is the most important single feature of soil because it has profound
effects on all physical and chemical properties. It is the permanent
condition referring to the coarseness or fineness of the soil
particles. Soils can be classified according to texture. This results
from the inherent and marked property differences between sand and
grains remain in the soil because they are composed of inert minerals
resistant to decomposition or change. Sand grains do not absorb ions
and contribute nothing to chemical soil fertility.
particles are derived from the chemical alteration of rock minerals
such as orthoclase. Clay particles in soil:
- Resemble small sheaves of paper in shape giving
it its slippery quality
- Allow absorbed ions to be released readily to
- Provide the main source of plant nutrients
- When wet, are surrounded by a thin film of
a given volume of soil the minute clay particles have a much greater
surface area than the same volume of sand and therefore, retains more
moisture. There are many kinds of clay, their composition depending
on the amount of desilicification which has occurred however,
clay particles are usually a mixture of magnesium, potassium,
hydrogen, calcium, sodium, and other elements. Clays like all
colloids have the power of retaining water by imbibition and
the amount of water imbibed varies with the type of clay. Clays in
dry areas generally absorb more water than clay in wet areas.
a soil is saturated with water and allowed to drain for 1-2 days it
is then holding the maximum amount of water under practical
conditions. Plants can extract this water easily and grow rapidly
but as water is used and the soil dries out, the plant is less able
to extract the water remaining in the smaller pores.
amount of water in wet ground is said to be the "available
moisture". Heavy clays may have a very high amount of "available
water" but it is less easily extracted compared with clay loam
soils. On the other hand sandy soils allow water to enter rapidly and
move readily through the soil.
aeration of soil is essential in providing oxygen for plant roots.
Although clay soils have a higher proportion of pore spaces than
sands most are filled with water so that generally, aeration is
poorer in clays.
is intermediate between sand and clay and therefore, is intermediate
for most of the above properties. Some silt soils are very fertile
because plant nutrients are being released from the weathering of
silt particles. Other silts fill the spaces between the sand grains
making the soil very dense. There is a difference between "texture"
and "structure" so that soils with the same texture may
have very different structures.
is assessed by rubbing the soil between the fingers. This is the
standard test in the field:
particles do not adhere or stick together and the soil is coarse to
SAND is soil where cohesion between the particles is just
LOAM is soil where the particles cohere but the soil is friable.
Individual sand grains can be felt.
SOILS are both friable and coherent. Sand grains cannot be felt
in a moist sample
CLAY LOAM SOIL is like a clay loam but sand grains can be felt
LOAM SOIL is somewhat friable but also plastic. If rolled
between the thumb and fingers will form a ribbon whereas a loam will
CLAY SOIL is like clay but sand grains can be felt
SOIL is tough and plastic. Long ribbons can be formed
LOAM SOIL is soft and floury when dry. It is distinguishable
from loams by a smooth feel and will not ribbon like a clay loam.
CLAY LOAM soil is like a clay loam but with the smooth feel of
a silty soil.
sand may contain not only quartz but also fragments of mica and
felspar. On the other hand, fine sand will contain almost only
quartz. Sand can be classified into:
- coarse sands
- fine sands
- very fine sands
- loamy fine sands.
loam contains roughly equal proportions of sand, silt, and clay.
Loams can be classified into:
- sandy loams
- fine sandy loams
- silt loams
- silty clay loams
clay soil the particles are very small and can be described as dust. As
soils age their bedrock characteristics disappear. Soils from
shale or sandstone parent rocks may improve with age. They can be
- sandy clays
- silty clays
1 shows the "texture triangle" which represents the
various combinations of clay, sand and silt.
soil sample will contain obvious organic matter and mineral
particles. The mineral particles can be classified as follows:
- fine (clay)
- medium (silt)
- coarse (gravel) - fractions.
international agreement on soil particles recognises the following
2mm diameter = stones
- 0.2 mm = coarse sand
- 0.02 mm = fine sand
- 0.002 mm = silt
<0.002 mm = clay.
is a temporary condition referring to the arrangement of the soil
particles. Farmers often describe soil by depth, structure, texture,
colour, and mode of origin. For example, a farmer may say that his
soil is "deep, friable, heavy red basalt". The valuer
should understand such everyday terms and the "jargon" of
the farmer as the valuer is concerned with the opinion of the market
place when determining market value. Structure is the arrangement of
soil particles into aggregates or PEDS. Structure can be
most desirable structure is a crumb structure which is friable enough
to cultivate early but cohesive enough to remain stable when wet. Crumb
structure is promoted by the presence of calcium and magnesium
on the clay particles which causes them to flocculate or stick
together and by organic matter which produces glues to bind sand and
clay together. Soil structure can be destroyed by mechanical means
such as excessive cultivation, compaction, heavy rainfall or
irrigation. Soils with good structure have a high proportion of large
pores resulting in high infiltration rates and permeability, good
drainage, and aeration. Well structured soils are easy to cultivate
and relatively resistant to erosion. Structure does not affect water
availability as this depends solely on texture.
wet climates excess water percolates through the soil so that over a
long period of time, leaching occurs of all soluble material from the
arid regions soluble materials are drawn upwards with the capillary
rise of moisture and tends to concentrate at the surface. In semi
arid regions salts are washed down in the wet season and drawn up in
the dry season. They tend to concentrate as nodules or concretions
at certain depths in the soil.
of lime, gypsum or ironstone are most common. Sometimes they join
together to form hard pans which limit root growth, water penetration