Without proper amounts of water to leach these salts (known as the leaching fraction) from the upper soil horizons, the growth of the plants can be slightly to severely impacted. These soils suffer from the accumulation of salts due to limited leaching. The Food and Agriculture Organization of the United States estimated 45 million hectares of the 230 million hectares of irrigated croplands are salt-affected. In dry soils of semi-arid regions salts can accumulate in the top horizons of the soil. ![]() In agricultural ecosystems, leaching is an important balance between preventing salt accumulation and removing nutrients from soil. Leaching can transport chemical compounds like dissolved substances or larger materials such as decomposing plant materials, fine rock fragments, and microbes throughout the Critical Zone. In addition, the physical movement of water can dislodge and move particles. Compounds on the surface of minerals can be become dissolved. As the water passes through the rock and soil, it interacts with the surfaces of the materials. Leaching is actually two important actions occurring simultaneously: (1) chemical interactions with surfaces and (2) physical movement of water. Although this process seems trivial, leaching is one of the key processes of the Critical Zone, controlling the rate and direction in which compounds move. Currently, leaching primarily describes the process of water carrying soluble substances or small particles through soil or rock. The word “leaching” is believed to have been derived from either late Middle English “leche” or Old English “leccan” meaning to moisten and to allow leaking. Leaching is not to be confused with the use of leeches as medicine. Rain falls on vegetation, pools on the ground, and leaches into and down through the soil profile. ![]() Rain falls on a cross section of a tree atop a soil profile. Water leaching through the Critical Zone.
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