Agriculture Reference
In-Depth Information
The situation discussed above may be generalised to other elements. Stevenson (1986)
states that, while some local variation occurs, the ratio C 140:N 10: S: 1.3:P 1.3 remains
relatively constant across a wide range of environments. Where C:organic P and C:S ratios
are less than 200 and 400, respectively, net mineralisation will occur. Net immobilisation
of P and S will occur at C:P and C:S ratios above 300 and 400, respectively.
3.1.4.4
Roles of symbiotic micro-organisms
As considered above, certain micro-organisms (for example,
Rhizobium
and related
species,
Frankia
spp., Actinobacteria) enter into symbioses and looser associations with
many legumes and some non-legumes to fix atmospheric N a variable portion of which
may become available for use by the host plants. The biological N fixation
process underpins the agricultural economies of many areas and offers the species
involved a competitive advantage in N-deficient environments. The other micro-
organisms important in mediating nutrient acquisition for their hosts are the diverse
groups of fungi that form mycorrhizal relations with many higher plants.
For most plant groups in most environments, it is not likely to be profitable to con-
sider their nutrition independently of the actual or potential activities of these organisms.
Their roles are addressed in greater detail in Chapter IV.3.2.1.2 and IV.3.2.1.3.
3.2
Organic resources
In soils, energy flows either through the consumer or decomposer foodwebs (see
definitions in Chapter III). The decomposer foodwebs are the most complex in that
decomposing organic resources are progressively transformed through sequences
of ingestion-egestion processes which form what Swift
et al.
(1979) have aptly called
cascade structures
. Consequently, organic resources are extremely structurally and
chemically diverse and, of the enormous range of compounds known, almost all are
likely to be found in soils. These resources are here characterised by their quality,
availability in time and space and, where possible, quantified.
Organic resources present in soils are distributed among the following four major
classes of materials:
Dead leaves, woody litter and reproductive structures which fall to and may accu-
mulate on the surface of the mineral horizons;
Living and dead roots;
Organic matter accumulated in the soil profile; and
Soluble organic compounds, including bark, foliar and litter leachates, root exudates,
polysaccharides and other materials produced by both micro-organisms and inverte-
brates.
1.
2.
3.
4.
3.2.1
RESOURCE CLASSIFICATION
In ecological systems, the quality and availability of resources in time and space largely
determine patterns of consumer usage and the organisation of the consumer communities.
