All water contains some dissolved salt, sodium chloride. Irrigated plants leave some salt in the soil; saline irrigation water leaves even more salt. This salt accumulates in the soil, especially if the soil drains poorly. The buildup of salt makes it more difficult for plants to absorb moisture from the soil and for leaves to transpire water.
Effects of Salt
Salt in water is an electrolyte, meaning it is able to conduct electricity. To assess the salinity of a soil, a sample taken from the area near the roots is measured for its EC, or ability to conduct electricity. Samples taken below the root zone are measured to determine the salinity of the water that leaches from the soil. When plant roots move salty water into the plant it can stunt the plant's growth and burn its leaves. Seedlings and small plants are more susceptible to salt damage. High sodium can reduce the water that the roots are able to take up, leading to waterlogged soil. Plants receive fewer necessary nutrients. Weeds that are tolerant to salt move in.
Plants can only shed or transpire pure water through the stomata or pores in their leaves. Although a soil may look wet, a high EC rating of salty water may mean less pure water is available to plants. The yield drops when the leaves of a crop are unable to transpire water.
Effects of Sodium
Salt, or sodium chloride, in irrigation water is just one source of sodium. Sodium nitrate and sodium borate, also called borax, are other common sources of sodium. Excessive sodium in water is measured by the ratio of sodium to magnesium and calcium, SAR (sodium adsorption ratio). When irrigation water with a high SAR ratio is used over a long period of time, it adds to the sodium that may be present from other sources. Sodium becomes attached to soil particles, pushing them apart; calcium and magnesium both hold them together. Soil that is dispersed tends to crust and block the penetration of water that plants need; this is a special problem with clay and other soils with a fine texture. Magnesium and calcium are used as soil amendments to counter excessive sodium.
Effects and Soil
Water evaporates; dissolved salt does not. Irrigating with salty water leads to the water in the soil becoming increasingly salty, especially if the soil does not drain well. As salt accumulate in soil, it needs to be irrigated more often to help flush it out. Sand or soils with gravel that drain easily have fewer problems with accumulating salt. In a phenomenon called the "wickling effect", spots and areas that dry out tend to accumulate salt from evaporating water.
Effects of Sprinkling
Sprinkler irrigation with saline water causes more problems with both sodium and chloride. Leaf burn can be reduced by sprinkling on cool, cloudy doors or at night. Irrigating with salty water using sprinklers,especially those that form mists, can burn plant leaves, especially those with broad leaves. Turf is less affected.