Agriculture Reference
In-Depth Information
The most important of the inorganic components in terms of soil behaviour is
undoubtedly the highly-diverse group of substances characterised generally as clays.
The term clay may refer to three distinct entities. In the textural sense, clay refers to
the important colloidal particles less than 0.002 mm in diameter. More loosely,
it may also refer to a class of soils with a high proportion of such particles. It may also
be applied to the phyllosilicate clay minerals discussed below. Here, the term clay will
refer to clay-sized particles unless otherwise qualified. The phyllosilicate and other
mineral particles in the clay size range have large surface areas relative to their masses
and, in soils with appreciable clay contents, they control many reactions important
to biological processes. Surface area is also closely correlated with a range of other
properties that regulate the physical and chemical characteristics of the soil mass and
influence plant growth (Farrar and Coleman, 1967).
Organic materials found in soils may be divided into the living organisms considered
later in this topic and non-living materials of biological origin. The latter comprise
a diversity of materials including roots and other plant and animal remains in all stages
of subdivision and decomposition. In addition, dead fungal hyphae, spores, bacteria and
other larger constructs of microbial (
e.g.,
the sporocarps of mycorrhizal fungi) and faunal
(
e.g.,
termitaria) origin are frequently present. During the later stages of decomposition
of plant materials, less resistant materials are lost leaving only cell wall outlines and
fragments which, in the terminal stages of decomposition, may appear amorphous under
microscopic examination. Other directly-derived biological materials
include such
