A primary reason for studying the fate of a herbicide in soil is because of its potential effect, beneficial or detrimental, on plants. Herbicide concentrations in soil often can be accurately analyzed by chemical or physical procedures. But such quantitative measurements sometimes are not well correlated with plant response because of a number of interacting soil and environmental factors. If the question is not “How much herbicide residue is present in the soil?”, but rather “How much potential exists for herbicidal effects on plants?”, then the use of plants as one aspect of studies on herbicide persistence can be valuable. This paper addresses factors influencing the response of plants to herbicide residues under field conditions.

Among the numerous research reports on the fate of herbicides in soils, the literature dealing with the degradation of herbicides under field conditions is probably most difficult to interpret. Few have attempted to evaluate critically the purposes of these studies. This review will not be a comprehensive recitation of previous studies on herbicide degradation under field conditions, but is aimed at pointing out certain problem areas in the approach to research on this subject. The problems relate both to a lack of clear understanding of the degradation process and a lack of appropriate methodology for studying degradation under field conditions.

Most herbicides are applied preemergence onto bare soil or during the early stage of plant development. Therefore, the major part of the active ingredient either reaches the soil surface immediately or later with decaying plant material. The further fate of the herbicide depends largely on the physicochemical behavior of the respective compound, the amount and method of application, and a number of soil, plant, and climatic factors influencing the persistence and bio availability of organic compounds in a given soil (5, 7). Especially in the upper 2-cm soil layer, drastic changes in temperature and moisture content during a growing season have a great influence on the degradation and adsorption of herbicides in soil (10, 31).

The spatial variation of measured pesticide concentrations is often a complicating factor in interpreting the results from field studies. This is particularly true when the study objective is quantitative evaluation of pathways of pesticide loss (leaching, degradation) or efficacy. The variation can result from a lack of uniformity in pesticide or water application (an extrinsic factor) or from spatial differences in various physical, chemical, and biological processes (intrinsic factors) that act to transport and transform pesticides in the field. Most research attention to date has not focused on the spatial variation of these basic processes, but rather on the total spatial variation (extrinsic plus intrinsic) in the measured concentrations of residual pesticide.

Modeling is increasingly being used as a tool for the evaluation of the environmental fate of pesticides. Sorption, leaching, degradation, and volatilization are some of the processes being integrated through the use of simulation modeling techniques. Several research programs are focusing their attention on such issues (16, 17, 18, 32, 35), with regulatory agencies involved in management of pesticides also taking a modeling approach (3, 7). Because of the extreme complexity of agroecosystems, it is obvious that the use of simulation models will continue to be the most expeditious, reliable, and cost-effective means of integrating the various processes acting upon a pesticide to determine its fate. For example, modeling will help to summarize and interpret efficacy trials and will provide the vehicle for transferring experimental results to unstudied situations, such as the potential environmental fate of an applied herbicide. However, proper development, testing, and responsible use of a modeling approach must be based upon a thorough, comprehensive understanding of interdependent and dynamic natural processes.

Much of the success of modern agriculture is due to pesticide technology. Because weeds pose the greatest threat to crop yields, the largest class of pesticides used today is herbicides. Herbicides include products whose active ingredients represent a wide range of chemical families; yet most constitute a minimal potential hazard to man and the environment. Criteria typically used to determine potential hazard are mammalian and piscine toxicity, persistence, and bioaccumulation. By evaluating these criteria and assigning numerical values it is possible to devise a ranking of pesticides such as that developed by Weber (51). This list identifies those pesticides most likely to have some undesirable activity, either before or after they are applied. All herbicides ranked well below organochlorine insecticides in hazard rating and most were less hazardous than the organophosphate and carbamate insecticides. However, any substance, when used improperly, has the potential to cause problems, and for this reason all chemicals should be handled with caution.