As water moves down through the soil, it carries away some of the nutrients that plants use, such as nitrates and sulfur. This process is called leaching. Under normal circumstances, minor levels of leaching occur with typical rainfall, and the breakdown of organic materials on the surface resupplies the soil. In the case of excessive rainfall or irrigation, the effects of soil leaching can be more dramatic.
Like a swimming pool, soil maintains a pH level. In essence, lower pH means higher acidity. For the purposes of agricultural plant production, such as farming, a slightly acidic soil typically generates the best results. When leaching removes too much nitrate content from the soil, however, the pH drops too far and the soil become over-acidic. Soil acidification yields numerous negative consequences in itself, including alteration to the types of soil microbes, surface water contamination and declining populations of earthworms.
As water carries nutrients away from the topsoil, some of the nutrients stay in the lower levels of the soil. The rest of the nutrients find their way into the groundwater, leading to a permanent loss of those nutrients for surface plants. This excess nitrate content in groundwater poses some threats to human health. Infants lack the ability to properly process nitrates and convert it into nitrite, which bonds with hemoglobin and limits oxygen distribution in the body. Also of concern, a number of pesticides enter groundwater through the leeching process. Pesticide exposure has serious health consequences ranging from birth defects to cancer.
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Soil salt removal presents one beneficial application of leaching. High salt content in soil limits the ability of seeds to germinate, as well as plant growth and crop yields. Controlled leaching, typically through irrigation, removes or reduces to the total salt content in the soil, allowing for healthier crops. In some cases, the process also requires a method of artificial drainage.
While not directly responsible for erosion, the effects of nutrient leaching provide the opportunity for erosion to occur. For example, the acidification of soil can limit the types of plants that grow in a particular area, which leads to poorly developed root systems. This poor root system development, combined with the reduction in earthworm populations, which contribute to the quality of soil, increases the likelihood of runoff and wind removing topsoil.