SOIL-PH DECLINE IN RELATION TO ROTATION, TILLAGE, STUBBLE RETENTION AND NITROGEN-FERTILIZER IN SE AUSTRALIA

The decline in topsoil (0-0.1 m) pH (CaCl2) over 11 years (1979-90) was measured in a rotation, tillage, stubble and nitrogen fertilizer experiment on a Chromic Luvisol at Wagga Wagga in S.E. Australia. The rotations consisted of annual wheat cropping (WW) with and without nitrogen fertilizer (100 kg N/ha/year), alternating lupin-wheat (LW) and subterranean clover-wheat (CW). The initial mean pH at the site was 4.9 and the experiment was preceded by subterranean clover-based pasture for most of the previous 19 years. An initial rapid decline in soil pH under all treatments over the first 8-9 years was followed by a 2-3 year period when no further decline was detected. The annual rate of pH decline over the first 8-9 years varied from 0.06 for WW to 0.09 units for WW with added N fertilizer. Apparent steady-state for WW after 11 years was approximately 0.5 pH units higher than for WW with added N fertilizer. There was no difference between CW and LW in the rate of decline or in the apparent steady-state reached. Six years' stubble burning in a LW rotation promoted a slightly higher pH than where stubble was retained. However, there was no significant effect of tillage in either LW or CW rotations. By 1990 the addition of N fertilizer to WW had increased the concentration of exchangeable aluminium by 100% and of manganese by 24%. The inclusion of a legume in the rotation increased the concentration of aluminium but did not affect manganese. However, burning stubble in the LW rotation slightly decreased manganese concentrations.