<P> In agriculture, leaching refers to the loss of water - soluble plant nutrients from the soil, due to rain and irrigation . Soil structure, crop planting, type and application rates of fertilizers, and other factors are taken into account to avoid excessive nutrient loss . Leaching may also refer to the practice of applying a small amount of excess irrigation where the water has a high salt content to avoid salts from building up in the soil (salinity control). Where this is practiced, drainage must also usually be employed, to carry away the excess water . </P> <P> Leaching is an environmental concern when it contributes to groundwater contamination . As water from rain, flooding, or other sources seeps into the ground, it can dissolve chemicals and carry them into the underground water supply . Of particular concern are hazardous waste dumps and landfills, and, in agriculture, excess fertilizer, improperly stored animal manure, and biocides (e.g. pesticides, fungicides, insecticides and herbicides). </P>

What is leaching and why is it a problem