block, or granular.
Figure 4.5 shows typical soil structure types. A ped is a unit of soil particles sticking together as an
aggregate, crumb, prism, block or grain, which has been formed by natural processes (as opposed to
clods, which are formed artificially).
Figure 4.5: Various types of soil structures
4.3 Soil consistency
Consistency is a measure of soil’s arability and is revealed in the resistance that soil has against
distortion or breakage when subjected to pressure, tearing and traction. The consistency of the soil is
the result of the cohesive and adhesive forces (see Section 4.8.1) present therein. Therefore, any process
that will change the two forces will also influence the consistency. Consistency is influenced by the
texture, structure and organic material content. As example: soil with a crumbly structure or high
organic content, tends to break easier than soils with massive structures and low organic material
content. Soil aggregation reduces the obvious cohesive and adhesive characteristics of soil particles
because of the relatively weak powers with which the aggregates, e.g. crumbly fragments are bonded.
Organic materials have a similar effect, partly because of the structure stabilising effect thereof and
partly because of the fact that humus does not contain strong cohesive and adhesive abilities.
4.8
Irrigation User’s Manual
Consistency is also influenced by the soil water content. An example of such an effect is the way in
which water can soften a hard clot. The concept is that as the water filters into the soil, it also moves
into the cohered particles to reduce the cohesive forces. Unless the particles of the soil aggregated are
bonded by the cementation, adding a small amount of water will tend to change it from a hard to a more
breakable condition. By adding more water, the soil will first become plastic and then adhesive. In the
deformable, plastic condition, cohesion is dominant over adhesion, but in the adhesive condition, the
opposite applies.
4.4 Bulk density
The bulk density is the mass of the dry soil per unit of total volume. The total volume is determined
before the sample is dried. Values vary roughly between 1 000 and 1 800 kg/m3, although higher values
may be found in compacted soil. At this stage, it must be emphasised that there are various other terms
for the concept “bulk density”, such as i) volumetric density and ii) matric density. They all however
refer to the same concept, namely the dry mass per unit volume soil in the natural undisturbed field
condition. Two methods, which are generally used to determine bulk density, are the clot and wax, and
cylinder methods.
The bulk density (ρb) of soil can be determined with the aid of the following equation:
ρb =
Mass dry soil
Volume dry soil
( 4 .1)
4.5 Porosity and pore size
The porosity is the percentage of the total volume of soil, which is not filled by solid particles and in
which the water for plant absorption is stored. It is therefore the “tank” in which water is stored in the
soil. It is dependent on the structure, texture, consistency and bulk density of the soil. These pores may
be filled with air or water. Porosity therefore refers to the relation between the total pore volume and the
bulk volume of the soil, but does not refer to the size of the pores. A knowledge of pore sizes is
therefore of more value than the porosity figure. Two size classes are recognised, namely:
•
Macro pores (> 60 µm) are important for the renewal of air within the soil peds, as well as for
the movement and distribution of water.
•
Micro pores (< 60 µm) are important for the storage of capillary soil water.
The total porosity of the soil is not always as important as its relative pore sizes are. For example, clay
soils tend to have a higher total porosity than sand, because they have a greater percentage micro pores
which are relatively small and thereby contribute to greater water holding, slower water infiltration and,
at times, to inadequate aeration. The volume of readily available water (RAW) in sandy soils is usually
lower than that in clay soils, but sand will release the water more freely than clay.
A direct measurement of a soil’s porosity is however very difficult. Therefore, the following relation
between bulk density, particle density and porosity is used:
Volume solid phase = ρb / ρd × 100%
Porosity (%) = (1 - ρb / ρd) × 100
Where:
ρb
ρd
=
=
(4.2)
bulk density, and
average particle density of the soil (The particle density of quarts,
namely 2 650 kg/m3 is usually used)
The productivity of soil is influenced by the relation of solid particles to macro pores. In ideal soil, the
relation is 50 : 25 : 25. An arbitrary class indexing of the pores according to their size, is shown in
Table 4.5.
Soil
4.9
Table 4.5: Size classification and functions of pores in soils
Class
Macro pores
Meso pores
(medium pores)
Micro pores
Equivalent diameter (µm)
> 100 µm
(> 0,1 mm)
30 – 100 µm
(0,03 – 0,10 mm)
(< 0,03 mm)
Capillarity
None
Drainage rate
Fast
Moderate
Slow
High to very high
Very slow to none
4.6 Infiltration
Infiltration may be described as the downward penetration of water into the soil. The infiltration rate
may be regarded as the rate at which the water penetrates the soil. If a dry soil is wetted, the
infiltration rate is initially high, but it decreases rapidly as the soil is wetted. (Figure 4.6 refers).
Figure 4.6: Infiltration curve
In the region of field capacity, (see Section 4.9.2) the infiltration rate stabilises, and it is this
infiltration rate that is important. Factors such as structure, texture, exchangeable sodium percentage,
drop size, density and soil moisture content of the soils will influence the infiltration rate.
Field observations indicated that crust formation is a determining factor. This crust is formed mainly
due to the external energy of rain and irrigation droplets which break down the soil particles and
rearrange them during wetting. This plays an important role in choosing a suitable irrigation system.
Soils with initial infiltration rates of higher than 125 mm/h, or basic infiltration rates of less than 3
mm/h, are regarded as unsuitable for irrigation. The values obtained on the same soils with different
methods, differ widely. Double ring, single ring, furrow and infiltrometer are the best known methods
for measuring infiltration.
4.7 Permeability
Permeability may be defined as the ease with which the gasses, water and plant roots can penetrate a
horizon or layer and move through it. In terms of irrigation, there are no minimum values according to
which soils may be evaluated. However, it is important where leaching or over-irrigation is deemed
necessary and the excess water must be drained off. Permeability is dependent on soil characteristics
such as porosity, structure, texture, and the state of the soil water level.
When referring to permeability of water, the term hydraulic conductivity is used. The hydraulic
conductivity (Kw) , describes the capability of the soil at a certain soil water content to conduct water
through the pores. A general rule is that water is conducted faster through a saturated soil, i.e. the
wetter the soil, the higher is the hydraulic conductivity thereof. In an unsaturated pore, Kw is always
4.10
Irrigation User’s Manual
lower than in a saturated pore. The reason for the first-mentioned is that the water must follow a
longer flow distance through an unsaturated pore than through a saturated pore. The longer the flow
distance, the larger becomes the effect of the friction resistance that the water molecules experience
and the slower the water will be conducted through the pore. In an unsaturated condition, it can occur
that a micro pore (completely saturated with water) will conduct water faster than a macro pore (which
is only partly saturated with water). The opposite applies when both pores are saturated with water.
4.8 Water holding capacity
The water holding capacity refers to the maximum amount of water that soil can hold under natural
conditions. This is discussed according to different soil water constants.
4.8.1 Principles of Capillarity
Water can be held by soil in two ways. The one way is where somewhere in soil, a layer occurs
that is impermeable. The result is that water which has been taken up by the soil, moves to the
impermeable layer and collects there. If enough water is added to such soil, a water table will
build up on top of the impermeable layer. From the above, it should be clear that the soil layer, in
which the water table occurs, is saturated - no air occurs and the soil is waterlogged. Such a water
table and waterlogged conditions can occur permanently or only temporarily.
Another way, in which water is hold in the soil, is by means of the process of capillarity and
adhesion. Capillarity is defined as the degree to which a porous material, such as soil, when dipped
in free water, such as in a glass, will absorb the water into the pores to a level that is higher than
that of the free water. In other words, water moves into the pores of the material (soil) and will
remain there even if the source of free water is removed. Capillarity is caused by two processes,
namely:
a)
b)
Adhesive forces: These forces are is the forces of attraction between the negatively loaded
surface of the soil particles and the polar water molecules.
Cohesive forces: The mutual forces of attraction between the water molecules.
The result of capillarity is that water, despite the gravity of the earth, is held in the soil. It will
become clear later that the less water there is present in the soil, the stronger it is held by the soil
and the stronger the counter-force must be to remove water from the soil again (such as a plant that
must extract water from the soil pores). On the other hand, if there is a lot of water in the soil, e.g.
if the pores are filled with water, only a slight force is required to extract water from the soil. This
water is so poorly bonded that it will spontaneously, under the influence of the earth’s gravity,
move from the soil.
The rising of water in a thin glass tube (also called a capillary tube) is a simple way of illustrating
the process of capillarity. The water rising in the tube forms a column in which water molecules
are held to the sides of the tube by means of cohesion and adhesion. The adhesion that exists
between the side of the tube and the water molecules is the primary cause of the rising process, in
that the water molecules are drawn to the side of the tube. Because of the mutual attraction that
exists between the water molecules, a number of water molecules, which are not directly in contact
with the side, are drawn up in the tube. This rising of water because of adhesion and cohesion is
presented schematically in Figure 4.7.
The weaker cohesion forces are, however, the limiting factor in capillarity, i.e. water will not keep
rising in a thin tube unlimited. The cohesion forces between molecules A and B in Figure 4.7
“carry” the rest of the water molecules in the column, e.g. molecules C and D. When the weight of
the water column becomes too high for the cohesion force, the water will stop rising. It should be
clear that capillary rising ceases as soon as the weight of the water column and the limiting
cohesion force between the water molecules are in equilibrium with each other.
Soil
4.11
A B
A
C D D D D C B
Figure 4.7: The adhesive (A), cohesive (B) and gravitation forces (↓) that have an influence on the
molecules of water in a capillary tube.
4.8.2 Differences in the water holding capabilities of soil.
In order to remove water from the soil, a suction force greater than the capillary force with which
the water is held in the soil, is required. The capillary forces with which water is held in the soil,
is also called the matric suction force of the soil. Even if the suction force (exerted by roots) that
extracts the water from the soil is stronger than the capillary forces, all the soil water will not be
removed from the soil immediately. A soil is therefore not wet the one moment and dry the next,
because the water is released systematically. Imagine that all the pores in a soil is filled with water
and that no additional water is added to the soil. Under the influence of gravity, the poorly bonded
water will drain from the soil’s macro pores and the process will continue until the gravitational
forces of the earth and the capillary forces of the pores are equal. With the application of an
increasing higher suction force on the soil, the meso and then the micro pores will also begin to
lose their water. When all the water is released from the pores and the soil seems obviously dry,
there will still be water in the soil. This water occurs as thin layers around the soil particles and is
bound by very strong adhesive forces.
From the above description it is clear that water is released systematically. Because the pore
structures of soil differ, the manner in which water is released will also differ. The more pores in
the soil, the more water will be absorbed by it. The release of water under the influence of suction
force is however determined by the relation macro:meso:micro pores. The more micro pores in the
soil, the firmer the water is bound and the slower it is released. As the pore structure and size of
the soil is very well correlated with texture, a sand, loam and clay soil’s water release curves will
differ. This difference is shown in Figure 4.8. As will be emphasised later, the curves shown in
Figure 4.8, are known as soil water retention curves.
4.12
Irrigation User’s Manual
Figure 4.8: Soil water content as a function of texture and matric suction
4.9 Quantification of soil water
The amount of water present in the soil at any given moment, can be described in various ways. Some
of the terms are subject to a subjective interpretation, such as dry and wet, which refer to a general
condition of the soil, without quantifying the amount of water. Other terms, such as saturated and
unsaturated, are slightly more exact, but still do not quantify the soil water content fully. Saturation
refers to the condition where the pores of a soil is completely filled with water, while unsaturated
refers to any soil water content lower than that of saturation. These two conditions are shown
schematically in Figure 4.9. In illustration A, the soil pores are saturated with water, while in B, C and
D air is also present in the pores. The air is essential for roots to grow, as no root can grow under
saturated or waterlogged conditions.
A
B
C
D
Figure 4.9: Representation of the different water content terms: A = Saturation point, B = Field
capacity, C = Permanent wilting point and D = Air-dry soil
The soil water content of a certain layer of the entire profile is described on the basis of the following
descriptive terminology, namely: Saturated Water Content (0s), Field Capacity (FC), Permanent
Wilting Point (PWP), Total Available Water (TAW), Readily Available Water (RAW) and
Hygroscopic Water.
Soil
4.13
4.9.1 Saturation Point
The soil water condition is at saturaion point when all pore spaces (between particle spaces) in the
soil are filled with water and no air is present in the soil. The soil is in a “waterlogged” condition
and roots cannot grow under such conditions. The water will drain from the soil until the field
capacity of the soil can be reached.
4.9.2 Field Capacity (FC)
Field capacity can be defined as the amount of water that the soil can retain after it has been
saturated and allowed to drain to a stage where the drainage rate has become very small. Field
capacity is seen as the maximum amount of water a soil can retain against the earth’s gravity.
When FC is reached, most of the ground water in the macro pores have drained and only a portion
of the meso and micro pores are filled with water. The practical implications of field capacity is
that, if the soil is wetted to above its FC, seen from an irrigation point of view, it amounts to
wastage and loss of water, because the water is no longer available for the plant roots.
When field capacity is reached, the capillary forces, which tend to retain the water in the pores
and the gravitation force of the earth which tends to remove the water from the soil, are in
equilibrium. Local experimental information shows that the matrix potential, which indicates the
size of the capillary forces, is between -5 and -15 kPa when FC is reached. Experience has also
shown that the size thereof is not significantly influenced by texture. A clay and a sandy soil’s
matrix potential at reaching the FC does not differ much.
4.9.3 Permanent Wilting Point (PWP)
The permanent wilting point of a soil is the water content at which plants can no longer obtain
sufficient water to provide in their transpiration requirements. When the soil becomes drier, the
plant will wilt permanently, except if the soil water is replenished again by means of irrigation or
rain. Wilting occurs when a plant transpires faster than it can take up water through its roots. It is
important to note that plants often demonstrate temporary wilting symptoms, such as on hot days.
When the temperature decreases later in the day, the atmosphere’s relative humidity rises and the
transpiration rate is lower. The plant is then able to repair its turgor pressure and return to a
normal condition. At reaching the permanent wilting point, the water is held so firmly by the soil
matrix that the plant does not have the necessary energy to absorb it, even if the atmosphere cools
down. It is important to note that the force with which the water is held in the soil, is the factor
that will determine the PWP.
From the above description it is clear that PWP is a soil water condition determined by the plant.
Because of mutual differences in plant physiology, the PWP values of different plant types and
also cultivars will not necessarily be the same. The PWP of a drought resistant crop (e.g. olive
trees) will differ somewhat from a vegetable crop (e.g. tomatoes) with a high water requirement.
Although there is a difference between plants, the matrix potential at which permanent wilting
symptoms appear, is generally accepted as approximately -1500 kPa, but it can vary between -700
kPa and -3 000 kPa.
4.9.4 Total Available Water (TAW)
The difference in the soil water content between field capacity and permanent wilting point is
called the total available water. The concept total available water is a biological classification of
the soil water, because it indicates the biological usefulness thereof. In the same way, reference is
made to the free water in the macro pores as excessive water, because it has little benefit to the
plant. It impedes the aeration of soil and causes leaching losses of nutrients. On the other hand,
the term unavailable water describes the soil water that is so firmly bound by the soil, that the
plant cannot utilise it. The amount of total available water in soil as a function of the clay content
is shown in Table 4.6.
4.14
Irrigation User’s Manual
The amount of total available water in soil is calculated as follows:
Soil water content at field capacity (FC) :
Soil water content at permanent wilting point (PWP):
Difference (FC-PWP):
Because:
Therefore:
0,24 m3/m3
0,14 m3/m3
0,10 m3/m3
0,10 m3/m3 is the same as 0,10 mm/mm:
If the soil is 500 mm deep, there is 0,10 mm/mm × 500 mm =50 mm total
available water in the soil.
Table 4.6: Approximated Water Holding Capacity according to clay content
Clay content (%)
0-5
5 - 15
15 - 35
35 - 55
Water Holding Capacity (mm/m)
60 -100
80 - 130
100 - 130
130 -160
4.9.5 Readily Available Water (RAW)
Readily Available Water (RAW) is broadly seen as the water that is at a soil water tension
between -10 and -100 kPa and can therefore be readily absorbed by the plant. The amount of
water in mm available per layer, is calculated by reading off the difference in volumetric water
content at –10 and -100 kPa from the corresponding layer’s water curves (Figure 4.10) and
multiplying it with the thickness of the layer. Example:
Water content at -10 kPa
= 150 mm/m
Water content at -100 kPa
= 94 mm/m
RAW
= (150 – 94) mm/m
= 56 mm/m
If irrigation is to take place at 50% extraction of RAW, the soil water content will drop to
122 mm/m, which is equal to a tensiometer reading of 28 kPa.
Figure 4.10: Water retention curve of the readily available water of soil
Soil
4.15
4.9.6 Hygroscopic water
This is water that is held by means of adhesion forces around the soil particles. It is also known
as water absorbed by a dry soil from an atmosphere with relative humidity or water that remains
in the soil after “air-drying”. This form of water is not available to the plant and is bound by a soil
water matrix suction of less than -1 500 kPa or a higher numerical value. This means that soil
water is less available for the plant at a matrix suction of -1 600 kPa than at -1 500 kPa.
Figure 4.11: Graphic representation of the matrix suction with which soil particles retain the
types of soil water
4.10 Soil depth
The depth of soil up to a limiting layer is one of the most important properties of a soil in determining
its irrigability. The ideal is a well-drained soil with a soil depth of 1 500 mm or more. Nevertheless,
certain crops may do excellently under irrigation in soil with an effective soil depth of 900 mm. With
above average irrigation management, good harvests may be obtained in shallow soil with an effective
soil depth of 450 mm. Soil depth determine the reservoir or storage volume in which water and
nutrients are stored in the soil. A deep soil can therefore store more water than a shallow soil.
4.16
Irrigation User’s Manual
4.10 Calculation examples
Example 4.1a:
Calculation of the gravimetric water content of a soil.
Gravimetric water content of a soil reflects the amount of water in a soil on a mass basis. It can be shown as a
fraction, namely: (kg water per kg soil) or is mostly shown as a percentage: (kg water per 100 kg soil). In the
field it is determined by means of an equation [4.3]:
1. A soil sample is taken with the aid of a cylinder with known volume and the cylinder’s mass is determined
accurately on an electronic scale (to the nearest mg).
2. The wet soil sample is then dried and the dry mass is determined.
3. The gravitational water content is then determined as follows:
Pw(%)=(Mass wet soil – Mass dry soil) / × 100%
Mass wet soil
Mass dry soil
∴ Pw (%)
(4.3)
= 825 g
= 700 g
= (825 – 700)/700 × 100%
= 17,86% or = 17,86 g/100 g
Example 4.1b:
Calculation of the bulk density of the soil:
The bulk density of the soil in example 4.1a is calculated as follows (see equation 4.1):
Bulk density (pb)
= (Mass dry soil)/(Volume dry soil)
= (700 g/500 cm3)
= 1,4g/cm3
and that is equal to= 1 400kg/m3 (Please note: × 1 000 to convert from g/cm3 to kg/m3)
Example 4.1c:
Calculation of the porosity of the soil:
The porosity of the soil in example 4.1a is calculated as follows (see equation 4.2):
Porosity(%)
= 1 - (pb/pd ) × 100
= 1 – (1 400/2 650) × 100
= 47,17%
In volumetric units, the porosity = 0,4 717 m3/m3
This means that there will be 0,47 m3 air present in 1 m3 dry soil. It also means that the maximum water content of
the soil at saturation will be 0,47 mm/mm or 470 mm/m. Note that in order to convert volumetric water content from
mm/mm to mm/m, it is multiplied by 1 000.
Example 4.1d:
Calculation of the volumetric water content of a soil: The volumetric water content of the soil in example 4.1a is
calculated by means of equation 4.4, as follows:
P
ρ
Volumetric water content (θ v ) =  w × b
 100 ρ w
Where:
Pw
pb
pw
=
=
=
(4.4)
gravimetric water content of soil
bulk density of soil
density of water (equal to 1 000 kg/m3 or = 1g/cm3
Please note: Units of both pb and pw in the term
∴ Volumetric water content (θv)
and that is the same as



=
=
=
=
ρb
ρw
must be the same, namely: in kg/m3 or g/cm3
(17,86/100 × 1 400/1 000)
0,250 mm3/mm3
0,250 mm/mm
250 mm/m
Soil
4.17
Please note: If the calculations are given in volumetric units, it is easy to calculate it to depth, surface or volume unit.
I.e. the following units are all equal, namely:
m3/m3 = cm3/cm3 = mm3/mm3 = m²/m² =cm²/cm² = mm²/mm² = m/m = cm/cm = mm/mm
Example 4.1e
Soil water constants calculation – practical example:
The example below serves as illustration on how to calculate the available water content of a soil, irrigation
requirement and other constants. You have to calculate the irrigation requirements for an irrigation block (10 ha)
with a loamy soil with the following data at your disposal:
Soil data of irrigation block:
FC
[m3/m3]
0,35
0,42
Depth of soil layers [mm]
FC
PWP
Bulk density
[kg/m3]
0-300
1 400,00
300-600
1 480,00
= Field capacity of the soil
= Permanent wilting point of the soil
PWP
[m3/m]]
0,035
0,04
The gravimetric water content of each layer was determined in the crop’s growing season according to the method
described in Example 4.1a and the following data was obtained:
Depth of soil layers
Mass wet
Mass dry
Difference
[mm]
[g]
[g]
[g]
0-300
825
700
125
300-600
940
740
200
Volume of sample = 0,5 litre
The following can then be calculated with the aid of the above equations.
Gravimetric water
content [%]
17,86
27,03
Soil water data of the production block:
Depth of
soil layers
[mm]
Volumetric
water content
of layers
[mm/mm]
0-300
300-600
0,250
0,400
Water
content per
layer
[mm]
FC
[mm]
PWP
[mm]
75,0
105,0
120,0
126,0
Total
195,0
231,0
Notes regarding calculations in the above table:
10,5
12,0
22,5
1.
2.
3.
4.
5.
Total
available
water
(TAW)
[mm]
94,5
114,0
208,5
Plant available
water content
[mm]
Nett water
requirement
[mm]
64,5
108,0
172,5
30,0
6,0
36,0
The volumetric water content is calculated with the aid of Equation 4.4.
The water content, FC and PWP of each layer is calculated by multiplying the volumetric water content of a
specific layer with the thickness (300 mm). E.g. 0,250 × 300 = 75 mm
The total available water (TAW) is the difference between the FC and PWP.
The plant available water content is calculated as follows:
Water content (0 - 300 mm) = 75 – 10,5 = 64,5 mm
The nett water requirement is the amount of water required to wet the soil at current water content to field
capacity.
e.g.: For the 0 - 300 layer it is: 105 - 75 = 30 mm.
The soil in the example has an available water content of 208,5 mm in the 600 mm root depth.. At the time of
determining the water content, the soil, however, contained 172,5 mm, which means that 36 mm of the available
water was already abstracted from the soil. If the crop on the specific soil has an average daily water consumption of
4,5 mm/day, the crop will be able to grow for 38 days with the available water. Calculate as follows:
172,5 mm/4,5 mm/day = 38,3 days = ± 38 days.
The volume of water required to irrigate the block of 10 ha to FC again, is equal to 3 600 m3. It is calculated as
follows:
4.18
Irrigation User’s Manual
36 mm × 10 m3 /mm/ha × 10 ha = 3 600 m3.
5 Chemical properties of soil
Due to the fact that soil comprises inorganic particles, air and organic matter, certain chemical reactions
take place. The following chemical properties are of importance in the application of irrigation:
5.1 pH
The degree of acidity or alkalinity of a soil is normally expressed as a pH value. A pH of 7 is neutral,
less than 7 is acidic and higher than 7 is alkaline. Most crops prefer a slight to moderate acidic soil (pH
between 5,5 and 7). The pH of the soil is only an indicator of the soil reaction and is not normally
taken as a criterion when evaluating soils for irrigation. pH adjustments may be made within limits.
However, pH extremes are an indication that problems might occur under irrigation. The pH value of
soil, as measured according to the CaCl2 method, is about 0,5 pH units lower than when measured in a
1:2 soil water suspension.
The following general statements regarding pH apply:
•
A pH value of 7,5 and higher (determined according to the CaCl2 method) in a soil containing
lime indicates a sodic soil.
•
A pH value of 8,5 and higher (CaCl2 or H2O method) usually indicates an exchangeable sodium
percentage (ESP) of 15 or higher. A higher pH value indicates a sodium problem.
5.2 Sodium adsorption ratio (SAR)
The sodium adsorption ratio is determined by the ratio of Na to Ca and Mg (Na:(Ca + Mg)) in soils. A
high SAR-value may indicate sodium problems in the soil. The SAR of a soil is approximately equal to
(1 to 2 times) the exchangeable sodium percentage (ESP) of the soil. The ESP gives a very good
indication of the structural stability of a soil and the physical reaction that can be expected when the
soil is irrigated.
Soils, as indicated in Table 4.7, show unfavourable physical properties such as deflocculation.
Deflocculation and dispersion may be described as the separation of the various components from the
combined particles (e.g. soil aggregates) through physical and/or chemical processes. Apart from the
unfavourable effect that the ESP will have on the physical properties of the soil, certain cultivars show
a low tolerance for exchangeable sodium.
Table 4.7: An indication of the influence of ESP on harvest reduction
50% yield reduction by
ESP of 15%
(sensitive)
Avocado
Green beans
Citrus
Maize
Peaches
50% yield reduction by ESP
of 15 - 25%
(moderate tolerant)
Oats
Cotton
Lettuce
Red clover
Lemon
50% yield reduction by
ESP of 35%
(tolerant)
Beetroot
Barley
Lucerne
Onions
Carrots
5.3 Saline quality (brackishness)
A brackish soil is a soil with a soil water containing an excess of exchangeable sodium, as well as a
notable quantity of soluble salts, which impedes the growth of most cultivars. The presence of soluble
salts is probably the most important reason for the deterioration of soil under irrigation in arid areas. As
a result of the solubility of the salts, they may easily leach out if the drainage of the underlying soil is
Soil
4.19
adequate.
The higher the electrical conductivity, the higher the presence of salts. In soil studies, the electrical
conductivity is measured in milli-Siemens per metre [mS/m], and is an indicator of the concentration of
salts in solution. Irrigation water with a low saline content shows values of less than 25 mS/m, and that
with a high saline content returns values in excess of 75 mS/m. The electrical conductivity is an
indicator of the ability of a matter to conduct an electric current and is approximately the inverse of the
specific resistance [ohm].
Basically, there are three types of brackish soil, namely saline, sodium and saline-sodium. The
properties of the types of brackish soils are described in Table 4.8
Table 4.8: Properties of brackish soil types.
Properties
Saline brackish
Sodium brackish
Saline-sodium
brackish
Electrical conductivity of
a saturation extract
( at 250 C ) [mS/m]
> 400
< 400
> 400
ESP [%]
< 15
> 15
> 15
pH
< 8,5
between 8,5 and 10
< 8,5
Salts present
Calcium and
magnesium chlorides
and sulphates
Sodium carbonates
High sodium and salt
concentrations
Flocculation
Flocculated
Deflocculated
Flocculated
An excess of free salts does not only influence the permeability and internal draining of soils, but also
has a detrimental influence on the availability of soil water for plant roots. Some cultivars are more
resistant to salinity than others. Table 4.9 indicates the tolerance of cultivars, together with their
relative expected yields. In the evaluation of saline soils for irrigation development, particular attention
should be paid to:
•
quality of irrigation water, infiltration rate and permeability of the soil
•
drainability of the deep underlying soils and their ability to get rid of leaching water
•
the degree of brackishness and the availability of gypsum
If there is any doubt regarding any of the above, the advice of a specialist should be sought.
6 Organic properties
All soils contain organic matter. It plays an extremely important role in the determination of the physical,
chemical and biological properties of a soil, as well as the availability of water. A well-drained soil with
an organic content of 5% will, in all likelihood, contain a higher quantity of available water than a similar
soil with an organic content of 3%. This may be attributed mainly to the fact that organic matter improves
the quality of the soil as well as the porosity, and thus the ability to hold water. (Most irrigation soils in
South Africa have an organic content of less than 3%).
4.20
Irrigation User’s Manual
Table 4.9: The influence of soil salinity on the yield potential of selected crops (FAO 29, 1996)
Maximum electrical conductivity values recommended for the saturation extract of a soil
sample [mS/m]
Yield potential [%]
Field crops
100
90
75
50
Barley
800
1000
1300
1800
Broadbean
150
260
420
680
Cotton
770
960
1300
1700
Cowpea
490
570
700
910
Flax
170
250
380
590
Groundnut
320
350
410
490
Maize
170
250
380
590
300
380
Rice (paddy)
720
510
Sorghum
680
740
840
990
Soybean
500
550
630
750
Sugarbeet
700
870
1100
1500
Sugarcane
170
340
590
1000
Wheat
600
740
950
1300
Vegetable crops
Bean
Beet
Broccoli
Cabbage
Carrot
Celery
Cucumber
Lettuce
Onion
Pepper
Potato
Radish
Spinach
Squash, scallop
Squash, zucchini
(courgette)
Sweet potato
Tomato
Turnip
100
400
280
180
100
180
250
130
120
150
170
120
200
320
150
510
390
280
170
340
330
210
180
220
250
200
330
380
230
680
550
440
280
580
440
320
280
330
380
310
530
480
360
960
820
700
460
990
630
510
430
510
590
500
860
630
470
580
740
1000
150
250
90
240
350
200
380
500
370
600
760
650
Soil
4.21
Table 4.9: (Continued)
Maximum electrical conductivity values recommended for the saturation extract of a soil
sample [mS/m]
Yield potential [%]
Forage crops
Barley (forage)
Bermuda grass
Clover, alsike
Clover, ladino
Cowpea (forage)
Fescue, tall
Foxtail, meadow
Harding grass
Lucerne
Maize
Orchard grass
Ryegrass, perennial
Sesbania
Sudan grass
Trefoil, big
Trefoil, narrowleaf
birdsfoot
Vetch, common
Wheatgrass, fairway
crested
Wheatgrass, standard
crested
Wheatgrass, tall
Wildrye, beardless
100
600
690
150
90
740
850
230
75
950
1100
360
150
250
390
150
230
340
550
250
360
480
780
410
460
200
180
150
590
340
320
310
790
540
520
550
560
230
280
230
690
370
510
280
890
590
860
360
50
1300
1500
570
570
710
1200
670
1100
880
860
960
1200
940
1400
490
500
600
750
1000
300
750
390
900
530
1100
760
1500
350
600
980
1600
750
270
990
440
1300
690
1900
1100
150
160
150
200
200
200
280
260
260
400
150
180
170
170
150
100
680
250
240
230
220
210
130
1100
410
340
330
290
290
180
410
370
380
1800
670
490
480
410
430
250
Fruit crops
Almond
Apricot
Blackberry
Date palm
Grape
Grapefruit
Orange
Peach
Plum, prune
Strawberry
4.22
Irrigation User’s Manual
7 Factors influencing the irrigation of soil
When soils are investigated for irrigation purposes, there are a number of factors or aspects which must be
taken into consideration. A brief overview of these factors follow.
7.1 Colour of the soil
The colour of the soil can give an indication as to the irrigability of the soil. Soils coloured red, redbrown and yellow-brown are, in most cases, irrigable, depending on the depth of the soil and its
location. The soils should undergo a chemical analysis for the best recommendation, and the colour of
the soil alone should not be relied upon.
7.2 Effective soil depth
Soil depth is one of the most important aspects which should be investigated. It not only influences the
root development and soil water reservoir, but also the degree of drainage past the root zone. Overirrigation usually takes place and this water must be able to drain away without any problems. Dig a
profile hole, or use a soil auger to determine the depth of the soil down to the limiting layer. Basically,
a limiting layer may be described as a layer with a poor water conductivity which is harder or more
dense than the soils above. If the soil depth is more than 900 mm, it may be classified as irrigable. Soil
as shallow as 450 mm may be irrigated, provided cultivars with shallow root systems, such as planted
pastures, are cultivated there. The soil may also be ridged for the planting of fruit trees, for example.
However, this requires excellent irrigation management.
7.3 Texture of the soil
Soil texture is another important property which determines the irrigability of a soil. It influences, inter
alia, infiltration rate, permeability, water holding capacity, internal drainage and the erodability of
soils. The ideal texture is not too fine and not too coarse, and it must have a good distribution of
particle sizes. A too high sand content promotes wind erosion, causes low water holding capacity and
high infiltration rates. A too low sand content results in soils which are difficult to cultivate, having
low infiltration rates and poor drainage properties. Soils of all texture classes are irrigable, but the
following textures are undesirable within the effective root depth: clay, sandy clay, clay loam (> 35%
clay), silt clay, silt clay loam (> 35% clay) and coarse sand.
7.4 Clay content
A clay content of >35% may be considered for irrigation provided the colour of the soil is red and it
has a structure that is not stronger or greater than moderate, fine, angular and block under dry
conditions.
Vertic turf soils with a clay content of between 35% and 55% may be considered for irrigation when
they are of the Arcadia type. The Rensburg form poses a threat of waterlogging and is not
recommended for irrigation. It must be remembered that the finer the soil particle, the greater the
contact surface, the higher the volume of water held, the larger the volume of water that may be
absorbed, the smaller the air pores and the greater the volume of water that is available to the crop.
7.5 Soil structure
A moderately developed granular structure is preferable. The structure must be stable in water and
therefore not dispersive, or else soil crusting can develop. Soil crusting leads to aeration problems, low
infiltration rates and increases the erodability of the soil. A too strongly developed structure is
indicative of a high clay content with its accompanying problems.
Soil
4.23
7.6 Soil pH
Provided the pH of the soil lies between 7,5 and 5,5, and its conductivity is less than 300 mS/m, the
soil is suitable for irrigation. If these values are exceeded, the sodium adsorption ratio (SAR) and the
exchangeable sodium percentage (ESP) must be determined. Under these circumstances, it is advisable
to contact your local soil expert for advice. When the pH is lower than 5,5, the possibility of
aluminium poisoning exists.
7.7 Quality of irrigation water
The quality of the water used for irrigation is very important (See Chapter 5: Water). Previously
discussed aspects are applicable when water of a C2S1 classification is used. Where water of a poorer
quality is used, the soil must be deeper so that leaching of salts may take place.
Problems may occur if a C1S1 quality water is used with a high conductivity and many dissolved salts.
The water reacts with the salts, resulting in undesirable conditions such as lowering the infiltration
rate of the soil. Leaching should form part of the irrigation management program in cases of poor
quality irrigation water. To determine the need for leaching, a soil expert or a soil manual may be
consulted. An example is shown in Section 12.
8 Soil samples
Soil samples are necessary so that the physical and chemical nature of the soil may be determined.
Without the analysis of a soil sample, no reliable irrigation recommendations may be made. Furthermore,
if the soil sampling is not done accurately, the laboratory analysis will be worthless. The following method
should be followed to obtain a sample:
• If a variety of soil types exist, the growing strength of the plants will differ accordingly and if the
particular area is of such a nature that it justifies separate treatment, then the different soils should be
individually sampled.
• Samples may be taken using a spade or a soil auger. However, a profile hole is preferred, as a better
idea of root distribution and the limiting layer may be obtained. In the case of an orchard, holes must
be dug under the spread or drip area of the tree.
• Soils samples should preferably not be taken in areas that were fertilised recently. After the holes have
been dug, the different layers should be identified and sampled separately. Cultivation depth is
regarded as being the first layer. Soil layers under the cultivation layer should be differentiated
according to colour, texture and root distribution.
• If the various layers cannot be clearly distinguished, a soil sample should be taken every 300 mm
• Two samples should be taken to a depth of 600 mm in the case of soils with a uniform profile.
Shallower soils should be sampled up to the limiting layer. Each layer is sampled by digging out a
vertical sod of about 50 mm thick over the entire depth of that particular soil layer.
• Profile holes should be properly backfilled after sampling for safety reasons.
There is no general rule for the number of samples. A soil sample should preferably comprise 10 subsamples (mixed). The soil originating from the same layer, but from different holes, should be combined
and thoroughly mixed. A sample from this mix may then be sent to the laboratory.
If there are distinct differences between profile holes, the various types of soil should be defined and
regarded as separate soil groups. The boundaries of the chief soil types may be determined using a soil
auger and drilling according to a 50 m grid. The profiles may then be compared. Stones form part of the
profile and should form part of the sample. In stoneless ground, 1 kg of soil is needed and, in the case of
stony ground, 3 kg or more is necessary for analysis. The samples should be placed in a suitable container
and marked clearly with the name and address of the owner, field, and sample number, as well as the
sample depth. (Used fertiliser bags must not be used).
All relevant information, such as the slope and depth of the soil, should be supplied to the laboratory. The
4.24
Irrigation User’s Manual
time of sampling is important, and is usually done between the end of one growing season for a particular
cultivar and the start of the next, whether perennial or annual. The concentration of the chemical elements
is dynamic during the growing season of the plant, and only stabilises after the plant has died, in the case
of annual crops.
A list of organisations which carry out soil analyses may be found in Appendix A.
9 A typical soil analysis report
The following soil analysis covers a number of aspects. Soil analyses should preferably be passed on to soil or crop
experts for recommendations.
Example 4.2
Profile no PN 42
Map photo: 3 325 BC
Latitude & longitude 25º 40'34"/33º29'55"
Land type no:
Climatic zone no.
Height 65 m
Terrain unit: Terrace
Slope: 1%
Slope form: Straight
Aspect: N
Micro relief : None
Soil form: Valsriver
Soil serie: Marienthal
Surface rock: None
Surface stone: None
Flood occurrence: Nill
Vegetation/landuse: Cultivated, unknown
Water level: None
Described by: JPN
Description date: 1988-02
Underlying material: Unconsolidated, clayish
Weathering of underlying material:
Horizon
Depth
(mm)
Description
Diagnostic
horizons
Ap
0-170
Dry; dry brown-yellow 10YR6/6; moist brown to dark brown
10YR4/3 fine sand loam; weak medium sub-angular block;
somewhat hard; few pores; water absorption: seconds; general
roots; pH (water) 7.2; pH KCI/CaC12 5.9; resistance 1 300 ohm;
obvious smooth transition.
Orthic
B12
170-530
Dry; dry yellow-red 5YR5/6; moist dark red-brown 5YR3/4 clay;
moderate medium sub-angular blocky; somewhat hard; few pores;
unhardened free calcium, moderately effervescent; few friction
surfaces; general clay cutanes; water absorption: 8 seconds; few
roots; pH (water) 8.7; pH KCI/CaC12 7.5; resistance 260 ohm;
obvious smooth transition.
Pedocutanic
B22na
530-980
Dry; dry red-yellow 7.5YR6/6; moist yellow-red 5YR5/8 general
fine prominent blue-black sesquioxide stains; clay; moderately
medium sub-angular blocky; hard; unhardened free calcium,
moderately effervescent; general sesquioxide concretions; water
absorption: 10 seconds; pH (water) 9.1; pH KCI/CaC12 8.0;
resistance 160 ohm; gradual smooth transition.
Pedocutanic
Dry; dry red-yellow 7.5YR/8; moist strong brown 7.5YR5/6; few
fine obvious red-brown carbonate stains; few fine obvious blueblack sesquioxide stains; clay; moderately fine sub-angular block;
hard; unhardened free calcium; little effervescent; few friction
surfaces; general clay cutanes; water absorption: 10 seconds; pH
(water) 8.9; pH KCL/CaC128.0; resistance 130 ohm.
Pedocutanic
B31na
Soil
Drainage area: Sundays River
Profile no: PN42
Soil form/series: Vals River/Marienthal
Horizon
Depth [cm]
Ap
0-17
B21
17-53
4.25
B22
53-98
B31
98-135
0,3
2,1
9,4
8,4
7,5
6,1
66,1
4,2
4,0
9,1
8,9
10,4
11,0
52,2
0,5
1,6
8,7
9,3
10,3
9,0
60,3
287
260
8,7
7,5
21,9
11,7
0,38
0,34
0,63
1,49
0,04
0,11
Exchangeable cations [cmol + kg-1]
685
160
9,1
8,0
40,2
24,2
0,06
1,14
0,07
900
130
8,9
8,0
46,2
28,1
0,03
1,11
0,09
5,1
2,9
7,1
5,5
20,6
12,7
6,1
3,2
5,0
3,3
17,6
13,2
124
21
138
999
45
23
8
22
999
57
21
10,3
36
11,3
28,4
9,9
25,5
6,5
22,5
5,4
19,5
Clay mineral analyses [%]
26,5
22,6
19,8
17,5
36,6
28,0
25,4
21,9
82
9
46
5
9
49
Particle size distribution [%]
course sand 2 - 0,5 mm
medium sand 0,5 - 0,25 mm
fine sand 0,25 - 0,1 mm
very fine sand 0,1 - 0,05 mm
rough silt 0,05 - 0,02 mm
fine silt 0,02 - 0,002 mm
clay < 0,002 mm
Conductivity [mS/m]
Resistance [Ώ]
pH [H2O]
pH [KCl]
ESP
SAR
C[%]
Fe-SBD[%]
Al-SBD[%]
0,5
5,1
23,7
23,5
19,7
6,6
18,4
Chemical analyses
1300
7.2
5,9
10,4
Na
K
Ca
Mg
S value
T value [cation exchange capacity]
Modules of rupture [kPa]
Air-water permeability ratio
Liquid limit [%]
Plasticity limit [%]
Linear contraction [%]
Plasticity index units
-33 kPa
-80 kPa
-500 kPa
-1 500 kPa
Mica
Kaolinite
Kaolinite/Smectite
Interstratified
0,5
1,1
2,0
1,7
5,3
4,8
Physical analyses
3,0
2,6
6,8
5,6
18,0
13,7
38
137
43
21
8
22
Soil water content [mm/horizon depth]
83
9
8
82
10
8
4.26
Irrigation User’s Manual
10 Soil conservation
Where land is to be cultivated, it is necessary to investigate the possibility of erosion. If the possibility
exists, the necessary preventative measures should be taken.
Where irrigation is applied, erosion may occur as a result of high application rates as, for example, in the
use of low pressure centre pivots (at the last tower). Sometimes, soils are cultivated for irrigation, which
would not normally be cultivated under dry-land production, as in the case of steep slopes. A soil
conservation plan is a necessity in irrigation planning. The National Soil Conservation Manual, compiled
by the Department of Agriculture, may be consulted for the necessary details. Before new fields are
prepared, a permit must be obtained from the Directorate of Resource Conservation or the nearest
Agricultural Extension Officer. The Soil Conservation Act (Act 43 of 1983) compels any user of ground to
utilise the ground, or apply measures, in such a way as to prevent or combat erosion.
11 Drainage
Soils that do not have drainage problems under dry-land conditions may get waterlogged if irrigated. This
may be as a result of over-irrigation, shallow soils or low quality water and/or salts. If economically
justifiable, artificial drainage may be considered, or irrigation should be withdrawn from the specific field.
Before a soil is drained, it is essential that all possible factors causing the waterlogging is improved or
removed. In this regard, leaking earth dams, blocked natural drainage channels or water courses, dense
soil layers (resulting from tilling) with low infiltration rates as well as over-irrigation, play a considerable
role. The cause of the waterlogging and free water can be determined by means of a detailed soil survey
and a decision on the most suitable type of drainage system can then be made.
11.1 Types of drainage
Depending on the cause of the waterlogging, different approaches should be followed regarding the
method of drainage. Two main types are distinguished in practice:
i) Cut-off drainage
This is used where free water moves under gravity from a higher lying to a lower lying position in
porous, sandy or gravelly layers on dense soils (Figure 4.12). The cut-off drain is made more or
less perpendicular to the flow direction of the free water. The cut-off drain must however have a
gradient large enough to remove the water that accumulates in the drain, fast enough from the
landscape. Over its full length, the drain must be made at least 300 mm deep in the dense
underlying layer. It is essential that the lowest drainage level is continuous and has a consistent
gradient. It is also recommended to build this drain as an open drainage-ditch to get an idea of the
amount of water to be removed (stream strength). With the aid of the stream strength, the minimum
pipe diameter for removing the water can be determined if permanent pipe drains are considered.
ii) Subsurface drains
Actual soil water-levels occur on flat, low-lying landscape positions in sub-humid and humid
regions. The height of the water-level is, in many cases, controlled by the water-level in adjacent
river(s) or marshes. Because the lateral movement of such free water is very slow, cut-off drains
cannot be used. In such cases a network of drains that connect to each other in a way are needed.
The maximum depth of installation of subsurface drains is determined by the height of the waterlevel in the river or marsh where the drains deposit their water. In the case of high river waterlevels, the possibility of making the channel or river deeper must be considered. If the soil to be
drained has a relatively high permeability, it is economically beneficial to install the drains as deep
as possible.
Soil
4.27
The shallower the drains are placed, the greater the length of drains per surface unit needed to
obtain the same reduction of the free water-level in the soil than with a deeper placing.
A few examples of different subsurface drainage systems are shown in Figure 4.13. There are many
cases where relatively shallow, static water-levels on impermeable layers with a great extent occur.
The problem of waterlogging can sometimes be overcome in such cases by relinquishing the
limiting layer through deep soil preparation along the slope. The deeper such preparations are, the
deeper the new drainage level. This action is especially successful if tilling is done through the
limiting layer, into the underlying material with inherent better permeability, or which will remain
open longer.
Many soils, however, are found that are subject to waterlogging, resulting from their inherent low
permeability and/or physical instability that cannot be successfully drained by means of one of the
above methods. The practice followed in these cases is known as “banking up” or beds. The main
purpose of beds or banking-up walls is to remove excess rain-water or irrigation water collecting
on the ground surface, as fast as possible from the field.
Since an in-depth discussion of the different drainage systems and materials to be used is not the
purpose of this chapter, only a summary of the mentioned items are given in Table 4.10. The
spacing of subsurface drains is dependent on the hydraulic conductivity of the soil, as well as the
drain depth. Because the hydraulic conductivity of the soil is mainly determined by the texture
thereof, texture and drain depth can be used as a broad guideline for drain spacing. Drainage can
also be applied for reclamation of soils such as brackish soils. If excess water cannot drain
naturally from a soil profile, the installation of a artificial drainage system can be considered. Welldrained soil is essential if the soil or water requires that leaching of undesirable salts must take
place.
4.28
Irrigation User’s Manual
Figure 4.12: Example of a cut-off drain
Figure 4.13: Examples of different drainage systems
Table 4.10: Summary of different drainage systems and materials used
ITEM
Systems
TYPE OF SYSTEMS OR MATERIALS
Open channel
Stone drains
Pipe drains
Unglazed earthenware pipes
Materials for pipe drains
Glazed earthenware pipes
Cement pipes (for soils with no SO4²- in the soil water)
Smooth uPVC pipes with grooves (6 m lengths)
Pitch fibre pipes
Fluted uPVC pipes with grooves (in rolls)
Gravel or coarse river sand
Cover materials
(Especially for finely textured soils) Coal slate
Stone breaker dust
Fibreglass
“Styromull” (Synthetic small granules)
The User’s Manual for Subsurface Drainage (1984) of the Department of Agriculture can be consulted
for further information.
Soil
4.29
12 Leaching requirements
Irrigation with poor quality water not only has a negative effect on yields, but may also lead to the general
deterioration of good irrigable land. To prevent this, special management techniques, such as leaching,
should be used. The leaching requirement is the minimum amount of irrigation water that is needed to pass
through the root zone to control the salts within the tolerances of that particular cultivar, taking into
consideration the quality of the water used. The leaching requirement may be determined with the aid of
equation 4.5.
LR =
Where
LR
ECi
ECe
(3.5)
= leaching requirement [fraction]
= electric conductivity of irrigation water [mS/m]
= concentration limits for salinity sensitivity of particular crops without yield
losses [mS/m] (Refer Table 4.11)
NIR L =
Where
ECi
5(ECe) - ECi
ET
1 − LR
(3.6)
NIRL = nett irrigation requirement with leaching added [mm]
ET
= evapotranspiration [mm]
LR = leaching requirement [fraction]
Example 4.3
Determine the leaching requirement if only 10% crop losses (yield losses) on cotton are allowed. The EC of the
water used is 300 mS/m. The evapotranspiration for this example can be taken as 25 mm per cycle.
Solution
Refer Table 4.11:
Electric conductivity (limit values): 770 mS/m.
Percentage drop in yield per 1 mS/m increase in electrical conductivity 0,52%. For this, a 10% yield loss will result
in an increase of 10
= 19 mS/m in electrical conductivity.
0,52
From Equation 4.5:
300
LR =
5 (789) - 300
= 0,082
From Equation 4.6 the adapted nett irrigation requirement:
ET
1 − LR
25
=
1 − 0,082
= 27,2 mm per cycle
NIRL =
4.30
Irrigation User’s Manual
Table 4.11: Salt sensitivity of important cultivars
Crop
Apricot
Beetroot
Bermuda grass
Brussel sprouts
Cabbage
Cotton
Cumcumber
Dates
Grapes
Green beans
Lettuce
Lucerne
Maize (grain)
Maize (feed)
Onions
Oranges
Peaches
Peanuts
Pomelos
Potatoes
Prunes
Radishes
Spinach
Strawberries
Sudan grass
Sugar cane
Sugar beet
Sweet potato
Sweetcorn
Tomatoes
Wheat
Electrical
conductivity
(limit values)
[mS/m]
Percentage crop loss per 1 mS/m increase in
conductivity of saturation extract exceeding
limit values
[%]
160
400
690
280
180
770
250
400
150
100
130
150
170
180
120
170
170
320
180
170
150
120
200
100
280
170
700
150
170
250
600
0,24
0,09
0,64
0,092
0,097
0,52
0,13
0,036
0,096
0,19
0,13
0,12
0,12
0,074
0,16
0,16
0,21
0,29
0,16
0,12
0,18
0,13
0,076
0,33
0,043
0,059
0,059
0,11
0,12
0,099
0,071
If the EC-value of a soil is higher than the limit value of the crop to be cultivated, a soil scientist can be
consulted on how to rehabilitate the soil, e.g., if the soil has an EC-value higher than the limit value for the
relevant crop, it can only be cultivated if it can be leached to reduce the EC-value. By using a gypsum
treatment and leaching calculated and prescribed by a soil scientist, the soil can be rehabilitated.
Soil
4.31
13 References
1.
Brady, N. C. 1990. The nature and properties of soils (10th edition). Maxwell MacMillan
International Editions.
2.
Department of Agriculture. 1984. Gebruikshandleiding vir ondergrondse dreinering. RSA.
3.
Department of Water Affairs and Forestry. 1993. Water quality guidelines. Vol. 4 Agricultural Use.
4.
Food and Agricultural Organisation. 1996. Water quality for Agriculture. (FAO Bulletin No. 29).
Rome, Italy.
5.
Foth, H. D. 1984. Fundamentals of Soil Science. John Wiley and Sons, New York
6.
Hillel, D. 1980. Fundamentals of Soil Physics. Academic Press.
7.
Nell, P. 1996. Personal communication. ARC-ISCW.
8.
Van der Watt, H.v.H. et. al. 1995. A Glossary of Soil Science. The Soil Science Society of South
Africa.
4.32
Irrigation User’s Manual
APPENDIX A
ALASA MEMBERSHIP REGISTER FOR SOIL ANALYSES (31 AUGUSTUS 2002)
Organisation
Mpumalanga
Labserve Analytical Services
ARC-Institute for Tropical and Sub-tropical Crops
SGS Mineral Services Barberton
Gauteng
Agrilab
ARC-Institute for Industrial Crops (ARC-IIC)
Soil Science Laboratory
Dept. Plant production and soil science
Institute for Soil, Climate and Water
Central Analytical laboratories (Pty)
Madzivhandila College of Agriculture
Temo lab services cc
SGS Laboratory Services
North West Province
North West Agricultural Development Institute; Department of
Soil Science
Northern Province
Plant and Soil Analytical Laboratory
Department of Soil Science
Freestate
Glen Soil analysis lab (69)
Soil Science Laboratory ARC-Small Grain Institute
Omnia Fertilizer (Sasolburg)
Western Cape
Central Analytical Laboratories (Cal Cape)
Soil Science Laboratory
Address
Telephone number
P. O. Box 1920 Nelspruit 1200
Private Bag X11208 Nelspruit 1200
P. O. Box 648 Barberton 1300
(013) 741 2552
(013) 753 7000
(013) 712 6704
P. O. Box 1921 Tzaneen
Private Bag X82075 Rustenburg 0300
University of Pretoria, Pretoria 0002
(015) 307 4317
(014) 536 3150
(012) 420-3213
Private Bag X79 Pretoria 0001
P. O. Box 812 Ifafi 0260
Private Bag X5024 Thohoyandou 09500
Private Bag X07 Pretoria 0116
P. O. Box 5472 Halfway House 1685 Unit 5 Mufa
Park, Georgestr 399, Randjiespark, Midrand
(012) 310 2500
(012) 305 5003
(015) 962 4586/7/8
(012) 799 9721
(011) 811 2280
Private Bag X804 Potchefstroom
(018) 299 6511
University of the North
Private Bag X1106 Sovenga 0727
(015) 268 2188
Private Bag X01 Glen 9360
Private Bag X29 Bethlehem 9700
P. O. Box 384 Sasolburg 9570
(051) 861 1244
(058) 307 3501
(016) 970 7200
P. O. Box 927 Somerset West 7129 AECI Premises, De
Beers Rd, Somerset West 7130
Dept. Agriculture of Western Cape
Private Bag X1 Elsenburg 7607
(021) 852 7899
(012) 808 5286
Soil
Organisation
Bemlab Bk
Kwazulu-Natal
Address
P. O. Box 12457, Die Boord, Stellenbosch 7613
Telephone number
(021) 851 6401
SA Sugar Association Experiments Station
Eastern Cape
Grootfontein Agricultural Development Institute
Private Bag X02 Mount Edgecombe 4300
(031) 539 3205
Private Bag X529 Grootfontein L01 Middelburg OK
5900
Private Bag X6011 Port Elizabeth 6000
Private Bag 13184 Windhoek Namibia
(049) 842 1113
(041) 504 3631
(09-264-61) 208 7077
P. O. Box 1 Mhlume Swaziland L309
(09268) 313 1211
Agriculture Department of Port Elizabet Technikon
Namibia
Agriculture Laboratory Windhoek
Swaziland
Mhlume Sugar Co Ltd Agronomy Laboratory
Mozambique
Soil, Plant and Water Laboratory
P. O. Box 3658 Av. Das FPLM 2698, Mavalane,
Maputo, Mozambique
4.33