Agriculture Reference
In-Depth Information
Silt
Smallest
coarse sand grains
Root
hair
Silt
particles are those between 0.002 and
0.06 mm in diameter.
Smallest pore
space (0.05 mm
across)
between
coarse sand
grains
Coarse sand
Most particles in this size range are inert and non-
porous like sands, so they behave like very fine sand.
They have good water-holding capacity and plants can
take up a high proportion of this water (see Table 12.3).
However, some of the particles (usually around 15%)
have the properties of clay, so soils dominated by silt
do yield some nutrients. Similarly, they can hold on to
some nutrients like clay does, but to a lesser extent.
0.6 mm
0.2 mm
Largest fine sand has
largest pore space of
less than 0.05 mm
Fine sand
0.06 mm
Silt
0.002 mm
Clay
Typical
clay particle
(
100 000 actual size)
Stones and gravel
Stones
are particles larger than 2 mm in
diameter.
Particles bigger than sand are commonly known as
grit, gravel, pebbles, cobbles and boulders, according
to size and shape. The effect of stones on cultivated
areas depends on the type of stone, their size and
the proportion in the soil. In general, they make soils
difficult to cultivate. Digging is harder and the spade
is more easily blunted. They have detrimental effects
on mechanized work: tines and tyres are worn more
quickly especially if the stones are hard and sharp such
as broken flint. Stones interfere with drilling of seeds
and the harvesting of roots. Close cutting of turf is more
hazardous where there are protruding stones. A high
proportion of stone reduces the fertility of the soil as it
dilutes the soil components that supply nutrients and
hold the water. The amount of soil that roots can explore
is reduced according to the volume of stone present.
Figure 12.7
Relative sizes of sand, silt and clay (based
on SSEW classifi cation) with root hairs drawn alongside
for comparison. Note that even the smallest pore
spaces between unaggregated spherical coarse sand
grains allow water to be drawn out by gravity and so
allow some air in at fi eld capacity, whereas most pores
between unaggregated fi ne sand grains remain fi lled
wet and hard when dry. Unless there are cracks
between these blocks of packed clay particles, water
movement is very restricted. However, many types of
clay shrink on drying so introduce cracks. Alternatively,
cracks are introduced by cultivation or the action of
soil organisms (e.g. earthworms) to allow gravitational
('excess') water to leave the rooting zone.
These clay particles are also porous, so hold water
inside as well as on their surface. Consequently, clay-
rich soils are able to hold on to a lot of water (see
Table 12.3) but some of it is too tightly bound to be
released to plant roots (about 15%).
Clay continues to weather and release plant nutrients
especially potash. It also has the ability to hold on
to some nutrients in such a way that they remain
available to plants but protected against being leached
(washed down the profile) below the reach of roots
and eventually into water courses. This property
gives clay the ability to maintain nutrient levels in the
soil solution even when being taken up by plants. In
contrast, sandy soils are less well buffered against
the loss of nutrients by the leaching from the root
zone.
Soil texture
Soil texture describes the mineral composition of a
soil. In most cultivated soils the mineral content forms
the framework and exerts a major influence on its
characteristics.
Soil texture
can usefully be defined as the
relative proportions of the sand, silt and clay
particles in the soil.
A soil dominated by
X
sand particles is called '
a sand
' and feels 'gritty' or
abrasive
