EFFECTS OF PESTICIDE, SOIL, AND RAINFALL CHARACTERISTICS ON POTENTIAL PESTICIDE LOSS BY PERCOLATION - A GLEAMS SIMULATION

Potential pesticide loss in soil percolate is influenced by pesticide persistence and sorption by soil constituents (organic matter). Pesticide persistence, expressed as half-life (t1/2), changes with soil depth as microbial activity and soil properties change. Little is known, however, how these changes influence potential pesticide transport out of the root zone. Objectives of this study were to investigate relative differences in potential pesticide losses from the root zone by percolation due to 1) different soil surface and subsurface textures and pesticide t1/2, and 2) interactions between pesticide t1/2 and timing of rainfall after pesticide application. The GLEAMS (Groundwater Loading Effects of Agricultural Management Systems) model and a 50-year historical rainfall record at Tifton, Georgia, were used to simulate pesticide losses by percolation from three soils ranging in surface texture from sand to sandy clay loam. Hypothetical pesticides had surface t1/2 of 5, 15, 30, and 60 d and a range of subsurface t1/2 (2.5-360 d), and were applied to continuous corn (Zea maize, L.) at 2 kg ha-1 as surface spray at planting each year on 1 April. Simulated pesticide losses by percolation increased with increased surface and subsurface t1/2, and decreased with increased K(oc) (adsorption constant based on soil organic matter) values. Potential pesticide leaching was greatest for Lakeland sand and least for Greenville sandy clay loam. Rainfall timing affected simulated pesticide loss by percolation, especially for nonpersistent pesticides. For short pesticide t1/2 (0-5 d), excessive rainfall events within 1 t1/2 were largely responsible for simulated pesticide loss by percolation. Results indicate that changes in pesticide t1/2 in surface and subsurface horizons of different soils influence potential pesticide leaching from the root zone, and models (i.e., GLEAMS) can be used to provide comparative analysis of soil-pesticide-climate interactions. For example, depending on soil type and pesticide K(oc) and surface t1/2 values, potential leaching losses increased two to seven times as subsurface t1/2 increased six times.