Agriculture Reference
In-Depth Information
the pores and the aggregates. In terms of agricultural production. Dexter (1988) considers
it advantageous to have most pores in the size range 0.2 to to maximise water
storage but for 10 % of the soil volume to be in pores larger than to promote
aeration. In terms of aggregates, it is desirable that the whole range of size classes are
developed and are stable to mechanical stresses. Further, it is particularly important that
the aggregates exposed to raindrop impact and surface water flow are stable in water
since their too-ready slaking and dispersion may lead to excessive erosion of surface
soils, the blocking of soil pores and the formation of surface crusts.
1.3.2
CLASSIFICATION OF SOIL STRUCTURE
Soil structure has been classified in a variety of ways. Perhaps the best-known of
the morphological classifications is that of the USDA (Soil Survey Staff, 1993). Under
this system, the degree of structure in the soil, the
grade,
ranges from zero to strong on a
four-point scale, depending on the proportion of the total soil that is occupied by aggregates.
The peds are further classified on the basis of size, shape and degree of distinctness.
Nonetheless, a classification that is based on the effects of the soil forming factors
(a genetic classification), may be preferable and Bonneau and Levy (1982) have
suggested the classification presented in Table I.11.
Such systems are useful for characterising the macroscopic physical and chemical
structures found in soils and may imply other features of the soil environment. Few of
these systems satisfactorily incorporate the complexities of biological and other
small-scale structures found in soils.
1.3.3
AGGREGATE FORMATION AND STABILISATION
The soil components of colloidal size (clays and humic materials) must remain flocculated
for the soil to maintain its stability. Floccules initially form relatively-open structures
and undergo a process known as condensation to form more stable, overlapping arrange-
ments of the component panicles, usually clays. The formation of small, stable units of
multiple clay particles (assemblages, quasi-crystals, domains or, more generally, tactoids)
is followed by coalescence with organic matter and other cements to form increasingly-
larger aggregates.
Aggregates may be formed by the fragmentation of larger particles such as clods or
through agglomerative processes from smaller units. Aggregates may also be formed
through the action of the soil fauna, either by passage through the gut or through their
constructional activities.
1.3.3.1
Genesis and size of aggregates
The factors responsible for the initial formation of small aggregates include expansive
and contractive forces associated with wetting and drying or freezing and thawing.
Aggregate formation and stabilisation is therefore related to soil clay content and
the nature of the clay minerals present. Oades (1993) considers that biotic influences are
most important in coarse-textured soils but diminish with increasing clay content.
