FUNCTIONING OF THE SOIL SYSTEM - Soil Ecology - page 425

Utilisation efficiency is the proportion of nutrients released that is actually taken up
by the crop and is lower for organic than inorganic fertilisers. In three cropping systems
in the humid tropics, Sisworo et al. (1990) calculated efficiencies of respectively 9 to
40 % for inorganic, and 2 to 27 % for organic fertilisers.
Litter placement. Nutrient release may differ considerably between litter deposited
as a surface mulch, or buried within the soil. The superposition of a stratum of high
quality material over one of lower quality material may prevent losses by temporarily
immobilising nutrients leached from the high-quality material within the underlying
lower-quality material.
Manipulation of soil macrofauna and root systems. Experiments have shown that a
significantly higher proportion of nutrients mineralised from a decomposing surface
mulch may be found in crops in the presence of earthworms (Spain et al., 1992).
Mechanisms for this process may involve accelerated burial of litter under surface casts,
concentration of earthworm activity in the rhizosphere and modified pathways for
water flow within soil.
The spatial distribution and seasonal occurrence of root growth are key components
of synchrony. In some cases, nutrients that have been leached into the soil during
the early stages of plant growth may be taken up when the rooting system reaches
the horizons within which these nutrients are located.