EFFECT OF FERTILIZER TYPE, SAMPLING DEPTH, AND YEARS ON COLWELL SOIL TEST PHOSPHORUS FOR PHOSPHORUS LEACHING SOILS

The relationships between (i) soil test phosphorus (P) (Colwell sodium bicarbonate procedure) and the level of P applied (from 0 to 1000 kg total P ha(-1)) (relationship 1), and (ii) yield and soil-test P (rela tionship 2, the soil P test calibration), were measured in two field e xperiments on very sandy, P-leaching soils in the high rainfall (> 800 mm annual average) areas of south-western Australia. The soils were h umic sandy podzols, or haplohumods, comprising 97% sand (20 to 2000 mu m). The experiments started in April 1984 and were terminated at the end of 1990. Soil-test P, measured on soil samples collected to 5,10 a nd 25 cm depth each January in the years after P application, was rela ted to yields of dried clover (Trifolium subterraneum) herbage measure d later in each year. The four P fertilizers studied were single super phosphate, coastal superphosphate (made by adding, just before granula tion, extra rock phosphate together with elemental sulphur while manuf acturing single superphosphate), apatite rock phosphate, and Calciphos . Relationship (1) was adequately described by a linear equation (R(2) > 0.80, most being > 0.90). The slope coefficient estimates the extra ctability of P from the soil by the Colwell procedure, and is called e xtractability. Relationship (2) was adequately described by the Mitsch erlich equation (R(2) > 0.75, most being > 0.90). For relationship (2) , use of percentage of the maximum (relative) yield eliminated differe nces due to different maximum yields and yield responses (maximum yiel d minus the yield for the nil-P treatment). Soil test P ranged from ab out 4 to 150 mu g Pg(-1) soil. Soil test P and extractability were gen erally higher for samples of the top 5 cm of the soil than the top 25 cm, and were largest for single superphosphate and lowest for apatite rock phosphate. Both extractability (relationship (1)) and the curvatu re coefficient of the Mitscherlich equation (relationship (2)), differ ed for different P fertilizers and different soil sample depths. The c urvature coefficient also differed for different yield assessments (ha rvests) in the same or different years. Different soil P test calibrat ions were required for different P fertilizers, soil sample depths and harvest in the same or different years. It is concluded that soil P t esting provides a crude estimate of the current P status of P-leaching soils in Western Australia.