Agriculture Reference
In-Depth Information
I.3
SOIL RESOURCES
A wide range of resources are available to the organisms that grow in‚ or live within or
on the surface of the soil. These include mineral nutrients and a diverse range of organic
materials‚ both of differing degrees of availability.
Organic resources
consist predominantly of dead materials that accumulate on
the soil surface (leaf‚ wood and other litter‚ dead micro-organisms or animals) or deeper
within it‚ localised at specific places in the soil profile (dead roots and other organisms).
In addition to these dead materials‚ soils offer a considerable variety of other living
resources (roots and soil organisms) and their secretory products (root exudates‚ micro-
bial mucilages and invertebrate mucus) to the organisms that are mechanically and
physiologically equipped to utilise them.
Most mineral nutrient elements
are derived ultimately from soil parent materials
through weathering processes‚ although constant inputs of particular elements may also
be derived from atmospheric sources. Some of these nutrients accumulate in the living
biomass and are recycled through decomposition processes. A further part is retained
in the soil in mineral form‚ adsorbed more or less tightly to clay particles or organic
materials by electrostatic forces‚ or mechanically trapped within them.
3.1
Inorganic Nutrients
The major elements that combine to form organisms are‚ in order of importance:
hydrogen‚ oxygen‚ carbon‚ nitrogen‚ phosphorus and sulphur. These elements possess
distinctive chemical properties which allow them to contribute to the continuing creation
and maintenance of life (Taube‚ 1992). To be capable of this‚ they must be abundant‚
stable and capable of reacting together to form the complex molecules that comprise
structural elements‚ mediate the internal and external exchanges of materials and energy‚
transmit their genetic information and form the semi-permeable membranes responsible
for the necessary isolation of organisms from their environments.
In addition to the six elements considered above‚ only a small proportion of the total
of 91 naturally-occurring elements are known to be required for the normal development
of organisms. The nutrient elements defined as essential for an organism are those with-
out which it is not able to complete its life cycle (
i.e
.‚ produce viable propagules)‚ or
that form part of a molecule which is itself known to be essential (Epstein‚ 1972).
The designation of an element as an essential nutrient implies that it has an indispensable
structural or biochemical role. However‚ not all the roles of the essential elements are
known in detail and most nutrient elements have multiple roles. Numerous complex
interactions occur between the nutrient and non-nutrient elements. For some essential
elements‚ partial substitution may occur by another not known to be so and certain
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