Mineralogy in Relation to Phosphorus Sorption and Dissolved Phosphorus Losses in Runoff

Abstract

The relationship between soil test P (STP) and dissolved reactive P (DRP) in runoff has been shown to vary with soil type due to differences in soil properties. This study was conducted to determine the effect of mineralogy on P sorption behavior and DRP losses in runoff using simulated rainfall. Nine different soil types were sampled from four different fields to provide a range in STP. Unamended soils were packed into runoff boxes for use in a rainfall simulation study (7.5 cm h−1 for 30 min). A mineralogical analysis and adsorption‐desorption isotherm was conducted on one representative sample from each soil type. Results indicated that P retention for adsorption and desorption in separated clay fractions and whole soils was well correlated to Al bearing minerals such as hydroxy‐interlayered‐vermiculite (HIV), gibbsite, and amorphous Al. However, P retention was negatively related to kaolinite content, which was also confirmed by isotherms conducted on pure clay minerals. Based on the isotherm results, all soils were split into two groups based on the ratio of HIV/kaolinite. Soils with a HIV/kaolinite ratio >0.5 had a significantly lower concentration of DRP in runoff for a given soil water soluble P level compared with soils with a ratio <0.5.