Phosphorus Intensity-Quantity Relationships in Soils Highly Contrasting in Phosphorus Adsorption Properties

Abstract

Methods to evaluate the phosphorus (P) leaching potential of agricultural soils are of increasing importance to develop sound guidelines for P-based nutrient management strategies. Previous work showed that the dynamics of P solubility in 0.01 M CaCl₂ closely resembled the dynamics of P solubility in drainage waters, with distinct change-points (concentrations of Olsen-P at which significant amounts of P begin to leach from soil to water) occurring in both conditions. In this study, we investigated the existence of a change point by evaluating the relationships between CaCl₂-extractable P and Olsen-P test values in soils collected from two sites selected to have a similar wide range of soil extractable P (Olsen-P concentration was in the range of 4–171 mg P kg⁻¹ for both soil groups), but very contrasting P adsorption properties (the P sorption index, a P sorption capacity related soil parameter, ranged from 335 to 669 mg P kg⁻¹ in one group of soils, and from 1124 to 1482 mg P kg⁻¹ in the other one). A close linear correlation between CaCl₂-P and Olsen-P was verified in samples from both the sites studied. However, a change point of 88 mg kg⁻¹ Olsen-P was evident for less P sorbing soils, whereas no change point could be determined in soils with the highest P retention, probably because the latter group of soils were not sufficiently saturated with P, even if having high content of Olsen extractable phosphorus.

Acknowledgments

Special thanks are due to Ms. Irene Perucchini for the help in analytical work.