Agriculture Reference
In-Depth Information
• Burrowing invertebrates, which reorganize soil particles. The most
important of these is the earthworm, which grinds and mixes organic
matter with clay and silt particles in its gut, adding extra calcium in the
process to form a
cast
(see fig. 2.14). Casts develop greater strength as they
dry.
• The physical binding of soil by fine roots and fungal mycelia, including
mycorrhizae. Particularly important in the stabilization of aggregates, this
binding is one of the beneficial effects of having a cover crop, including
perennial grasses, in the inter-rows of a vineyard.
• The activity of soil microorganisms, which can induce a change in metal
valency leading to the solution and subsequent reprecipitation of Mn and
Fe oxides (section 2.2.4.3). Fe oxides are usually positively charged and,
being attracted to negatively charged clay minerals, are particularly effective
cementing agents within soil aggregates.
Describing Soil Structure
Most soils have a structure that is readily observed in the field: this is the
macrostruc-
ture.
The USDA Soil Survey Manual (Soil Survey Division Staff 1993) presents
a widely used system for describing and classifying soil macrostructure. This sys-
tem, first published in 1951, is the basis for similar systems developed in other
countries, such as in Australia (McDonald et al. 1990). However, much of the
fine detail of particle interaction and aggregation is in the
microstructure
, which
can be observed only using some form of magnification, ranging from a hand lens
(
10) in the field to a light microscope (
50) or an electron microscope
(
10,000) in the laboratory. Descriptive terms for soil macrostructure are dis-
cussed in section 3.2.1.
Soil structure should be described from a freshly exposed soil profile or a large
undisturbed core. Samples from augers (see chapter 8) are unsuitable because of
the disturbance, as are those from old road cuttings and gullies that have been ex-
posed to the weather for some time.
3.1.2
Soil Aggregation
3.2
True aggregates are recognizable because they are separated by pores and natural
planes of weakness. They persist through cycles of wetting and drying, as distinct
from the less permanent clods formed by digging or cultivation. A
concretion
is
formed when localized accumulations of an insoluble compound, such as Fe ox-
ide, cement soil particles together or enclose them.
Nodules
are similarly formed,
but lack the symmetry and concentric internal structure of concretions.
Aggregate Type, Class, and Grade
Aggregates are described in more detail according to their
type
,
class
, and
grade
.
The main
types
of aggregate are described in table 3.1.
Aggregate
class
is based on size. Size classes range from
2 mm to
500 mm,
as measured by the average smallest dimension. The classes are described as fine,
medium, coarse, or very coarse. Refer to the specialist soil survey handbooks for
details.
3.2.1
