Effect of plant residue return on the development of surface soil pH gradients

In the field, surface soil pH gradients were observed under senescing plants over late spring and summer. A soil incubation experiment was conducted (119 days, 20 degreesC) to provide direct evidence of the influence of plant residue incorporation on soil pH. This was investigated in terms of plant residue type (wheat and subterranean clover) and dry matter addition rate (0, 6.25, 12.5 and 25.0 g kg(-1)), as well as the soil layer of incorporation (0-2.5 and 7.5-10 cm) and moisture regime (continuously moist and moist-dry cycles). During incubation, moist unamended soils slowly acidified. In contrast, the addition of plant residue resulted in a rapid (day 0-7) increase of soil pH due to the association, and particularly oxidation, of added organic anions. This was followed by a gradual (day 7-119) pH decline attributed to the mineralization and subsequent nitrification of added organic N. The addition of 12.5-25.0 g kg(-1) of cereal crop residues, and 6.25-25.0 g kg(-1) of legume-based pasture residues, resulted in a net alkalization of the surface 2.5 cm of soil. It was therefore concluded that surface soil pH gradients observed in the field were largely attributable to an increase of pH at the surface 2.5 cm in response to plant residue return. The magnitude of such gradients will be particularly large with the return of large quantities of plant residues of high ash alkalinity in soils of relatively low initial pH and biological activity, and when the surface of the soil is exposed to moist-dry cycles.