In order to provide a suitable root environment for cultivated plants
the soil must be constructed in such a way as to allow good gaseous
exchange, whilst holding adequate reserves of available water.
There should be a high water infiltration rate, free drainage and an
interconnected network of spaces allowing roots to find water and
nutrients without hindrance. There should be no large cavities that
prevent thorough contact between soil and roots and allow roots to
dry out in the seedbed. The soil should be managed so that erosion is
minimized. Good structural stability should be maintained so that the
structure does not deteriorate and limit crop growth.
The plant roots and soil organisms live in the pores between the solid
components of the growing medium. In the same way that a house
is mainly judged by the living accommodation created by the bricks,
wood, plaster, cement, etc., so a soil is evaluated by examining the
|Figure 17.12 Tilth. The ideal tilth for most seedbeds is
made up of soil aggregates between 0.5 and 5 mm diameter.
Within these crumbs are predominantly small pores (less than
0.05 mm) that hold water and between the crumbs are large
pores (greater than 0.05 mm) that allow easy water movement
and contain air when soil is at field capacity (×5 actual size).
Pores greater than about 0.05 mm in diameter, called macropores
drain easily to allow in air within hours of being saturated (i.e. fully
wetted), whereas the smaller pores, micropores
continue to contain only
water. The roots remove more water from these micropores allowing
more air back into the soil (see soil water). Ideally, there should
be a mixture of pore sizes allowing good water holding,
free drainage, gaseous exchange and thorough root
exploration, as shown in Figure 17.12.
An important indicator of a satisfactory growing medium
is its air-filled porosity or air capacity, i.e. the percentage
volume filled with air when it has completed draining,
having been saturated with water.
is the mass of soil per unit volume and
it can be measured by taking a core of soil of known
volume and weighing it after thorough drying. In normal
mineral soils results are usually between 1.0 and 1.6 g/ml.
The difference is largely attributable to variation in total
pore space. Finer textured soils tend to have more pore space and therefore lower bulk density than sands, but for all soils higher
values indicate greater packing or compaction
This information is not only useful to diagnose compaction problems,
but can also be used to calculate the weight of soil in a given volume.
Assuming a cultivated soil to have a bulk density of 1.0 g/ml, the weight
of dry soil in one hectare to a plough depth of 15 cm is 1500 tonnes;
when compacted the same volume weighs 2400 tonnes. Similarly,
of a typical topsoil with a bulk density of 1.0 will weigh 1 tonne
(1000 kg) when dry and up to half as much again when moist.
The pore space does not depend solely upon the size of the soil particles
as shown in Figure 17.8, because they are normally grouped together.
, or peds, are groups of particles held together by the
adhesive properties of clay and humus. The ideal arrangement of small
and large pores for establishing plants is illustrated in Figure 17.12
alongside a dusty tilth with too few large pores and a cloddy tilth that
has too many large pores.
A soil with a simple structure
is one in which there is no observable
aggregation. If this is because none of the soil particles are joined
together, as in sands or loamy sands with low organic matter levels,
it is described as single grain
structure. Where all the particles are
joined with no natural lines of weakness the structure is said to be massive
A weakly developed
structure is one in which aggregation is indistinct
and the soil, when disturbed, breaks into very few whole aggregates, but
a lot of unaggregated material. This tends to occur in loamy sands and
sandy loams. Soils with a high clay content form strongly-developed
structures in which there are obvious lines of weakness and, when
disturbed, aggregates fall away undamaged. The prismatic, angular
blocky, round blocky, crumbs and platy structures which are found in
soils are illustrated in Figure 17.13.
Development of soil structures
Soil structure develops as the result of the action on the soil components
of natural structure-forming agents , freezing and thawing, wetting
and drying, root growth, soil organisms, as well as the influence of
- frost leads to the shattering of clods by producing a frost mould. It
is largely confined to the surface layers and is advantageous in the
management of clays;
- drying soil can affect the whole rooting depth. Cracks usually open
up in heavier soils as the clay shrinks;
- earthworms and other soil organisms play an important part in
loosening soil, maintaining the network of drainage channels and
stabilizing the soil structure;
- roots have a major effect on the drying of deeper soil layers, but they
also play an important part in soil structure by growing into the cracks
and keeping them open. They help establish the natural fracture lines.
In strong structures, a close-fitting arrangement of prismatic (see
Figure 17.13) or angular blocky aggregates is readily seen.
|Figure 17.13 Soil structures. The soil profile on the left is composed of soil particles aggregated into structures that produce good growing
conditions. Examples of structures that create a poor rooting environment are shown in the profile on the right.
In soils with low clay content the roots are vital in maintaining an open
structure. The exploring roots probe the soil, opening up channels where
the soil is loose enough and producing sideways pressure as they grow.
On death, the root leaves behind channels stabilized by its decomposed
tissue for other roots to follow. Fine granular
structures are developed
under pastures by the action of the fibrous rooting over many years. The
soil structure is greatly improved by the rootball. Its physical influence
is most easily appreciated by shaking out the soil crumbs from around
the root of a tuft of well-established grass and comparing them with the
structure of soil taken from a nearby bare patch.
Freshly exposed land is often referred to as raw;
when weathered it
. Once mellow , a seedbed is more easily prepared.
The weathering process and influence of cultivation tend to produce
rounded blocky structures
and rounded granules
in the cultivated
of soil by hand or mechanized implements is undertaken to
produce a suitable rooting environment for plants, to destroy pests and
weeds and to mix in plant residues, manures and fertilizers.
However, the use of cultivators can lead to the formation of platy layers
, which are characterized by the lack of vertical cracks and form an obstruction to root and water movement. The surface of soils is
also compacted to create surface capping by associated traffic, whether
by feet or tyres, if undertaken in the wrong conditions (see soil
develop in some soils as a result of fine material
cementing a layer of soil together. In some sandy soils rich in iron oxide,
these oxides cement together a layer of sand where there has been a
fluctuating water table, to produce an iron pan
Soil aggregates with little or no stability collapse spontaneously as
they soak up water, i.e. they slake. Those high in fine sand or silt are
particularly vulnerable to slaking. Aggregates with better stability
maintain their shape when wetted for a short time, but gradually pieces fall
off if left immersed in water. Aggregates with good structural stability
are able to resist damage when wet unless vigorously disturbed. Soils with
a high level of clay content have better stability than those with low levels.
Stability is also increased by the presence of calcium carbonate (chalk),
iron oxides, and, most importantly, humus
The soil surface or seedbed should be carefully managed to produce the
required crumb structure.
|Figure 17.14 Soil cap.
Sandy soils are easily broken down to the right size with cultivation
equipment. Heavier soils are less easy to cultivate and benefit from
weathering to produce a frost ‘mould’.
The fineness of a seedbed should be related to the size of
seeds, but ideally consists of granules or crumbs between
0.5 and 5 mm in diameter (see Figure 17.12). Cloddy
surfaces lead to poor germination, as well as poor results
from soil herbicide treatments. The rain on the soil surface
breaks down tilth. As soil crumbs break up, the particles
fill in the gaps; this reduces infiltration rates. As the surface
dries, a cap
is formed (Figure 17.14). Thus fine
‘dusty’ tilths should be avoided and the soil crumbs should
be stable so that they can withstand the effect of rain until
plants are established. This is particularly important on fine
sandy and silty soils, which tend to have poor structural
stability. In general, fine tilths should be avoided outdoors
until well into spring when conditions are becoming more
favourable and growth through any developing cap is rapid.