Texture is a good indicator of water holding capacity. Soil texture is the relative proportions of sand, silt and clay in the soil. Soils with more clay and organic matter tend to hold water and therefore chemicals longer.
The presence of macropores, soil cracking, worm holes and soil structure can affect water movement under some site conditions. These can enhance or retard water movement, depending on factors such as tillage, rainfall, chemical application method, cropping patterns, depth of macropore and others. Sandy soils have large "pore" spaces between individual particles, and the particles provide relatively little surface area for "sorption," or physical attachment of most contaminants. Large amounts of rainfall or excessive irrigation water can percolate through these soils, and dissolved contaminants can move rapidly down through the soil and into the groundwater.
Clay soils, on the other hand, are made up of extremely small particles that slow the movement of water and dissolved contaminants through the soil. Contaminants also stick tightly to clay surfaces.
While held securely to soil particles, contaminants are broken down by bacteria and reactions with minerals and natural chemicals in the soil. Most of this chemical and biological breakdown takes place in the loose, cultivated surface layers, where the soil tends to be warm, moist, high in organic matter, and well-aerated.
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