Effect of surfactant on leaching of pendimethalin in Florida Candler fine sand.

Herbicides are an essential part of modern horticultural and agricultural production systems. For long-term weed control, application of preemergence herbicides is made directly to the soil that is a common medium for growth of crop plants and weeds. Also, when herbicides are applied postemergence, some of the herbicide drifts away from the foliage onto the soil. In light, sandy soils, high precipitation can further increase the chances of herbicide leaching into subsoil profiles, especially if the herbicides are poorly adsorbed, for example, atrazine and simazine (Hall et al., 1989) . Groundwater contamination caused by herbicide leaching is a concern in major agricultural regions of the world (Hallberg, 1988) . Most herbicides are applied at much higher rates than recommended to offset losses that occur from their site of action in the plant root zone. Preemergence herbicides for weed management in citrus require soil incorporation, either by irrigation or rainfall into the zone of weed seed germination (the top several inches of soil). Soil incorporation is essential for the area under the canopy of citrus trees since mechanical incorporation may potentially damage citrus roots and low volume irrigation lines. Additionally, it is difficult to achieve complete and even distribution of the herbicide by mechanical means under the canopy. In the absence of soil incorporation, herbicide effectiveness is reduced because of volatilization and photodecomposition (Jain and Singh, 1992). Herbicide leaching is influenced by several factors, including adsorption of herbicides to soil colloids and soil type (Alva and Singh, 1990), uniformity of water flow (Boesten, 1987), water solubility of herbicides, and physicochemical and biological characteristics (soil pH, soil colloids, bulk density, and pore distribution) (Anderson, 1996). When a pesticide enters the soil, some of it will stick to soil particles, particularly organic matter, through a process called sorption. As more water enters the soil through rainfall or irrigation, the pesticide molecules will move down and may enter soil-water through a process called desorption (Hornsby, 1999). For strongly adsorbed herbicides, soil organic matter content is generally the most important factor—the higher the organic content the less leaching. Leaching of weakly adsorbed acidic herbicides, for example, sulfonylureas and imidazolinones, is much greater in high pH soils (Brown and Kearney, 1991) . One of the most useful indices for quantifying pesticide sorption on soils is the partition coefficient (PC), which is defined as the ratio of pesticide concentration bound to soil organic matter particles to concentration that is dissolved in the soil-water. Thus, pesticides with small PC values are more likely to be leached than those with large PC values (Hornsby, 1999). Beyond the nature of the soil and the amount of water that percolates through it, the extent of leaching also depends on the herbicide properties (Weber, 1972, 1987). More than 12 different pesticides have been detected in the groundwater of at least 24 states in the USA as a result of routine agricultural use (Hallberg, 1986). Approximately 344,000 ha of citrus are cultivated in Florida, which contributes significantly to the state’s agricultural revenue (Anonymous, 1996). Florida’s climate allows weeds to grow year-round (Singh and Tan, 1992), and efficient control is only achieved by herbicide use that often requires repeated applications (Muraro and Oswalt, 1996). Herbicide leaching may result in damage to citrus S. D. Sharma (&) M. Singh Institute of Food and Agricultural Sciences, Citrus Research and Education Center, University of Florida, 700 Experiment Station Road, Lake Alfred, FL 33850, USA e-mail: shiv901@ufl.edu