COLLOIDAL IRON OXIDE TRANSPORT IN SANDY SOIL INDUCED BY EXCESSIVE PHOSPHORUS APPLICATION ¹

Abstract

Colloidal movement in the soil profile is of great concern because of its suspected role in facilitating transport of contaminants to groundwater. Laboratory studies were conducted to determine the effects of excessive phosphorus (P) application on the transport of colloidal iron oxides in the surface layer of a Wabasso sand (sandy, siliceous, hyperthermic alfic haplaquods). Soils were packed in columns, and the surface layer was treated with five phosphorus application rates. A total of five pore volumes of deionized water were applied to soil columns under conditions of alternating saturated and unsaturated flow, and leachates were collected and analyzed for iron (Fe) and P concentrations. Excessive P application altered characteristics of the soil and the stability of iron oxides in the soil solution and, thus, accelerated transport of colloidal iron oxides in the soil. Concentrations of Fe in the leachates, ranging from 0.1 to 33.4 mg/L, depended on the rate of P application and varied with leaching sequence. The Fe in the leachates was mainly particulate Fe, which averaged 38.0–94.1% of total Fe in the leachates. The highest concentration of Fe in the leachate generally occurred in the soils receiving P levels as high as 1- to 3-fold of maximum phosphate adsorption calculated from a Langmuir equation. Elevated Fe transport in the form of iron oxide also increased the migration of P and trace elements in particulate forms.