Expanding the use of diffusive gradients in thin-films (DGT) for assessing phosphorus requirements of different crop types

The DGT soil test for assessing plant-available phosphorus (P) in soil has now been calibrated over three growing seasons using field-based P response trials across a range of crops (wheat, barley, canola and peas). Regression analysis of relative early dry matter production and grain yield responses demonstrated that the DGT method predicted wheat responsiveness to applied P more accurately than Colwell P and resin P. Using regression the DGT method explained 75 % of the variation in response for early dry matter and 74 % for grain. No other significant regression relationships were obtained for the other soil tests, apart from resin P which explained 40 % of the variation in response for grain. The performance of the Colwell-P test was assessed after modification using the phosphorus buffering index (PBI) and correctly predicted the crop response to P for 17 of the 30 experiments, compared to 20/30 for resin P and 26/31 for DGT. These observations suggest that the DGT technique can assess plant available P in soils with significantly greater accuracy than traditional soil P testing methods. Phosphorus response trials with other crop types including barley, canola, field peas and chickpeas have shown that different crop types have varying abilities to utilise residual P in the soil. Importantly DGT has consistently been the most accurate method for predicting P response for each of these crop types.Sean Mason, Ann McNeill and Mike J. McLaughli