Pesticide transfer is sometimes essential for pest control. For example, to be effective, certain pre-emergence herbicides must move within the soil to reach the germinating weed seeds. However, too much movement can move a pesticide away from the target pest. This can lead to reduced pest control, injury of non-target species including humans, and contamination of surface water and groundwater. Pesticides can be transferred in five ways: volatilization, runoff leaching, absorption, and crop removal.
Volatilization is the conversion of a solid or liquid into a gas. Once volatilized, a pesticide can move in air currents away from the treated surface. Vapor pressure is an important factor in determining whether a pesticide will volatilize: The higher the vapor pressure, the more volatile the pesticide. Environmental factors such as high temperature, low relative humidity. and air movement tend to increase volatilization. A pesticide tightly adsorbed to soil particles is less likely to volatilize; therefore, soil conditions such as. texture, organic matter content, and moisture can influence pesticide volatilization. Formulations can also help reduce volatilization. Granular, flowable, and wettable powders are less susceptible to volatilization than emulsifiable concentrates and soluble powders.
Runoff is the movement of water over a sloping surface. Runoff occurs when water is applied to the soil at a faster rate than it can enter the soil. Runoff water can carry pesticides in the water itself or bound to eroding soil particles.
The severity of pesticide runoff is influenced by the slope or grade of an area; the erodibility, texture, and moisture content of the soil; and the amount and timing of rainfall and irrigation. Pesticide runoff is usually greatest when a heavy or sustained rain follows soon after an application. Over-irrigation can lead to the accumulation of excess surface water and, especially with chemigation, pesticide runoff. Runoff can also occur if a pesticide is applied to a saturated soil, resulting from a previous rain or irrigation, followed by a light rain or irrigation.
Vegetation or crop residue tend to slow the movement of runoff water. Certain physical and chemical properties of the pesticide are also important, such as how quickly it is absorbed by plants or how tightly it is bound to plant tissue or soil.
Practices used to reduce pesticide runoff include monitoring weather conditions, careful application of irrigation water, using a spray mix additive to enhance pesticide retention on the foliage, and incorporating the pesticide into the soil. Reduced-tillage cropping systems and surface grading? in addition to contour planting and strip cropping, can also reduce pesticide runoff. Finally, dikes or a border of untreated vegetation can slow the movement of runoff water and help keep it out of wells, sinkholes, water bodies, and other sensitive areas.
Leaching is the movement of pesticides through the soil as opposed to movement over the surface. Pesticide leaching depends, in part, on the chemical and physical properties of the pesticide. For example, a pesticide held strongly to soil particles by adsorption is less likely to leach. Solubility is another factor because a pesticide that dissolves in water can move with water in the soil. The persistence, or longevity of a pesticide also influences the likelihood of leaching. A pesticide that is rapidly broken down by a degradation process is less likely to leach because it may remain in the soil only a short time.
Soil factors that influence leaching include texture and organic matter, in part because of their effect on pesticide adsorption. Soil permeability, or how readily water moves through the soil, is also important. The more permeable a soil, the greater potential for pesticide leaching: A sandy soil is much more permeable than a clay soil.
Pesticide leaching can also be influenced by the method and rate of application; the use of tillage systems that modify soil conditions; and the amount and timing of water a treated area receives after application. Typically, the closer the time of application to a heavy or sustained rainfall, the greater likelihood some pesticide leaching will occur.
Monitoring weather conditions and the amount and timing of irrigation can help minimize pesticide leaching. Careful pesticide selection is important because those that are highly water soluble, not readily adsorbed, and not rapidly degraded are the most likely to leach. Labels must be carefully inspected for application instructions such as rates, timing, and method. Labels may also contain statements that advise against the use of the pesticide when certain soil, geologic, or climatic conditions are present.
Pesticides can leach through the soil to groundwater from storage, mixing, equipment cleaning, and disposal areas. Under certain conditions, some pesticides can leach to groundwater from normal applications.
Absorption or uptake is the movement of pesticides into plants and animals. Pesticide absorption by target and non-target organisms is influenced by environmental conditions and by the chemical and physical properties of both the pesticide and the soil. Once absorbed by plants, pesticides may be broken down or remain inside the plant until the tissues decay or the crop is harvested.
Crop removal transfers pesticides and their breakdown products from the treatment site. Most harvested food commodities are subjected to washing and processing procedures that remove or degrade much of the remaining pesticide residue. Although harvesting is more typically associated with food and feed products, it is easy to forget that pesticides can be transferred during such operations as tree and shrub pruning and turfgrass mowing.