A retardation-based model for phosphorus transport in sandy soil

Animal-based agricultural is practiced in many areas of the world on sandy soils in which leaching is the major transport process for phosphorus (P). Disposing of large quantities of animal wastes on these soils can overload their P retention capacity and lead to elevated concentrations of P in ground and surface water. Subsurface zones may represent sizeable reserves for storing P, but to effectively utilize these zones, a way of predicting how much P will be adsorbed before solution P concentrations exceed acceptable limits must be known. This article describes a simple transport model that simulates P leaching through soil that uses soil bulk density, volumetric water content, solution P concentration, and column length as inputs. Phosphorus is partitioned between soil and solution phases using Freundlich equations whose parameters are calculated from a single-point adsorption measurement. The model was used to simulate P leaching from 40 columns containing samples of A, E, or Bt horizons of soil from the Suwannee River basin of Florida and Georgia, in the United States; some of which had been loaded with P from dairy and poultry manure applications. The degree of P saturation, DPS, based on acid ammonium oxalate-extractable P, Fe, and Al, ranged from 4% to 140% for these soils. No soil with a DPS of <50% had an initial column leachate P concentration that exceeded 0.10 mg L. The relationship between measured and simulated retardation of P for soils with a DPS of <50% was good (R = 0.873), but the measured retardation exceeded simulated retardation by about 30%. This was attributed to the fact that the Freundlich parameters were based on a 24-h equilibration between soil and solution, whereas the column studies covered a period of about 2.5 years. The relationship for soils with a DPS of >50% was not as good (R = 0.333) but could be improved considerably (R = 0.661) by setting the initially adsorbed P in the model equal to the oxalate-extractable P and adjusting the desorption exponent in the Freundlich equation to allow for greater desorption of P. The model was able to capture several important aspects of P retention in sandy soil despite its simplicity and small number of inputs.