Sodic soils—also called alkali—are soils rich in sodium. Since sodium compounds are highly soluble, sodic soils are only found in arid areas, and especially in areas with clay soil. Sodic soils usually have a pH higher than 8.5. Sodium destroys the structure of clay, allowing all the clay particles to bond to one another without clumping or forming crumbs. Unstructured clay is almost impervious to water. As a result, of this lack of drainage, the rain that falls evaporates rather than sinking into the water table, so the soluble compounds remain in the soil.
In addition to causing poor drainage, many forms of sodium are toxic to plants. Sodium chloride, common table salt, is frequently a component of sodic soils. Only a few adapted plants can tolerate it.
Sodic soils are usually white or light-colored. The Bonneville Salt Flats of Utah is an alkali salt flat. Its surface is white with sodium, potassium, and magnesium salts. Rain covers it with water each winter, but the water is unable to drain away, so remains in place until it evaporates, leaving a fresh crust of white salt.
In some areas, the sodium salts are sodium carbonate, which is highly corrosive, dissolving organic matter in the soil. The resulting dissolved humus forms dark deposits on the soil surface, which earn the name of “black alkali”.
Sodium can also enter the soil from irrigation water. Water drawn from slightly brackish sources, or from water softeners, will make soil sodic. Water softeners “soften” the water by replacing the calcium in the water with sodium. Softened water should never be used for irrigation water. If you have softened water, draw water from outside faucets (which is not softened) to water your house plants.
To test whether a drainage problem is due to sodic conditions, dig a hole a foot or so deep and fill it with water. Let it drain to saturate the soil, then fill it again and time the rate at which the water drops. After it has drained, dig a cup of powdered gypsum into the soil at the bottom of the hole and fill it with water again. Let it drain, then fill it again and again time the rate at which it drains. If it drains faster after adding the gypsum, the slow drainage was due to sodium.
Improving Sodic Soil
The calcium in gypsum (calcium sulfate) replaces the sodium in the soil, helping the clay in the soil to form a strong structure and improving its drainage. Till 25 pounds of powdered gypsum per 100 square feet into the soil as deep as practicable, then irrigate the soil thoroughly. The leaching is an important step to carry the now-free sodium down and out of the root zone. Water with at least several inches of water at one time. If puddling and runoff occurs, pause the irrigation until the puddles are absorbed, then continue.