Movement and Absorption of Pesticides in Soil

A pesticide is a toxic chemical substance or a mixture of substances or biological agents that are intentionally released into the environment in order to avert, deter, control and/or kill and destroy populations of insects, weeds, rodents, fungi and other harmful pests. Different pesticides have been used for crop protection for centuries (Imadi et al., 2015).

Pesticides released into the soil environment are subjected to various reactions with soil and environmental processes including adsorption, transfer and degradation. Transfer refers to processes that move the pesticides away from their target sites.

These processes include volatilization, spray drift, runoff, leaching, absorption and crop removal. This unit intends to explain the movement and absorption of pesticides in soil.

Fate and Behaviour of Pesticides in Soil

Pesticides reaching the soil are subjected to diverse processes that sometimes may be beneficial in terms of control of target pests. For example, the leaching of some herbicides into the root zone can give better weed control.

Conversely, releasing pesticides into the environment sometimes can be quite harmful, as not the entire applied chemical reaches the target site.

For example, runoff can move a herbicide away from target weeds resulting in wastage of chemical and poor weed control as well as in more chance of damaging other plants and polluting soil and water.

In addition, some of the pesticide may drift downward and outside of the application site. The fate processes of pesticides fall into three major types: adsorption, transfer, and degradation.

Adsorption

The adsorption process binds pesticides to soil particles and it often occurs because of the attraction between a chemical and soil particles. For example, positively charged pesticide molecules are attracted to and can bind to negatively charged clay particles.

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Adsorption of herbicides to soil colloids occurs due to the attraction between charges on soil colloid surfaces and herbicide molecule.

In most situations, the charges are relatively weak and thus the process is reversible. An equilibrium is reached between the amount of herbicide bound to colloids and that found in solution (Hartzler, 2018).

A number of factors influence pesticide adsorption. Soils rich in organic matter or clay are more adsorptive than coarse, sandy soils, partly because a clay or organic soil has more particle surface area, or more sites onto which particles can bind.

Adsorption is also affected by soil moisture content. Dry soils tend to adsorb more pesticide than wet soils because water molecules compete with the pesticide for the binding sites.

Adsorption to soil particles varies amongst pesticides. For example, paraquat and glyphosate bind very tightly, whereas others bind only weakly and are readily desorbed or released back into soil solution. Reduced pest control could be a problem arising from pesticide adsorption.

For example, weeds may not be controlled if a herbicide is held too tightly to soil particles and cannot be taken up by the roots of the target weeds. Some pesticide labels recommend higher application rates when the pesticide is applied to adsorptive soils.

Plant injury can be another problem resulting from adsorption of pesticides to soil particles. Injury can result if a pesticide used for one crop is subsequently released from the soil in amounts large enough to cause injury to a sensitive rotational crop. This pesticide ‘carry-over’ can also culminate in the presence of illegal residues on rotational food or feed crops.

Adsorption is particularly important because it influences whether other processes are able to affect pesticides.

Pesticide Transfer

Pesticide transfer is sometimes necessary for pest control. For instance, certain pre-emergence herbicides need to move within the soil to reach germinating weed seeds for them to be effective. However, too much movement can move a pesticide away from the target pest.

This can culminate in reduced pest control, contamination of surface water and groundwater, and injury of non-target species, including humans. Five ways through which pesticides can be transferred are volatilization, runoff, leaching, absorption or uptake, and crop removal.

Volatilization

The conversion of a solid or liquid into a gas is called volatilization. A pesticide, once volatilized, can move in air currents away from the treated surface. An important factor that determines whether a pesticide will volatilize is vapour pressure. The higher the vapour pressure, the more volatile the pesticide.

Environmental factors that tend to increase volatilization include high temperature, low relative humidity, and air movement. A pesticide tightly adsorbed to soil particles is less likely to volatilize, thus soil conditions such as texture, organic matter content, and moisture can thus influence pesticide volatilization.

Volatilization can culminate in reduced control of the target pest as less pesticide remains at the target site. Vapour drift, the movement of pesticide vapours or gases in the atmosphere can lead to injury of nontarget species. For example, herbicide vapours can injure nontarget plants.

Pesticide volatilization can be reduced by avoiding the application of volatile pesticides when conditions are unfavourable, such as very hot, dry days or when the soils are wet. Labels on pesticide containers often provide warnings if there is a volatility hazard under certain conditions.

Labels for volatile pesticides may suggest incorporating the pesticide into the soil by tillage or irrigation during or shortly after application. This helps to reduce the amount of exposed pesticide on the soil surface, thereby reducing volatilization. Low-volatile formulations are available for some pesticides.

Runoff

Runoff refers to the movement of water over a sloping surface. This occurs when water is applied faster than it can enter the soil. Pesticides can be transported in the water itself or bound to eroding soil particles.

The magnitude of pesticide runoff depends on the slope or gradient of an area; the erodibility, texture and moisture content of the soil; and the amount and timing of rainfall and irrigation. Ordinarily, pesticide runoff is greatest when a heavy or sustained rain follows application.

Over- irrigation can result in excess surface water as well as in pesticide runoff, particularly when an irrigation system is used to apply a pesticide.

The movement of runoff water tends to be slowed down by vegetation or crop residue. Certain physicochemical properties of the pesticide, such as how quickly plants absorb it or how tightly it is bound to plant tissue or soil, are also important.

Herbicide runoff can result in direct injury to non-target plants. Insecticide and nematicide runoff into surface waters such as streams and ponds can be particularly harmful to aquatic organisms.

Pesticide runoff also can lead to groundwater contamination and can cause injury to crops, livestock or humans if the contaminated is used downstream.

Practices to mitigate pesticide runoff include monitoring of weather conditions, careful application of irrigation water, using a spray mix additive to enhance pesticide retention on foliage, and incorporating the pesticide into the soil.

Reduced-tillage cropping systems and surface grading, in addition to contour planting and strip cropping of untreated vegetation, can slow the movement of runoff water and help keep it out of wells, sinkholes, water bodies and other sensitive areas (Fishel, 2003).

Leaching

Leaching occurs when pesticides move through the soil rather than over the surface. Leaching is governed partly by the pesticide’s physicochemical properties. For example, a pesticide held strongly to soil particles by adsorption is less likely to leach.

Solubility is another factor influencing leaching. A pesticide that dissolves in water can move with water in the soil. Furthermore, the persistence 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 influencing leaching include texture and organic matter partly because of their effect on pesticide adsorption. Another important factor is soil permeability that indicates how readily water moves through the soil.

Soil permeability is directly related to leaching because the more permeable a soil, the greater propensity for pesticide leaching. For example, a sandy soil is much more permeable than a clay soil.

Pesticide leaching also can be influenced by the method and rate of application, the use of tillage systems modifying soil conditions, and the amount and timing of water a treated area receives after application.

Ordinarily, the closer the time of application to a heavy or sustained rainfall, the greater the likelihood that some pesticide leaching will occur.

A certain amount of leaching may be essential for control of a target pest. Nevertheless, too much leaching can result in reduced pest control, injury of nontarget species and groundwater contamination.

Pesticide leaching also can be mitigated by monitoring weather conditions and the amount and timing of irrigation. Proper pesticide selection is vital because those pesticides that are not readily adsorbed, not rapidly degraded, and highly water-soluble are the most prone to leaching.

Prior to pesticide application, labels must be read carefully for instructions on the rates, timing and methods of application. The label may also advise against applying the pesticide when certain soil, geologic or climatic conditions present.

Pesticides can leach not only through the soil to groundwater from storage, mixing, equipment cleaning and disposal areas but also from normal applications.

Absorption or Uptake

Absorption or uptake refers to the movement of pesticides into plants and animals. Absorption of pesticides by target and non-target organisms is influenced by environmental conditions and by the physicochemical properties of the pesticide and the soil.

For example, a large portion of herbicide present in the soil is bound to soil colloids (clay, organic matter), and this herbicide is less readily available to plants than the herbicide present in the soil solution.

Conditions that favour movement of the herbicide into soil solution tend to increase absorption by plants (Hartzler, 2018). Pesticides may be broken down or they may remain in the plant until tissue decay or harvest after plants absorb them.

Crop Removal

The presence of pesticides in soil may lead to their residues in plants grown in contaminated soil although the rate of uptake may differ for pesticides that are equally persistent. Accumulation of a pesticide in a plant is usually dependent upon the concentration of the residues in the soil.

The total amount of the pesticide in plant may increase with time if the compound is long-lived (Khan, 1980).

Crop removal transfers pesticides and their metabolites 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 (Fishel, 2003).

Degradation

As soon as a pesticide is applied, it begins to break down or degrade into simpler compounds, which are usually less toxic. Each pesticide has its own speed of degradation, which depends on the active ingredient, the formulation, and environmental conditions.

There are both advantages and disadvantages to a long degradation time. The longer a pesticide takes to break down, the longer it is present to control the insect, weed, or disease for which it was applied. This is called residual activity.

One disadvantage of extended residual activity, or persistence, is that the pesticide may also be available for leaching or runoff over a longer period. Three types of pesticide degradation are microbial, chemical, and photodegradation.

Microbial Degradation

Microbial degradation is the breakdown of pesticides by fungi, bacteria, and other microorganisms utilizing pesticides as a food source. Most microbial degradation of pesticides occurs in the soil.

The rate of microbial degradation is affected by soil conditions such as moisture, temperature, aeration, pH, and the amount of organic matter affect, because of their direct influence on microbial growth and activity.

Another factor that can influence microbial degradation is the frequency of pesticide application. Rapid microbial degradation is more likely when the same pesticide is used repeatedly in a field.

Repeated applications can actually stimulate the buildup of organisms capable of effectively degrading the pesticide. The amount of pesticide available for pest control is reduced as the population of these organisms increases and degradation accelerates.

In extreme cases, accelerated microbial degradation has been responsible for certain products being withdrawn from the market. The effectiveness of these chemicals is greatly reduced by microorganisms soon after application.

To reduce the possibility of very rapid pesticide breakdown is by using pesticides only when required and by avoiding repeated applications of the same pesticide. In order to minimize the potential for microbial degradation problems as well as for pest resistance, it is necessary to alternate between different classes, groups or formulations of pesticides.

Chemical Degradation

Chemical degradation is the breakdown of pesticides by processes not involving microorganisms. In addition to the physicochemical properties of the pesticide, temperature, moisture, pH and adsorption determine the chemical reactions that take place and how they quickly proceed.

Hydrolysis, a breakdown process involving the reaction of a pesticide reacts with water, is one of the most common pesticide degradation reactions. A large number of organophosphate and carbamate insects are particularly susceptible to hydrolysis under alkaline conditions. In fact, some are actually broken down in a matter of hours when mixed with alkaline water.

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Instructions on product labels may indicate warning against mixing a pesticide with certain fertilizers, other pesticides or water with specific characteristics. Pesticide degradation and potential incompatibility problems can be prevented by observing these precautions.

Buffers or other additives may be available to modify spray mix conditions and prevent or reduce degradation in some situations. By not allowing a spray mix to remain in a spray tank for a long period, pesticide degradation and possible corrosion of application equipment can be avoided.

Photo degradation

The breakdown of pesticides by light, particularly sunlight is termed photodegradation. Pesticides on foliage, on soil surface, and even in the air can be degraded through photodegradation.

Factors affecting pesticide degradation include the intensity of the sunlight, properties of the application site, the application method and the properties of the pesticide. By incorporating the pesticide into the soil during or immediately after application, pesticide losses from photodegradation can be minimized.

In conclusion, the understanding of fate and behavior of pesticides in soil will enable agriculturists to fit pesticides to soil types and avoid the unpleasant consequences of ineffective control of target pests and environmental pollution.

The factors influencing the availability of a pesticide in the soil determine how effective a treatment will be. The understanding of pesticide behaviour in soil is useful in diagnosing performance problems in the field; and the differences in chemical characteristics among pesticides are relatively small, and therefore soil type and environment will exert a greater impact on performance than does the specific pesticide applied.