Influence of fallow, wheat and subterranean clover on pH within an initially mixed surface soil in the field

To ascertain the cause of the decrease in pH with depth through the surface 15 cm of moderately acidic soils, pH was monitored in layers of an initially mixed surface soil (to a nominal depth of 10 cm) during two consecutive seasons under fallow, wheat, and subterranean-clover plots. Variation of pH-influencing processes within soil layers to 15 cm depth was measured during the first season. Initially, soil pH was relatively uniform within the surface 7.5 cm, although there was an average 0.53 unit decrease of pH from 0-2.5 cm to 10-15 cm depth. Under all plots, residual lime reaction, net organic anion association and oxidation, net manganese oxidation and reduction, and particularly net N mineralisation and subsequent nitrification, tended to decrease with depth through the surface 15 cm of soil. In wheat and subterranean-clover plots, the alkalinity added with the return of 3.9-4.7 t ha(-1) of plant residue dry matter was predominantly released within the surface 2.5 cm of soil. The dominant pH-influencing processes were net N mineralisation and subsequent nitrification, and the return of alkaline plant residues. In the fallow plots, the surface 10 cm of soil tended to acidify due to nitrification. However in wheat and clover plots, alkalinity added to the surface 2.5 cm of soil from plant residues exceeded acidification resulting from nitrification at this depth. The magnitude of the pH gradient through 0-15 cm depth was therefore maintained under wheat, increased under clover, and decreased under fallow.