The origin and characteristics of saline and sodic soils were discussed at the end of chapter 6. There are a number of ways to deal with saline soils that don’t have shallow salty groundwater. One is to keep the soil continually moist. For example, if you use drip irrigation with low-salt water plus a surface mulch, the salt content will not get as high as it would if allowed to concentrate when the soil dries. Another way is to grow crops or varieties of crops that are more tolerant of soil salinity. Saline-tolerant plants include barley, Bermuda grass, oak, rosemary, and willow. However, the only way to get rid of the salt is to add sufficient water to wash it below the root zone. If the subsoil does not drain well, drainage tiles might need to be installed to get rid of the salty water leached from the soil. (However, this means that a high-salt water is being discharged into a ditch and may harm downstream water quality.) The amount of water needed to do this is related to the salt content of the irrigation water, expressed as electrical conductivity (ECw), and the salt content desired in the drainage water, expressed as electrical conductivity (ECdw). The amount of water needed can be calculated using the following equation:
Water needed = (amount of water needed to saturate soil) x (ECw/ECdw)
The amount of extra irrigation water needed to leach salts is also related to the sensitivity of the plants that you’re growing. For example, sensitive crops like onions and strawberries may have twice the leaching requirement of moderately sensitive broccoli or tomatoes. Drip irrigation uses relatively low amounts of water, so lack of leaching may cause salt buildup even for moderately saline irrigation sources. This means that the leaching may need to occur during the growing season, but care is needed to prevent leaching of nitrate below the root zone. For sodic soils, a calcium source is added—usually gypsum (calcium sulfate). The calcium replaces sodium held by the cation exchange capacity. The soil is then irrigated so that the sodium can be leached deep into the soil. Because the calcium in gypsum easily replaces the sodium on the CEC, the amount of gypsum needed can be estimated as follows: For every milliequivalent of sodium that needs to be replaced to 1 foot, about 2 tons of agricultural-grade gypsum is needed per acre. Adding gypsum to nonsodic soils doesn’t help physical properties if the soil is properly limed, except for those soils that contain easily dispersible clay and are also low in organic matter.