The relative effectiveness of superphosphate and rock phosphate for soils where vertical and lateral leaching of phosphate occurs

Laboratory studies have shown that up to 70% reactive rock phosphate dissolves in three soil types found in the high rainfall (> 800 mm annual average) area of south-western Australia. Three field experiments were undertaken on these soils to compare reactive apatite rock phosphate from North Carolina (NCRP) with single superphosphate (SSP) as fertilizers for subterranean clover (Trifolium subterraneum) pasture. Vertical leaching of phosphorus (P) occurs in one soil, a deep, very sandy, acid peaty sand. Lateral leaching of P occurs in the second soil, a shallow (3 cm) sand over a slowly permeable sandy clay loam. No leaching of P occurs in the third soil, a uniform, permeable red sandy loam with a moderate capacity to sorb P. All the soils remained moist to very wet for the 6 to 8 month growing season. Fertilizers were applied once only to different plots over a four-year period (1992 to 1995). Each year fertilizer effectiveness was determined relative to the effectiveness of freshly-applied (current) SSP using yield and P content of dried clover herbage and bicarbonate-soluble P extracted from the soil (soil test P) as indices of effectiveness. For the two P leaching soils, NCRP was less, equally, or more effective than current SSP in different years. This variation is attributed to the different extents of leaching of P from current SSP in different years which experienced different amounts of rainfall and associated leaching. For the non-P leaching soil, the effectiveness of current NCRP and the residual effectiveness of NCRP were from 5 to 80% the values for current SSP. When measured using soil test P, current NCRP and residual NCRP varied from 40%, as effective, to equally or 30% more effective as current SSP at one site, but were about 20% as effective at the other two sites. For the two P leaching soils in some years, the residual value of RP was higher than that of current SSP, presumably due to the rapid leaching of water-soluble P from the SSP. As measured using yield, P content and soil test P, the relative effectiveness of SSP consistently decreased with increasing time from application; the decreases were much less obvious for NCRP.