Phosphorus sorption capacity of alkaline Manitoba soils and its relationship to soil properties

The establishment of the P retention capacity of soil in Manitoba is essential for effective management of P in the region. However, the methods for determining the P retention capacity for neutral to calcareous soils in the Eastern Prairies are not well developed. The objectives of this study were to determine the P retention capacity of Manitoba soils and to generate equations that relate these capacities to other soil properties. One hundred and fifteen archived surface soils were selected and their physico-chemical properties were measured. These soils were used to generate a single-point P adsorption index by equilibrating 2 g of soil in 20 mL of 0.01 M KCl solution containing either 150 (P-150) or 400 (P-400) mg P L-1. A subset of 26 of these soils was used for multipoint isotherms with P concentrations in the range of 0-1000 mg P L-1. The data obtained were fitted to the Langmuir isotherm and the adsorption indices were correlated with the various soil properties that were then used to developed predictive equations of the P retention capacity of the soil. The values of the adsorption index, P-150, obtained from the single point adsorption study using 150 mg P L-1, ranged between 88 and 891 mg P kg(-1), while that of P-400 ranged between 100 and 1250 mg P kg(-1). A better correlation was obtained between P-150 and soil properties compared with P400. For the 26 soil subset, the adsorption indices, Smax 1 to Smax 6; obtained from the Langmuir isotherm, ranged from 300 to 1330 mg kg(-1). A good correlation was obtained between the single point index and the multipoint isotherm (r = 0.93). Hence, S-max for the 115 soils was estimated from the relationship between P-150 and S-max 3 of the 26 soils. The best relationships between the adsorption parameters, P-150 and Smax and the soil properties were obtained with the sum of Mehlich-3 extractable Ca and Mg (R-2 = 0.66) and the sum of exchangeable Ca and Mg (R-2 = 0.64). Mehlich-3-Ca and -Mg each explained 56% of the variation, while clay content explained 40% of the variation in the P retention capacity of these soils. Unlike the widely reported influence of Al and Fe in acid soils, our study showed that the retention of P in Manitoba soils was influenced more by Ca and Mg and soil texture.