Agriculture Reference
In-Depth Information
IV.1 DECOMPOSITION
1.1
General processes
Decomposition is the general process whereby dead organic materials are transformed
into simpler states with the concurrent release of energy and their contained biological
nutrient - and other - elements in inorganic forms. Such forms are directly assimilable
by micro-organisms and plants and the remaining soil organic matter may be stabilised
through physical and chemical processes‚ or further decomposed. Two important
features of decomposition processes are their overall rates and the transfers of organic
materials to different macro- or microsites within the ecosystem.
1.1.1
THE ROLE OF ORGANIC MATTER IN SOIL FUNCTION
Decomposition may be defined as the sequence of organic matter transformations
occurring after the death of organisms. These transformations involve two simultaneous
but complementary processes: mineralisation and humification. Mineralisation is the
catabolic process through which the elements contained in organic form within biologi-
cal tissues are converted to inorganic forms such as nitrate‚ phosphate and sulphate ions.
In the contrasting anabolic process‚ humification‚ organic molecules are condensed into
degradation-resistant organic polymers which may persist little-altered for decades or
centuries. Both processes occur simultaneously and are important aspects of soil fertility.
Mineralisation determines the fluxes of plant- and micro-organism-available nutrients
and their distributions in time and space; humification regulates the accumulation of
stabilised organic matter within the soil.
Soil organic matter contributes to fertility in three ways:
(i) Cementation of soil within aggregates: part of the soil organic matter occurs in
colloidal forms and cements soil particles together to create solid structural units known
as aggregates (Section I.1.3.1). These are surrounded by inter-connected pore spaces that
permit the movement of water‚ solutes and gases through the soil matrix and contribute
substantially to erosion resistance.
(ii) Retention of cations: as a material possessing a pH-dependent‚ net negative electro-
static charge at soil pH values‚ soil organic matter aids the retention of positively-charged
cations‚ especially in the acid pH range where soil minerals may retain lesser amounts.
Properties (i) and (ii) are common to soil organic matter and clay minerals (see Chapter
I.1.1.1.2)‚ although some differences exist. For example‚ in soils more acid than pH 4.2‚
soil minerals retain few cations whereas humic molecules still retain an appreciable
cation exchange capacity down to pH 2.5 (Bonneau and Souchier‚ 1982).
(iii) Conservation of nutrients in organic forms: organic matter conserves both nutrients
and energy in forms that are neither readily assimilable by micro-organisms nor
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