Geoscience Reference
In-Depth Information
100
0
(b)
(a)
80
20
Clay
60
40
Clays
Silty
clay
40
60
Silty clay
loam
Clay loam
Sandy clay
loam
20
80
Loa
m
Silt loam
Sandy
loam
Silt
Sand
0
100
0
100
80
60
40
20
Sands
Light loams
Per cent sand
Texture or particle size influences many chemical,
physical and biological properties of soil. Larger particles
have larger pores between them and therefore allow more
rapid infiltration and
drainage
of water. Finer particles
have finer capillary pores which, in contrast, hold water
in the soil and thus improve the soil's
water-holding
capacity
. Coarse-textured soils are quickly drained of
rainfall and are not able to hold much water for plant
growth. They are 'droughty' soils, and lack of available
water can be a limitation on their productivity and choice
of crops. However, because the solid phase has a much
lower heat capacity than water, coarse-textured soils heat
up much more rapidly in spring, and thus have longer
growing seasons. In contrast, fine-textured clays have
greater water-holding capacities, and thus show fewer
symptoms of drought. Indeed, the farming problem here
is often to remove excess water by artificial drainage in
order to improve soil aeration. Because plant roots need
oxygen for respiration, soil waterlogging can be a serious
limitation. Equally, wet clay soils will be slow to heat up,
given their high heat capacities. Thus soil texture is
important in water capacity and movement, soil tempera-
ture and aeration (
Chapter 8).
Chemical properties of soils are also dependent on soil
texture. The fine separate of clay determines most of the
chemical properties of soils. Particles with a diameter
smaller than 0·002 mm (2 micrometres or 2μ) are classed
as
colloids
, or are said to be in the
colloidal state
. Colloidal
properties arise from the very large surface area associated
with a small mass. There is an indirect relationship
between particle size and the surface area of the particles.
Assuming for simplicity that particles are spherical and
that the volumes of solid particles are equal, the surface
area of soils can be compared by using the value
R
(ratio
of surface area to volume for a particle of radius
r
): thus
when
r
= 1 mm,
R
= 3 mm
-1
and when
r
= 0·001 mm,
R
= 3000 mm
-1
.
Soil structure
Under field conditions, properties determined by soil
texture may be considerably modified by
soil structure
.
Solid mineral particles exist in a definite arrangement, and
the pore spaces between them are filled partly with water
and partly with gases. The arrangement of individual
particles into larger
aggregates
or
peds
of various sizes
and shapes is the
soil structure
. Although 'texture' and
'structure' seem to be used interchangeably in popular
media usage, there is a real difference in meaning. Clay and
silt are not spread uniformly throughout the soil. The fines
coat sand particles. Individual clay units join together into
clay domains
rather like the leaves of a book. This is a kind
of parallel orientation, with the clay domains forming
coatings around sand particles or soil structure units. A
further distinction between texture and structure lies in
the role of organic matter. Humic colloids considerably
influence the properties derived from texture. Organic
matter can improve the water-holding capacity of sand
and can improve the drainage properties of clay. This is
achieved by promoting structure formation.
Figure 19.2
