SOIL PHOSPHORUS SORPTION CHARACTERISTICS IN AN INTENSIVE FINISHING SYSTEM ON A FEEDLOT AND A NATURAL PASTURE

Abstract

The intensive system of beef production is an environmental issue due to the high inputs of phosphorus (P) to the soil through manure. The objective of this study was to compare sorption isotherm parameters and the degree of soil P saturation in soils under pasture and feedlot. P maximum-adsorption capacity (Xm) was determined by Langmuir sorption isotherm, as well as the equilibrium P concentration at zero-net P sorption (EPC), the P sorption index (PSI) by the one-point sorption isotherm, and the degree of soil P saturation (GSP) using Xm or PSI. The Xm sorption parameter could not be obtained for the soil under a feedlot system, but a value of 357.1 mg P kg-1 was obtained for soil under a pasture based system. The EPC value in the soil under pasture was 0.24 mg P L-1, while it reached 19.18 mg P L-1 in the feedlot system. GSP values differed depending on the calculation method used; higher values were obtained when using the Xm of the Langmuir equation compared to values using PSI (11.6% and 7.5% for GSPLangmuir and GSPPSI, respectively). The GSPPSI in feedlot largely exceeded that of the pasture (55.7% and 11.6%, respectively). High values of EPC and GSP in the feedlot suggest that soils under this system are vulnerable to P loss by runoff.