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Why does evaporation cause salt to build up in irrigated soils?

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Large wheeled irrigation sprinkler sprays water over green crops in a field, with mountains and cloudy sky in the background. — Irrigation Specialized irrigation equipment watering crops in a field.

In irrigation agriculture, salt buildup occurs because all irrigation water contains dissolved salts, which are collected as the water runs off over land and through soil. Unlike pure water, which evaporates or is transpired by plants, these salts remain behind in the soil.1234

As evaporation and transpiration remove the water, the concentration of these residual salts increases. Because most plants exclude these salts at their roots, the salts accumulate in the soil profile. In arid regions, where there is insufficient rainfall to flush these minerals downward, the salt concentration can reach levels that inhibit plant growth by causing dehydration and preventing nutrient uptake.1324

To maintain a favorable salt balance and prevent land from becoming barren, farmers must ensure that the amount of salt leaving the area—typically through leaching with excess water—equals or exceeds the amount introduced by the irrigation supply.132