Leaching refers to irrigating with more water than is needed for plants to grow. The purpose of leaching is to flush salts accumulated from irrigation and evapotranspiration down to levels below the roots of plants, where the salts cannot be absorbed by and harm the plants.
The portion of irrigation water that mobilizes the accumulated salts is known as the leaching fraction. To calculate the leaching fraction, divide the depth of water passing down below the roots by the depth of water applied at the surface of the soil.
When using recycled water for irrigation, both the salinity of the water and the salinity of the soil must be considered to estimate what is termed the leaching requirement: the amount of water needed to keep salts within the plants' tolerance range. The more saline the recycled water, and the more saline the soil, the more the water needed for adequate leaching.
Rainfall must be taken into account, too. In years of average or heavy rainfall, leaching may occur even without irrigation. On the other hand, when the rainfall is less than average, application of irrigation water will, of course, be necessary.
The type of irrigation used also makes a difference. With sprinkler irrigation, salts increase relatively uniformly with depth of soil. With drip irrigation, salts tend to accumulate around the perimeter of the area watered.
The soil has an effect as well. It must be permeable enough to allow an adequate amount of leaching to occur.
Bear in mind the water table, too. A shallow water table can create problems. For example, turfgrasses near the coast are particularly salt-stressed due to the shallow, saline water table, as well as to the concentration of salt in the air. In most areas it will be prudent to avoid irrigating so much that it causes the water table to rise.
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