Microbes play a major role in the formation of soil structure (Lynch and Bragg, 1985; Tisdall, 1991). Fungi and actinomycetes produce hyphal threads that bind soil particles together. Extracellular polysaccharides produced by bacteria and fungi bind soil particles together, assisting in building soil structure. Humic materials from microbial action form organic matter/clay complexes. This action reduces erosion, allows for good water infiltration, and maintains adequate aeration of the soil. Soil aggregation can be increased by the addition of residues resulting in additional microbial activity (Gilmour et al., 1948). The carbon and nitrogen pools delimit microbial biomass and decomposition rates and polysaccharide production (Knapp et al., 1983). The ability of fungi and bacteria to influence aggregation varies with species and is substrate dependent (Aspiras et al., 1971). Limited nitrogen in soil solution reduces biomass while increasing polysaccharide production, which can lead to increased aggregation (Elliott and Lynch, 1984). Soil stability may be managed by addition of different amendments to stimulate microbial activity (Jordahl and Karlen, 1993).
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