Development and evaluation of an improved soil test for phosphorus, 3: field comparison of Olsen, Colwell and Resin soil P tests for New Zealand pasture soils

Soil tests suitable for estimating the phosphorus (P) status of soils ferti lised with soluble or sparingly soluble P fertilisers (reactive phosphate r ock) were evaluated using the New Zealand Ministry of Agriculture Technolog y (NZMAFTech) 'National Series forms of phosphate trials' on permanent past ures located throughout NZ. This included a common core of treatments compa ring Sechura phosphate rock (SPR) with triple superphosphate (TSP). At each site, a re-application of twice maintenance TSP was superimposed on one-ha lf plots that previously had received six annual applications of increasing amounts of P (0, 0.5, 0.75, 1.0 and 2.0 times the maintenance rate) in the form of TSP or SPR. Before the re-application of TSP, soil samples (0-30 a nd 0-75 mm depths) were collected from each plot. All the trials were run f or 1 year during which seven to ten harvests were taken. Pasture response w as expressed as percent increase in yield obtained with re-application over the previous treatment. The 0.5 NaHCO3 based (Olsen P) extractant with different combinations i.e. soil volume (Olsen (v)), soil weight (Olsen (w)), shaking time variations ( Olsen (16 h)) and soil:solution ratio (Colwell), and Resin P soil tests wer e conducted on soils taken from the plots prior to re-application of TSP. T he Olsen (v), Olsen (16 h) and Colwell P values increased with increasing r ates of P applied in all soils with values for sparingly soluble P material s being less than where soluble P fertiliser had been previously applied. T he Resin P values showed similar increases with P applied regardless of the solubility of previously applied fertiliser. When the yield increases caus ed by TSP application to all treatments (irrespective of fertiliser source) were regressed against soil test values, Resin P explained 76% of the vari ation in yield response, compared to 50% by Olsen (v), 42% by Olsen (w), 39 % by Olsen (16 h) and 40% by Colwell P. Partitioning the data according to fertiliser source slightly improved the coefficient of determination for Re sin P for both the soluble (R-2=0.81) and sparingly soluble (R-2= 0.80) P f ertilisers. With 0.5 M NaHCO3 (Olsen) extractants, R-2 values consistently indicated a poorer prediction for the SPR treatments. A Resin P model was a ble to account for more variance in yield response to re-applied TSP, than an Olsen P model because the Olsen model underestimated the yield response to re-applied TSP on the PR treatments. The Resin test is more suitable tha n the current Olsen test for assessing the plant available P status of soil s previously fertilised with fertilisers of varying solubility.