Soil leaching refers is the movement of nutrient elements from topsoil through the soil profile. Leaching causes significant nutrient losses, particularly in humid regions with high precipitation. Leaching is a natural process caused mainly by precipitation, acidification and nitrogen saturation. Human activity greatly influences some of these factors and the amount of leaching that occurs.
Soil leaching occurs most directly because of precipitation. Whenever more water enters a system through precipitation than is lost through evaporation and transpiration, the excess water leaves the area by moving downward and collecting in groundwater or emerging in surface water. This downward-moving water carries dissolved substances, such as nutrients, from the surface and topsoil. Commonly lost substances include iron, potassium, calcium, aluminum and organic matter.
Soil acidification increases the amount of leaching that occurs. Soils are naturally acidic or become increasingly acidic because of acid rain and air pollution, organic matter buildup or the presence of excess nitrogen. Acidic solutions dissolve compounds and displace nutrients. A greater magnitude of nutrients, such as calcium, magnesium and potassium, leaches out of the topsoil if the soil water becomes increasingly acidic.
One serious soil-leaching problem relates to nitrogen, primarily in the form of nitrates. Nitrates naturally occur, but are often applied to croplands as fertilizer. Nitrates leach easily, depending on precipitation and soil type. Nitrogen saturation increases acidification and the leaching of aluminum and nutrients. Positively charged ions of calcium, magnesium and potassium are drawn to the negatively charged nitrates as they leach out of the topsoil.
Several factors influence or indirectly cause soil leaching. Soil type affects leaching. Clay soils better retain nutrients than do sandy soils. If vegetation is present, more nutrients become tied up on site. Human activities including air pollution, some fertilization practices, harvesting of trees or other biomass, and mining all increase soil acidification. Overfertilization or improper application processes can swamp land with excessive nitrates that leach following rain events or snow melt.