Agriculture Reference
In-Depth Information
diverse products as plant root lysates and exudates, the faeces, excreta and secreta of ani-
mals, the cutaneous mucus secreted by earthworms and gels produced by micro-organisms.
In terms of chemical structure, an enormous variety of biologically-derived materials
occur in soils. These range in complexity from simple aliphatic materials to polymers
of high molecular weight and great complexity. Their temporal existence in soils ranges
from the most ephemeral, with turnover times of a few hours or days, to highly degra-
dation-resistant polymers whose age may be measured in tens of thousands of years.
Gases derived from the metabolism of micro-organisms and roots comprise an
important part of the soil atmosphere and the state of aeration of the soil determines
the pathways of many chemical and biological processes. These gases may be present as
components of the soil atmosphere, partly dissolved in the soil solution and in some form
of dynamic equilibrium with the soil atmosphere, or adsorbed onto other soil components.
Such gases are of pedogenetic importance and may interact with roots to influence plant
growth characteristics.
Through the form in which it is present, the tenacity with which it is held, and the
pathways of its flow into, through and out of soils, water conditions the whole evolution
and functioning of soils. Beyond its direct biological role in the hydration of plants and
the other living inhabitants of the soil, it is the major agent of formation and change in
soils. Apart from physical changes resulting from freezing and thawing, the effects of
water are due largely to reaction (hydrolysis) and the dissolution and transport of a wide
range of substances in solution and in suspension. This movement may take place
vertically in either direction, laterally within the profile, or from one part of the landscape
to others lower-lying. In regions where it has been active, glaciation has played an
enormously important role over geological times in modifying landscapes, and in
determining the characteristics and distributions of the soils present in these areas.
Because of its importance in pedogenetic processes and the biological functioning
of soil, the physical processes associated with the status, entry and egress of water into
the soil are treated in a separate section. However, the properties of the soil solution
are arbitrarily considered in the section on the soil components.
The information presented here on soil components is of an introductory nature only;
more detailed information may be obtained from the works of Birkeland (1984), Bonneau
and Souchier (1982), Brady and Weil (1996), Duchaufour (1997,1998) and Jenny (1980).
1.1.1.1
The mineral components - soil texture
A useful way of categorising soil inorganic material is by determining its texture which
is defined as the proportions by mass of the mineral particles that fall into different size
classes, the particle size distribution. Texture is an important characteristic of soil
since it influences many aspects of its field behaviour including a number of properties
important to the growth of higher plants. These include the amount of water held by
the soil and its capacity to retain plant nutrient elements.
Texture directly determines water-holding capacity; soils with a high proportion
of clay size particles (that is, of fine texture) may normally be expected to have lower
infiltration rates but a greater capacity to store and supply water than those of coarse
texture (that is, possess a high proportion of the larger particles). However, the moisture
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