Agriculture Reference
In-Depth Information
low availability in highly-weathered soils due to absorption by iron and aluminium.
Thus, measures of the total amounts of many elements in defined parts of a profile may
be of little value in predicting their availability to the biota although they may have some
use as indicators of the overall reserves present.
3.1.4.2
Release from dead plant materials and soil organic matter
The rates of nutrient release from decomposing plant remains depend on environmental
conditions and the nature of the materials under consideration. Where leaves are shed
naturally, a substantial proportion of the more mobile nutrient elements contained in
the living tissues are normally withdrawn prior to abscission. This contrasts with the
situation in such crops as sugarcane which, because they are harvested when the leaves
are still actively photosynthesising, leafy residues may still contain somewhat greater
quantities of elements. Such processes are analogous to those that occur through herbivory.
In pastures and other systems, the grazing of plant materials that possess a relatively
high nutrient status results in a direct transfer of a large proportion of the included
nutrients from the above ground biomass to the soil through their deposition in excreta
and secreta (Swift
et al.,
1979).
Table I.20 presents changes in the standing crop biomasses of selected nutrient
elements in sugarcane harvest residues decomposing as a surface mulch in the humid
tropical environment of northeastern Australia (Spain and Hodgen, 1994). The biomass
of dead plant material initially declines steeply and then less rapidly; such changes are
characteristic of many decomposing plant residues although such materials differ enor-
mously, depending on the chemical and.physical characteristics of the plant materials
concerned and environmental conditions. Percolating rainwater leaches away a portion
of the nutrient elements and other materials solubilised through microbial activity.
