Phosphorous Fertilizer Recommendations Considering Single-point P Sorption Capacity and Soil P Test Extraction Efficiency

In order to make accurate phosphorous (P) fertilizer recommendations, P buffering capacity (PBC) has to be determined, which is commonly done using batch equilibrium multiple-point or single-point sorption isotherms. However, these sorption isotherm approaches do not simulate field conditions. A further complication in soil P fertilizer recommendations is that a variety of soil P extraction tests are used that differ significantly in P extraction efficiency. Therefore, a need exists to compare multiple-point and single-point sorption indices and a more field-relevant soil P incubation protocol to assist in making accurate P fertilizer recommendations. Ten soils varying in P sorbing capacity were used in this study. The relationship between a multipoint Freundlich isotherm and a single-point estimate of P sorption was determined. The single-point estimate at an application rate of 500 mg P kg(-1), using transformed and general variations of the Freundlich equation, correlated significantly with the multipoint isotherm (R-2 = 0.98, p < 0.05). Soils were treated with realistic agricultural P application rates (0, 50, 100, 150 and 200 mg kg(-1)) and incubated for 7 days, 30 days and 6 months, respectively, at field water capacity. Bray II, Olsen and Mehlich III soil extractable P was determined after each incubation period. A strong linear relationship was observed between applied P and Bray II, Mehlich III and Olsen extractable P. Sorbed P determined from the incubation study correlated significantly with the multipoint isotherm PBC (R-2 = 0.68, p < 0.05). Therefore, it is possible to make fertilizer recommendations considering PBC and specific P extraction tests, using a single P application (100-150 mg kg(-1)) and incubating at field capacity for 7 days.

Automated summary

Accurate phosphorous (P) fertilizer recommendations require determining P buffering capacity (PBC), with single-point estimate of P sorption showing significant correlation with multi-point isotherm. Specific P extraction tests and soil P incubation protocol can enhance the accuracy of fertilizer recommendations.