Plant residues are a major source of organic matter used by microorganisms for growth and activity. Soil nitrogen and other nutrients released from crop residues and soil organic matter result primarily from activity of these micro-organisms. So any change in the amount (microbial biomass) or activity of these micro-organisms can change the rate of biological soil nitrogen supply, according to Grains Research and Development Corporationsupported, University of Western Australia (UWA) PhD student, Frances Hoyle. In WA there are two primary constraints to biological activity and, hence, organic matter breakdown. The more important of these, according to Hoyle, is water availability as there is little or no biological activity in dry soils. The secondconstraint is lack of a suitable food source.
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