Salts in soil occur as electrically-charged forms of atoms or compounds known as ions. Weathering minerals in soil release these salts; they are present in fertilizer, and they well upward from shallow groundwater. They accumulate in arid or semi-arid regions where there is not enough water to leach them from the soil. The effect of salt in irrigation water is benign to severe, depending on the amount of salt that is built up in the soil and ability of individual plants to tolerate it.
Effects on Plants
When the salt in irrigation water exceeds a plant's tolerance, the plant's growth is stunted. Plants injured by salt show symptoms resembling those caused by drought even though the soil is moist. They wilt. They grow poorly. If the salt concentration becomes high enough, the plant can die
Plants expend more energy to get water from the soil that has salt in it. Salty soil must be kept wetter for plants to get the same amount of water as they would in soil that is not salty. The energy they expend to get water would otherwise be used to grow, flower and fruit.
Effects Increased by Toxic Minerals
A low level of salt in irrigation water can affect plants if the soil contains chlorine, boron and sodium, which have toxic properties.
Effect on Use of Fertilizers
High levels of salt in the soil can upset the balance of nutrients in plants and interfere with their ability to absorb fertilizers that contain salts.
Effects on Specific Plants
Not all plants respond the same to salt in irrigation systems. Alan D. Blaylock of the University of Wyoming gives a list of some popular vegetables and plants grouped by their ability to tolerate salt.
Plants that are sensitive to salt include apples, cherries, carrots, currants, onions, gooseberries, parsnips, pears, plums, roses, raspberries and strawberries.
Plants moderately sensitive to salt include broccoli, cabbage, cantaloupe, cauliflower, celery, corn, cucumber, kale, lettuce, peas, peppers, potatoes, pumpkin, radish, spinach, squash, tomato, turnip and watermelon.
Plants that are moderately tolerant of salt include beets, crested wheatgrass, evergreens, juniper, Russian olive, perennial ryegrass, tall fescue, squash and zucchini.
Plants that can tolerate salt include alkalai grass, asparagus, creeping bentgrass and iceplant.