What Is the Meaning of Leaching?

The term "leaching" often comes up in discussions of soil and fertilizer. What exactly is leaching? In soil, leaching is a natural process that can be helpful or harmful, depending on the circumstances.

Leaching refers to the process by which chemicals, nutrients and other soluble matter in soil are dissolved by excess water and washed away.

Leaching of soil occurs naturally and is part of the process of soil formation. When soil becomes saturated with rainwater, water travels downward, into groundwater or into springs and eventually rivers, lakes and streams. In the process, substances such as iron, aluminum, calcium and organic matter are carried deep into the soil or washed away altogether.

Leaching is a beneficial technique for washing excess salts and fertilizers from the soil of potted plants. When you apply fertilizer repeatedly to potted plants, salts from the fertilizer build up in the soil and can burn plant roots. You can sometimes see a ring of salt on the outside of clay pots or around the top of the soil. The College of Agriculture & Life Sciences at the University of Arizona recommends leaching potted plants every four to six months to prevent salts from accumulating. To leach a potted plant, pour approximately twice the amount of water as the pot can hold on the top of the soil and let it drain completely.

Leaching becomes a problem when pesticides and fertilizers from the soil are leached into water supplies. The leaching of nitrogen is of particular concern. Nitrogen, an important plant nutrient, occurs naturally in soil as a result of decomposing plant and animal matter and as a result of soil amendments such as animal manure and nitrogen fertilizers. During times of heavy rainfall, nitrogen is converted into nitrate, a mobile form of nitrogen which moves easily with the flow of water. When nitrate is leached into groundwater and streams, it becomes potentially harmful to pregnant animals and their young. When it enters private wells and water supplies, nitrate poses a threat to humans---especially babies less than 1 year of age. According to the University of Missouri Extension, nitrate poisoning in infants causes internal suffocation or "blue baby syndrome."

Organic farming methods can be helpful in reducing the amount of pesticides and nitrogen leached into water supplies. The USDA National Institute of Food and Agriculture reports that, in a study by Washington State University, nitrogen leaching was four to five times greater in trees fertilized using synthetic fertilizers than those fertilized using organic methods. Some ways you can reduce harmful soil leaching are by increasing the organic content of your soil, using natural slow-release fertilizers, and avoiding the use of pesticides whenever possible.