Field studies on calibrations relating soil test phosphorus to plant yield

Abstract

Soil test phosphorus (P) calibrations relate soil test P to plant yield and are used to estimate the current P status of soils for fertilizer recommendations. The procedures developed for commercial soil P testing are designed to generate the large volume of data required to determine the calibrations for different soils, environments and enterprises encountered in agriculture. However, there are many uncontrolled variations for important factors affecting the calibrations so that it is not the best way of making sense of the data. This paper summarises field studies using an alternative method of determining soil test P calibrations when variations of some of the important factors were controlled. For each experiment, the calibrations differed for different years, different P fertilizers (superphosphate, rock phosphate) and different plant species grown in the same year. They also differ for the Colwell, Olsen, Bray 1, calcium-acetate-lactate and Truog reagents. Alternative ways of estimating the current P status of soils also have problems, are more difficult to do, and cannot generate the large amount of field data required to estimate the current P status of soils. Soil P testing will continue to be the most widely adopted method of providing the estimates, but the errors and limitations of the procedure need to be known.