Evaluation of phosphorus retention equations for Manitoba soils

Abstract

One of the key factors in phosphorus management is the P retention capacity (PRC) of the soil. In our previous study, we formulated several equations for estimating the phosphorus retention capacity of Manitoba soils. The objectives of the current study were to evaluate these equations using independent soil samples and to evaluate the influence of manure application on the predictive ability of these equations. Forty representative surface soil samples (20 soils with history of manure application and 20 without manure application history) were collected from across Manitoba. The P retention index (P150) and Langmuir adsorption maximum (Smax) were determined in the laboratory. The measured P retention capacities were then compared with those estimated using the formulated equations. Surprisingly, P150, which was obtained from a single measurement, was more robust than Smax that was obtained from at least 17 measurements as the equations provided a better estimate of P150 than Smax. Equations that were based on soil particle sizes (either percent clay or percent sand) provided poor estimates of soil PRC for the whole soil collection. However, when the soils were grouped on a pH basis, soil particle size worked better for soils with pH <7 than for soils with pH ≥7. The equations also worked better for soils with pH similar to those of the soils that were used to formulate the P retention equations. The accuracy of the predicted P sorption capacity improved with the manured soils due to the direct influence of manure application on soil properties that influence P retention such as Ca and Mg. Overall, the combination of CaM3, MgM3 and AlOx provided the best estimate of the PRC of Manitoba soils. Key words: Soil phosphorus, calcareous soils, phosphorus sorption, phosphorus retention, phosphorus retention capacity, estimating phosphorus retention capacity