'\)fj1

FIGURE 2 Production, decomposition, and recycling of plant residues in soiL (From Fuller, 1975a.)

Continued soil productivity depends upon the replenishment and maintenance of some organic matter. The maintenance of a given, fixed level is often not practical, nor is it advised. The practice of incorporating readily available plant residues, manures, straw, leaves, composts, and municipal wastes and sludges into the soil is more important than the establishment of a fixed level. In irrigated soils, including those of arid lands, the organic matter level may increase substantially above that under native conditions (Fig. 3), as a result of crop production and! or organic waste application. Only recently, municipal solid waste leachate in landfills has also been shown to slowly generate stable fulvic and humic compounds (Artiola and Fuller, 1982).