Phosphorus Sorption Capacity of Six Iowa Soils before and after Five Years of Use as Vegetative Treatment Areas

Accumulation of phosphorus in soil is a major factor limiting the operational life of land application waste disposal systems. Moreover, for nutrient management purposes and evaluation of potential environmental problems it is necessary to understand the impact of manure application on soil phosphorus sorption characteristics. In this study laboratory experiments were conducted to investigate the impact of feedlot runoff effluent application on phosphorus sorption capacities, equilibrium phosphorus concentrations, and phosphorus buffering capacities of six Iowa soils. Soil samples were collected from vegetative treatment areas that had received feedlot runoff application for five years and from a paired grassed area that did not receive the effluent application. Subsamples of each soil were incubated with a series of twelve phosphorus solutions ranging in concentration from 0 to 200 mg P/L to determine the sorption characteristics of the soil. Sorption data were fitted to the Langmuir sorption model to determine the phosphorus equilibrium concentration, the phosphorus buffering capacity, and the maximum phosphorus sorption capacity of the soil. Sorption parameters of the vegetative treatment area and grassed area that didn’t receive the feedlot runoff were then compared to evaluate the impact effluent application had on soil phosphorus sorption properties. Results indicated that vegetative treatment areas generally had elevated phosphorus equilibrium concentrations in relation to the grassed area that didn’t receive the effluent application, indicating an elevated risk of loss of dissolved phosphorus. In most cases the ability of the soil to sorb phosphorus was significantly increased as was the remaining phosphorus sorption capacity of the soil. These results indicate that vegetative treatment area life could be greatly extended due to soil property modifications that occur as a result of system use.