EFFECTS OF SURFACE-APPLIED PHOSPHORUS AND SUPERPHOSPHATE ON THE SOLUTION CHEMISTRY AND PHYTOTOXICITY OF SUBSURFACE ALUMINUM - SAND SOLUTION AND SOIL EXPERIMENTS

Experiments using split-root sand/solution culture and reconstituted soil profiles were undertaken to examine the influence of P supply to surface roots of subterranean clover (Trifolium subterraneum var. Woogenellup) on root penetration into Al-toxic subsurface strata. In sand/solution culture increasing P supply to surface (0-40 mm) roots enchanced root penetration into Al-toxic nutrient solutions in the subsurface (40-100 mm), but only at low to moderate concentrations of Al. At high concentrations of Al, surface P supply had no effect on root penetration into the lower compartment. Results from the soil experiments confirmed the observations obtained in sand/solution culture. Topdressed P (as single superphosphate-SSP) enhanced root penetration into the Al-toxic subsurface stratum. Plants suffering P stress showed poor root growth in the acidic subsurface stratum. Gypsum in the SSP confounded P treatment effects at high rates of P application. The gypsum quickly dissolved and was able to move through the limited surface soil (0-40 mm, pH 5.0), to the acidic subsurface stratum, as evidenced by Ca concentrations in soil solution. This raised the ionic strength and reduced the pH of the soil solution, and raised solution Al concentrations. Under conditions of limiting leaching, and where the subsurface horizon was very acidic (pH 3.9), the gypsum in SSP caused yield reductions at high rates of application and produced a quadratic yield response curve to topdressed SSP.