Factors Influencing Pesticide Degradation in Soils
Several factors determine the length of time herbicides persist. These factors fall into three categories: soil factors, climatic conditions, and herbicidal properties. Factors from each category strongly interact with one another.
1. Soil factors
Soil factors affecting pesticide degradation include soil composition, soil chemistry, and microbial activity. Soil composition is a physical factor determined by the relative amounts of sand, silt, and clay in the soil (the soil texture), as well as by the organic matter content.
An important chemical property of soil that can influence herbicide persistence is pH. The microbial aspects of the soil environment include the types and abundance of soil microorganisms present in the soil.
2. Soil composition
Soil composition affects pesticide breakdown through soil-herbicide binding (adsorption and desorption).
Generally, soils high in clay, organic matter, or both have a greater potential for adsorption because of increased binding of the herbicide to soil particles, with a corresponding decrease in availability of the pesticide in soil mostore to be accessed by microbial degrading agents.
This “tie-up” results in decreased in microbial breakdown, initial plant uptake and herbicidal activity. More herbicide is held in reserve, potentially injuring susceptible crops in the future.
In general, medium- and fine-textured soils with an organic matter content of more than 3 percent have the greatest potential to bind or hold herbicides and injure sensitive rotation crops.
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Coarse- to medium- textured soils with a lower organic matter content (less than 3 percent), are less likely to retain herbicides and have carryover problems. Under the right circumstances, however, herbicide carryover can occur in any type of soil.
3. Soil pH
Soil pH can influence the persistence of some herbicides, especially the triazines and sulfonylureas. Chemical and microbial breakdown, two ways herbicides degrade in soil, often are slower in higher-pH soils.
In particular, the chemical degradation rate of the triazine and sulfonylurea herbicide families slows as the soil pH increases, particularly above 7.0. In addition, in higher-pH soils, lesser amounts of these herbicides are bound to soil particles, making more available for plant uptake.
So in higher-pH soils, the triazine and sulfonylurea herbicides persist longer, and more is available for plant uptake. (Some triazine and sulfonylurea herbicides do not persist and carry over regardless of how high the soil pH is.) Low pH also can affect the persistence of both the triazine and the sulfonylurea herbicides.
Soil pH levels below 6.0 allow a more rapid dissipation of both these herbicide families. In acid soils, herbicides like atrazine become bound to soil particles, making them unavailable for weed control; but at the same time, they are chemically degraded more quickly.
This makes liming an acid soil important for achieving an adequate performance form these two herbicide families. In contrast, a low soil pH increases the persistence of the imidazolinone herbicides imazaquin (Scepter) and imazethapyr (Pursuit).
As the soil pH drops below 6.0, imazaquin and imazethapyr become increasingly bound, or adsorbed, to soil particles. Adsorption of these herbicides appears to reduce their availability to soil microorganisms, the primary mechanisms of degradation.
Even though adsorption is greater in lower- pH soils, the herbicide can still be released several months later, becoming available for plant uptake and potentially injuring a sensitive follow crop. Sorption processes may affect biodegradation mainly bymodifying chemical bioavailability.
A positive relationship between sorption coefficient (Kd) and half-life has been reported for many ionizable pesticides (Kah and Brown, 2006).However, several factors might counterbalance the influence of sorption on degradation, and the link between sorption and degradation is not always obvious (Barriuso, etal., 1997; Radosevich etal., 1996; Shaw and Burns, 1998).
Degradation processes by soil microorganisms probably are the most important pathways responsible for the breakdown of herbicides. The types of microorganisms (fungi, bacteria, protozoans, etc.) and their relative numbers determine how quickly decomposition occurs.
Microorganisms require certain environmental conditions for optimal growth and utilization of any pesticide. Factors that affect microbial activity are moisture, temperature, pH, oxygen, and mineral nutrient supply. Usually, a warm, well-aerated, fertilesoil with a near-neutral pH is most favorable for microbial growth and, hence, herbicide breakdown.
4. Climatic factors
The climatic variables involved in herbicide breakdown are moisture, temperature, and sunlight. Herbicide degradation rates generally increase as temperature and soil moisture increase because both chemical and microbial decomposition rates increase with higher temperatures and moisture levels. Cool, dry conditions slow down herbicide degradation.
Carryover problems are always greater the year following a drought. If winter and spring conditions are wet and mild following a previously dry summer, the likelihood of herbicide carryover is lower. Sunlight is sometimes an important factor in herbicide degradation.
Photodecomposition or degradation catalyzed by sunlight (photolysis) has been reported for many herbicides, especially in liquid solution (i.e., water) or on plant leaf surfaces. But for most of the more persistent soil-applied herbicides, once soil contact is made, losses due to photolysis are small.
The exception may be the dinitroanilines, including trifluralin (Treflan) and pendimethalin (Prowl). These can be lost if they remain on the soil surface for an extended period without rainfall. A sensitivity to sunlight and losses through volatilization are primary reasons for incorporating the dinitroanilines at application time.
5. Pesticide properties
A herbicide’s chemical properties affect its persistence. These properties include water solubility, vapor pressure, and the molecules’ susceptibility to chemical or microbial alteration or degradation. Leaching is one mechanism responsible for herbicide dissipation.
The solubility of a herbicide in water helps determine its leaching potential. Leaching occurs when a herbicide is dissolved in water and moves down through the soil profile. Herbicides that readily leach may be carried away from crop and weed germination zones.
Herbicide leaching is determined by other factors as well. These include herbicide-soil binding properties, soil physical characteristics, rainfall frequency and intensity, herbicide concentration, and time of herbicide application.
In general, herbicides that are less soluble in water and strongly attracted to soil particles are less likely to leach, particularly in dry years. The vaporpressureof a herbicide determines its volatility.
Volatilization is the process whereby a herbicide changes from a liquid or solid to a gas. Volatile herbicides (those with higher vapor pressures) generally dissipate more rapidly than herbicides with lower vapor pressures.Volatilization increases with temperature and moisture.
Most herbicides are relatively nonvolatile under normalfield-use conditions. The more volatile herbicides are generally incorporated to avoid gaseous losses.
Volatile herbicides include members of the thiocarbamate family EPTC (Eradicane, Eptam) and butylate (Sutan+); the dinitroanilinestrifluralin (Treflan) and ethalfluralin (Sonalan); and clomazone (Command).
A herbicide’s chemical structure dictates how the herbicide will degrade in soil. Some herbicides are rapidly decomposed by microorganisms if the right kind and number are present and if soil conditions are favorable for their growth. But herbicides vary greatly in their susceptibility to microbial decomposition.
The chemical structure of 2,4-D,for example, allows microbes to quickly detoxify the molecule into inactive metabolites, whereas atrazine is not as prone to microbial attack; hence, degradation is slower.
Some herbicides are prone to chemical reactions. Members of the sulfonylurea herbicide family, for example, are degraded through chemical hydrolysis as well as through microbial processes. Remember that for the sulfonylureas as well as the triazines, the rate of chemical hydrolysis is dependent on soil pH.
Although it is less sensitive than microbes to fluctuations in soil physical characteristics and often soil moisture, the rate of chemical reaction also will vary dependingon the surrounding soil environment.
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Several families of herbicides are degraded through both chemical and microbial pathways. Others less prone to chemical breakdown are lost primarily through microbial alteration.
In conclusion, there is the need for proper understanding of the types of degradation and factors that influence degradation in soils. This will inform the choice of pesticide to be applied based on soil and pesticide properties as well as the prevailing climatic condition.
Degradation of pesticide in the soil environment happens in three ways; viz: microbial, chemical, and photo-degradation;
Pesticide degradation in the soil is governed by several factors, which include soil properties, pesticide properties, as well as weather conditions.
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