SOIL
The Exciting World
Beneath Our Feet.
Physical properties of soil
Texture
Structure
Particle density
Bulk density
Pore space
Water relations
Plasticity
‘Soil tilth” is the term used in publications aimed at the general
population to refer to the physical condition of the soil.
Soil texture
The relative proportions, by weight, of the particle size fractions
(sand, silt and clay) that are < 2mm in ‘equivalent spherical diameter’.
Also called ‘particle size distribution’.
Soil texture is determined by sedimentation from suspension. The rate of
sedimentation of particles is related to their density, diameter and shape.
The diameters are calculated from falling times, assuming that the
particles are spheres – hence, ‘equivalent spherical diameters’
The size fractions are:
Sand – from 2 to 0.05 mm in diameter – individual particles can be seen
with the naked eye and felt by the fingers
Silt – from 0.05 to 0.002 mm in diameter – not visible by naked eye, but
visible under the light microscope. Not felt individually by fingers, but will
scratch fingernails and grit between the teeth.
Clay - < 0.002 mm (2 microns) – visible with the electron microscope
If I was allowed to know only one physical property of a soil, I
would want to know its texture.
Soil texture triangle
Twelve soil textural classes are recognized,
based on the percentages of sand, silt and
clay present (see figure).
Sands: particles act individually (do not
stick together). Soils dominated by them:
have large pores, low water holding
capacity, drain rapidly, and have low
fertility.
Silts: may form aggregates. Soils
dominated by them: have medium size
pores, high water holding capacity, drain
at moderate rate and have moderate
fertility.
Clays: have a high surface area per unit weight. Soils dominated by them: form
aggregates, have small pores, high water holding capacity. If dominated by
phyllosilicate clay minerals, they drain slowly and are sticky, plastic, and tend to be
fertile; if dominated by oxide clay minerals, they drain more rapidly and tend to be
non-sticky, non-plastic and of low fertility.
Soil texture is not easily altered; it can change over long
periods in intense chemical weathering environments.
Soil structure
Soil structure refers to the grouping of soil particles into aggregates
(‘peds’ to the soil classification people).
It refers to the spatial organization of the soil particles, and controls the
size distribution of pores and the nature of the pore spaces.
Types of soil structure
A) Structureless – single grain: particles act individually
- massive: soil act as one irregular mass
B) Spheroidal – rounded granular aggregates < 10 mm diameter
(also known as ‘crumb’ structure)
C) Blocky – angular: sharp edges and corners
- subangular: rounded edges and corners
D) Prism-like – columnar: rounded tops
-- prismatic: flat tops
E) Platy - have horizontal structures (rare)
Soil texture quite strongly influences the soil structure,
but not vice-versa.
Factors influenced by soil structure
moisture holding capacity
movement of air and water into and through soils
plant availability of soil water
root penetration
soil microorganism activity
heat transfer
ease of cultivation
seed germination
and more
Management of soil structure
Soil structure, the spatial arrangement of the soil particles,
is changing on an almost continuous basis. Changes for
the worse are very easy to accomplish, whereas
changes for the better require effort and diligence.
For example, compaction is easy to cause and makes a
soil more dense, thereby decreasing the amount of pore
space and the size of individual pore spaces. The soil
becomes harder for roots, water and air to penetrate,
harder to cultivate, and less productive.
Reducing the bulk density, and retaining the lower bulk
density, requires good management and persistence.
Factors affecting soil structure
Climate related: wetting and drying
freezing and thawing
Biological activity: plant roots
microorganism excretions
soil animals – insects, worms, etc.
organic residues
Management: cultivation (not too much, or when soil is too wet)
crop rotation
liming
fertilizers (when necessary for fertility reasons)
Other physical parameters
Particle density: Silicate dominated soils have particle densities between
2.65 and 2.70 gcm-3. The silicate and carbonate minerals all have densities
in or close to this range. Particle density does not change, except with major
production of iron oxides by extreme weathering.
Bulk density: The weight per unit volume of the whole soil, including the pore
space. Normal ranges: sand dominated = 1.2-1.8 gcm-3; clays and silts =
1.0-1.5 gcm-3. Bulk density is constantly changing.
Porosity: The percent of the soil volume that is pore space. Subject to almost
constant change.
Macropores: Drain under the force of gravity.
Micropores: Retain water against the force of gravity.
Water content: normally presented on a dry weight basis – the weight of
water as a percentage of the dry soil weight.
Percent saturation: The percent of the pore space that is occupied by
water.
Soil Water :
Water acts as a solvent and as a carrier of plant nutrients:
to the plants and in the plants.
Plants transpire a large amount of water: through their
leaves to the atmosphere.
Soil water helps in temperature regulation: both in the plants
and the soil surface zone.
Water must be available where and when the plant needs it.
Insufficient water is one of the major factors that limit crop
production.
Soil water
The physical structure of the soil is the dominant factor
controlling soil water relations, whether the concern is
infiltration, transmission, retention or water availability for plant
uptake.
Pore size distribution and pore interconnectivity are controlled by the
spatial distribution of the soil particles (i.e. by both the soil texture
and the soil structure)
Try to imagine the pore space in soils (it helps to close your eyes).
A balance such that about ½ of the pore space consists of
macropores (which drain rapidly) and ½ consists of micropores
(which retain water for plant use) is optimal.
Good soil management has been described by one soil physicist as
being
THE PRESERVATION OF MACROPORES
Soil water potential
Water is held in the soil and work must be done to remove
it.
Soil water potential
is a measure of how strongly water is held in the soil.
The strength with which the remaining water is held
increases as the soil water content decreases.
In other words,
the water that is closest to the particle surface is the most
strongly held, and
the water in small pores is held more strongly than that in
large pores.
Soil water
The soil water table is the reference point for soil water
potential.
Above the water table, the soil water is being pulled down
so it is under tension, or at a negative potential.
Below the water table, the soil water is under positive
pressure, or at a positive potential.
Because most plants have their roots in soil that is above
the water table, they are always working against the
negative potential to extract water.
Soil water
Two forces account for the retention of water by soil
solids:
Cohesion of water – the attraction between the hydrogen
atoms of one water molecule and the oxygens to each
of two neighbouring water molecules.
Adhesion of water – the attraction between a hydrogen
of a water molecule and an oxygen atom of the silicate
mineral surface.
These hydrogen bonds between water molecules and
the mineral surface cause the minerals to be strongly
hydrophillic: they attract water.
Hydrogen bonds and capillary rise
The hydrogen bond accounts for
the high melting and boiling points
of water, and for the unusually
strong capillary rise exhibited by
water when in contact with
hydrophillic surfaces.
The height of capillary rise is inversely proportional to the radius
of the capillary. Rise is greater in small capillaries than in large.
Hence water is more strongly held in the small pores than in the
large pores.
See Demo
Soil water potential
The most understandable units for soil water potential are
‘cm of water’ and ‘atmospheres’.
Because the soil water potential is negative above the
water table, a soil water potential of -10 cm of water
would be the equivalent of the downward force exerted
by a column of water 10 cm high pulling downward on
the water in the soil . (Drinking straw analogy).
A potential of -1 atmospheres would be the equivalent of a
10 meter long column of water pulling downward.
(To imagine such a force, remember the increase in the
pressure on your eardrums as you dive below the water
in a swimming pool, only as a ‘sucking‘ force.)
The soil water content vs soil water potential
relationship
Sponge demonstration
Field capacity (FC): The water content that is retained
against the force of gravity (-0.1 atm for sands; -0.5 atm
for clays; average potential for most soils -0.3 atm).
Macropores have drained
Wilting coefficient (WC): The water content at which
plants can no longer extract water (-15 atm).
Hygroscopic coefficient (HC): The water content of airdry soil. Only water remaining is strongly adsorbed to
particle surfaces.
Plant-oriented classification of
soil water
Superfluous water: the water that freely drains under the
force of gravity. While present, it excludes air and, as it
drains, it leaches nutrients.
Available water: The amount of water held between the
field capacity and the wilting coefficient. This becomes
increasingly difficult for plants to access, as the
remaining available water decreases. Organic matter in
the soil increases the amount of available water by
increasing the FC and decreasing the WC – i.e. by
changing the soil structure.
Unavailable water: The water remaining in very small
pores and as hygroscopic water.
Soil water
Potential (- atm)
0
l
0.3
l
1
l
15 31 100
l l
l
1000
l
10000
l
Physical
Perspective
Free
water
Hygroscopic
water
Location
Macropores
Micropores
Adsorbed water
Plant-related
Perspective
Superfluous
Available
Unavailable
Available Water vs Soil Texture
Soil water vs soil texture
30
Water content (%)
25
20
15
10
5
0
Sand
Sandy
loam
Soil texture
Loam
Silt
loam
Clay
loam
Field capacity
Clay
Wilting coefficient
Infiltration rate
The rate of downward entry of water into soil is influenced by:
Soil texture – more rapid in coarse soil
Soil structure – more rapid into soil with macropores that
open to the surface
Structural stability – unstable aggregates break down and
the fragments block pores
Surface condition – soil crusts with their small pores inhibit
infiltration
Layering – presence of layers of different texture decrease
the rate (clay over sand, or sand over clay)
Prior water content – faster into drier soils
NOTE: Dry organic soils tend to be hydrophobic. Water initially beads on
the surface, and enters very slowly for some period of time.
Soil water movement – saturated conditions
Soil water moves in response to gradients in the soil water potential.
The rate of water flow in a pore is proportional to the square of the pore
radius. This means that, for a equal cross-sectional area of pores, a
change of 10 times in the pore size leads to a 100 times difference
in the hydraulic conductivity – the ability to transmit water – under
saturated conditions.
Typical hydraulic conductivities under saturated conditions
Sands – 10-2 - 10-4 cm/sec
Silts - 10-5 - 10-7 cm/sec
Clays - 10-8 - 10-10 cm/sec
(i.e. a typical saturated sand will transmit water 1 million times faster
than a typical saturated clay!)
Soil water movement – partially saturated
conditions
Water flows much more slowly when the soil that is drier
than the field capacity.
Under conditions of partial saturation, the flow occurs only
through the water-filled pore space – i.e. through the
micropores and thin films on the particle surfaces of the
air-occupied pores.
At field capacity, the hydraulic conductivity is on the order
of 1 cm/day!
Excess water - Drainage
For soils that are normally too wet, drainage may be
accomplished by:
1) contouring the surface and/or constructing surface
drains to direct surface water to drainage ditches; or
2) by the installation of a subsurface drainage system.
Subsurface drains need to be more closely spaced and at
shallower depths in clay-rich soils than in sandy soils.
Insufficient water - Irrigation
Water is supplied when needed. In agricultural production,
if one waits for the plants to appear wilted before
irrigating, the water is being applied too late.
Need to have a way of knowing when about 1/2 to 2/3 of
the available water has been used, if sprinkler or flood
irrigation is used.
Drip irrigation is the most water-efficient means of applying
irrigation.
Salinization is a risk that accompanies irrigation in arid
regions, where salt accumulation can be seriously
problematic.
Questions,
re:water
???
Chemical properties and clay
mineralogy
Mineral surfaces
Essential elements
pH
Clay minerals; structure and properties
Cation exchange capacity (CEC)
Soil fertility
Mineral surfaces
Silicate, aluminosilicate and iron and aluminum oxide
and hydrous oxide minerals, in widely varying amounts,
constitute the major mineral constituents of soils.
The nature of their surfaces is very important to their
interaction with the elements that are dissolved in the
pore water.
Carbonates may be present in soils developed from
limestone and in soils of semi-arid and arid regions.
When present, they control the pH of the soil (maintain
basic / alkaline conditions) which affects element
solubility, but otherwise they have little interaction with
elements in solution.
Essential elements
Plants require a range of elements for proper growth.
These must be present where and when the plants need
them.
Macronutrients: Elements required in large quantities
From the air: carbon, hydrogen, oxygen
From the soil: nitrogen, phosphorus, potassium, calcium,
magnesium and sulfur
Micronutrients: elements required in small quantities
From the soil: iron, manganese, boron, molybdenum,
copper, zinc, chlorine, cobalt and nickel
pH
pH is the measure of the acidity or alkalinity of solutions.
It is defined as:
pH = - log10 [H+]
A pH change of 1 unit represents a 10X change in [H+] in solution
NOTE: a lower pH indicates a higher H+ concentration
The pH range extends for 0 to 14.
pH 7 is neutral
pH <7 is acidic
pH > 7 is basic (alkaline)
Most soils fall within the pH 3 to pH 11 range
Humid region soils: mostly pH 4 to 7+ range
Arid regions soils: mostly pH 7- to 9 range
pH
If I were limited to knowing only one chemical property of a soil, I would
ask for its pH. WHY?
1.
2.
3.
4.
5.
6.
Basic soils are usually more fertile than acidic soils.
Available calcium and magnesium contents are higher.
Availability of phosphorus is strongly pH dependent.
Availability of many micronutrients is pH dependent; most have
minimum concentrations below which there is deficiency and
maximum concentrations above which plants exhibit toxicity
symptoms
Al and Mn (manganese) toxicities may occur below about pH 4 – this
is a major problem in the humid tropics.
Microorganism activity (beneficial and detrimental) is pH dependent.
pH is manipulated by lime addition (higher) or sulfur addition (lower).
Clay mineralogy
The clay size fraction dominates the physical and chemical
properties of soils to an extent that is out of proportion to the
percentage present.
This dominance is due to their small size, regardless of their
mineralogy
The aluminosilicate clay minerals are particularly effective in
dominating the properties because, in addition to being of small
size, they are plate shaped and as a result have a very large
exposed surface area per unit weight.
These plate-shaped minerals are known as
layer silicates or phylosilicates.
In order to fully appreciate why phyllosilicates are so important
it is necessary to understand their structure at the atomic level.