About Salt-Affected Soils
Salt is a naturally occurring element in soil. Although small amounts of salt are not harmful, high levels of salt can harm soil structure and make the soil inhospitable to plants. After salt has accumulated in soil, no chemical treatment or soil conditioner can remove it. Salt-affected soils, however, often can be reclaimed through careful soil management practices.
The most common salts found in soil come from sodium, potassium, calcium, magnesium, sulfate, carbonate and bicarbonate. Salt enters soil naturally, through the weathering of rocks and from dust and rainwater. It often accumulates due to poor drainage. Salt also is transported to the soil surface from the water table through capillary action. Soils in low-lying coastal areas acquire salt from seawater. Man-made causes of salt-affected soil include the salting of roads in winter, excessive use of fertilizers and irrigation of crops with water containing dissolved salts.
Saline soils contain high amounts of soluble salts, particularly calcium and magnesium, in plants' root zone. These soils often have white, salty deposits on their surface. Although water can penetrate saline soils, the high salt content interferes with plants' ability to absorb water. Therefore, plants often wilt, even though adequate moisture is present. Excessive salt also interferes with seed germination, stunts plant growth and reduces crop yields.
Sodic soils are high in sodium and often have a black, oily appearance. Their surface is typically hard and crusty, and their pH level tends to be high -- often exceeding 9. High sodium levels cause soil particles to disperse, destroying soil structure. Sodic soils are toxic to plants because they do not allow water to penetrate and drain easily, and they are usually low in nutrients.
The first step in restoring salt-affected soil is correcting drainage problems. Digging trenches that allow water to run off, or installing underground, perforated pipes, may be needed to improve water flow in the soil. Saline soils usually are corrected by applying excess amounts of water, a process known as leaching. Leaching causes dissolved salts to drain from the root zone. Correction of sodic soil is often difficult and expensive. Sodic soil usually is corrected by replacing the sodium with calcium and then leaching the soil. Calcium is added by mixing gypsum or calcium chloride into the soil, but the process can be costly. About 5 tons of pure gypsum must be added per 1 acre to correct 12 inches of sodic topsoil, according to a Colorado State University Extension Service online article.
Although leaching removes some salt from soil, it cannot make the soil salt-free. Heavily affected soils can take years to correct. The use of salt-tolerant plants helps to protect soil during the restoration process. Prevent salt accumulation through good soil management practices, such as maintaining proper drainage and soil moisture, using fertilizers prudently and avoiding heavy tillage. Irrigation water should be tested to ensure it is low in salt. The addition of peat, compost or sawdust helps to improve soil structure, but manure can have a high amount of salt.
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