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Leaching of Soils

By Mike Charmaine

Leaching of soil can be defined as the loss of any soluble substance (such as salts or nutrients) as well as colloids by the flushing of fresh water through it. When soil becomes so wet that it cannot hold any more water, it will percolate downward with all the soluble material. The substances lost generally get redeposited in a lower layer.

Useful Effects of Leaching

Leaching of soil can be a desirable or an undesirable phenomenon, depending upon the final outcome it has on the cultivability of the land. If leaching is being done to purge the soil of dissolved salts to make it more arable or increase its agricultural usefulness, then it is considered good and is artificially induced by flushing the soil with fresh water. When water-soluble fertilizers have been used in growing plants, it can eventually lead to residual salt content in the soil, and leaching in this case will reduce such toxic material from the soil composition.

Harmful Effects of Leaching

Leaching can also result in the loss of much-needed nutrients such as nitrates and other minerals that are essential for plant growth, which is undesirable because it can make the soil unsuitable for cultivation. Further, as these dissolved salts or nutrients percolate downward, they might contaminate groundwater, which in turn can have adverse effects on the health of animals and humans consuming it.

Factors Affecting Leaching of Soil

The rate at which leaching of soil occurs depends on factors such as rainfall, the ambient temperature and the protective vegetation cover. Other factors are water quality and the composition and structure of the soil.

Leaching Requirement

The amount of water required to bring down the salinity of the soil (measured in terms of the electrical conductivity of the soil solution), in accordance with the salt tolerance of the crops to be grown, is technically defined as the leaching requirement.

Model to Measure Leaching

A mathematical model can be built to measure the rate of leaching of soil based on various parameters affecting it, such as the initial salt concentration, time of leaching exposure, amount of water supplied and the rate at which water can percolate through the soil (defined in terms of leaching efficiency, which is a function of the soil composition and structure).