Agriculture Reference
In-Depth Information
Functional pools
Soil organic matter may be classified into a number of pools that differ in their overall
chemical compositions and in their degree of physical protection (see I.1.3.3, Figure
I.13). Such protection of organic matter occurs when it is adsorbed onto the exterior
surfaces of clay minerals, intercalated in their inter-layer spaces, within the inter-
particle spaces of small aggregations of clay particles or within compacted soils.
Separate pools of organic matter with different residence times in the soil have been
defined on this basis. Jenkinson
et al.
(1987) and Parton
et al.
(1983) distinguish the
following five pools which include two classes (1-2) of plant materials and three (3-5) of
soil organic matter:
1. Readily decomposable “metabolic” plant material with a turnover time of 0.1 to 1 year;
2. Resistant structural plant material (2-5 year turnover time);
3. Active soil organic matter (
i.e
., microbial biomass) (2-4 year turnover time);
4. Slow (or physically stabilised) soil organic matter (20-50 year turnover time);
5. Passive (or chemically protected) soil organic matter (800-1200 year turnover time).
An approximate correspondence has been found between soil organic matter fractions
separated by various methods and the functional pools (Figure I.1.13)
1.2
The soil atmosphere
The mixture of gases and water vapour forming the atmosphere within the water-free
pore space of the soil differs considerably from that above the surface. As discussed
