Efficacy of different phosphate application methods in relation to phosphate sorptivity of soils

Abstract

The efficacy of top-dressing, shallow and deep subsurface placement, and uniform mixing of phosphate through the top 17 cm of soil was compared on six soils varying in sorptivity from 100 to 10000 ml g-1 in a glasshouse experiment. Shallow banding at 5 cm depth was the most consistently superior method of application, whether effectiveness was measured as yield response in the first crop (wheat), residual effects in the second crop (clover), or cumulative recovery of applied phosphate during 27 months of wheat and clover. Top-dressing was equally effective in the most weakly sorptive soil but inferior in the most strongly sorptive soil. Deep placement at 15 cm and top-dressing were generally the least effective methods. The almost equivalent effectiveness of phosphate-soil mixing to shallow banding, regardless of sorptivity, suggests that the important factor in maximizing fertilizer effectiveness is its positional availability in the root zone rather than restriction of chemical immobilization by fertilizer concentration in bands. The apparent recovery of applied phosphate from most soils declined most sharply in the second year after application and thereafter remained constant or even increased up to 27 months after application. Total recovery of applied phosphate was similar over the normal range of sorptivities (100-2500 ml g-l) but it was lower on the very highly sorptive soil.