Agriculture Reference
In-Depth Information
than 1 the multi-lamellate structures of plant cell walls and cell membranes may be
still recognisable (Foster, 1985).
Living organisms secrete copious metabolites (see Chapter III). The most important
of these are:
(i) Root exudates and mucilages, which form a thick film over the actively-growing
part of the roots;
(ii) Polysaccharide capsules and sheaths produced by bacteria and fungi;
(iii) Mucus and other secreta deposited by such invertebrates as earthworms and ter-
mites on the walls of their galleries, or admixed with ingested soil in order to facilitate
digestive processes.
Finally, a considerable proportion of soil organic matter comprises small particles
of uncertain origin falling in the micron to sub-micron size range. These particles are
amorphous or granular in structure and include translocated and precipitated organic
materials leached from the surface, humic substances synthesised
in situ
by microbial
activity, or formed through spontaneous chemical reactions. These panicles include
much of what are chemically characterised as humic substances which may assume
quite different shapes depending on the ionic environment,
i.e
., pH and electrolyte
concentration (Ghosh and Schnitzer, 1979).
