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.