Testing soils for phosphorus

The major purpose of testing soils for phosphorus (P) is to determine the quantity of supplemental P required to prevent economic loss of crop value because of P deficiency. A second purpose for testing soils for P is to monitor the quantity of available P present over time. This chapter aims to review the facets of testing soils for P including interpretation of the test results. In the process, an attempt is made to indicate what existing soil test methodology can and cannot do. A greenhouse experiment on millet demonstrated that P uptake increased as soil moisture increased at a soil temperature of 27 °C, but not at 16 °C. The quantity of labile P in a soil is just one of several factors that determine the plant availability of soil P. The chapter discusses factors more specific to soil test P calibration and interpretation.