Soil properties exert significant influences on the fate and transport of pesticides in croplands. Pesticide movement in soil is largely controlled by several factors including soil texture, soil moisture, organic matter, and soil pH.
Soil texture describes the relative percentage of sand, silt and clay. Soil permeability, which is a measure of how fast water can move vertically through the soil, is affected by soil texture. Soils with coarse sandy textures are generally more permeable.
Soils with higher permeability have greater potential for ground-water contamination than less permeable soils. Finer-textured clay soils have much more surface area, more adsorptivity, and limit pesticide and water movement better than sandy soils do.
Soil moisture affects how fast water can travel through the soil. If soils are already wet or saturated before rainfall or irrigation, excess moisture will runoff.
Soil moisture also influences pesticide breakdown. Pesticide degradation is slow in dry soils. The rate of pesticide transformation generally increases with water content.
In very wet soils, such as rice paddies, the rate of diffusion of atmospheric oxygen into the soil is limited and anaerobic pesticide transformation can prevail over aerobic degradation.
Poor oxygen transfer at high moisture content can, however, accelerate or retard the degradation of pesticides (Pal et al., 2006).
Soil structure is another property that reflects the manner in which soil particles are aggregated and cemented.
A soil with a weak structure is more likely to be eroded and have lower infiltration rates, and hence, sorbed pesticides are more likely to be discharged through runoff.
Recent evidence indicates that at times soil macropores and cracks have a major effect on the movement of pesticides in soils (National Academy of Sciences, 2018). Macropores are formed by earthworms and decayed root systems, while cracks are formed by soil shrinkage.
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Under particular water application rate conditions, both water and chemicals in the dissolved and particulate forms tend to preferentially move through the macropores and cracks and reach the water table in a shorter period.
Organic matter is the single most important factor affecting adsorption of pesticides in soils. Organic matter content of soils may be increased by the addition of manure and incorporation of crop residues.
Many pesticides are adsorbed (bound) by soil organic matter, which reduces their rate of downward movement.
Soils high in organic matter tend to hold more water, which may make less water available for leaching.
Soil pH can have a significant effect on the adsorption and therefore on mobility of many pesticides, particularly herbicides.
pH is a measure of the availability of hydrogen ions (H+) in a solution. As the pH decreases below 7 (acid conditions), the concentration of hydrogen ions found in the solution increases. Many herbicides can incorporate hydrogen ions into their molecular structure, therefore changing the charge of the herbicide molecule.
At soil pH’s below 7, atrazine may pick up hydrogen ions from the soil solution causing the atrazine to take on a positive charge. The positive charge on the atrazine molecule under acid conditions increases the attraction between the herbicide molecule and negatively charged soil colloids.
At soil pH’s above 7 most of the atrazine maintains a neutral charge and thus the herbicide is less tightly adsorbed and more available to plants (Hartzler, 2018).