PHOSPHORUS RETENTION BY THE Ap HORIZON OF A SPODOSOL AS INFLUENCED BY CALCIUM AMENDMENTS 1

The low capacity of acid sandy soils to bind P is one reason for the leaching and runoff of P from these soils. Various soil amendments have been used to reduce P leaching. This study was conducted to show the effect of CaCO3, dolomite, gypsum, and CaCl2 on P retention by the Ap horizon of a Spodosol. Phosphorus sorption was studied in a batch experiment at an initial P concentration range of 0 to 10 mg L−1. The amendment rate used was 0.05 mmol of either Ca or Ca + Mg (and also 0.1 mmol in the case of CaCO3 and gypsum) per g of soil. The influence on P retention of time after soil amendment (1 h to 1 week) was assessed. P adsorption was described by Langmuir and Freundlich equations. All amendments, except for CaCl2, increased soil pH and P retention by soil. CaCO3 was more effective then dolomite, the carbonates (CaCO3 and dolomite) were more effective in their interaction with the soil, and gypsum increased P retention by itself, its effect was weakened upon interaction with soil. Sorption of P in soil amended with gypsum was faster than sorption in soil amended with CaCO3. The effect of the amendments under study was mainly to provide Ca ions, which must be accompanied by an increase in pH to increase P retention.