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Crop species, and even varieties within a species, vary in their tolerance of soil salinity. Salinity can cause yield reductions across a range of salt concentrations.
Soil salinity is measured by the electrical conductivity of the soil solution and is usually expressed as deciSiemens per meter (dS/m). The soil salinity threshold (ECt) is the electrical conductivity (or total salinity) above which yield reduction is expected for a given crop. Yield loss is usually expressed as the expected percentage decrease in crop yield for a specific level of soil salinity and crop type compared to the anticipated yield of that crop in non-saline conditions.
Salinity stress can affect permanent and multi-year crops (i.e., alfalfa stands) as salts accumulate in the soil. Prolonged exposure to salinity stress can cause water stress, leaf-tissue damage, reduced photosynthetic rates, impaired water transport in the plant, reduced nutrient and water uptake, and premature tissue death.
In some soils, under certain management conditions, salts may leach through the soil and out of the rooting zone of the plant. When excess irrigation (described as the leaching fraction or the amount of water applied in excess of soil water-holding capacity and crop consumption) or rainfall moves through the soil profile, soluble salts can be removed from the root zone.
In saline soils, osmotic problems can impair plant water uptake as the concentration of salt interferes with the plant’s ability to take up and move water through the crop.
Salinity first impacts a crop through the prevention of seedling germination or survival.
Salinity stress introduced during the growing season can reduce plant function and impact yield by impairing water uptake. For example, a short-term reaction to salinity is characterized by acute water stress and an overall reduced rate of root and leaf elongation.
Some salts (most commonly boron, chloride, and sodium) may be toxic to plants at certain concentrations in the soil. These specific ion toxicities may appear even at lower total salt loads if the toxic element is present at sufficient concentration.
The presence of excessive salts can also prevent a plant from accessing nutrient ions. For example, excess sodium in the soil has been reported to reduce potassium uptake by the crop and cause the plant to leak accumulated potassium back into the soil. This is caused by molecular level changes in root cell chemistry triggered by sodium. Soil iron, manganese, and other micronutrients will become less available for plant uptake when excessive salts are present. Under sodic soil conditions, it’s critical to carefully monitor sodium absorption ratio (SAR) or exchangeable sodium percentage (ESP).
In sodic soils, high levels of exchangeable sodium can cause the dispersion of soil clays. Clay dispersion causes reduced water infiltration and aeration and increases the potential of soil crusting.
Increases in soil salinity can lead to reduced water intake and indirect risks such as soil degradation or drainage problems. These problems are exacerbated when sodium is the dominant cation. Poor drainage can lead to salt accumulation as salts are not easily leached past the root zone.
Increases in soil exchangeable sodium can cause clay dispersion or deflocculation, which leads to reduced water infiltration, the collapse of soil structure, drainage problems, and soil crusting.
Classified as soils with high sodium levels but low total salt concentrations. Sodic soils can be identified through soil testing.
Sodic soils are soils with a pH greater than 8.5, an electrical conductivity, or EC, less than 2.0 dS/m, and an ESP greater than 15 (SAR>13). High concentrations of sodium can become toxic to some crops and can cause soil physical degradation.
Saline-sodic soils are soils with high total salt concentrations and high exchangeable sodium content. Saline-sodic soils can be identified through soil testing.
Saline-sodic soils are soils with a pH less than 8.5, an electrical conductivity, or EC, greater than 2.0, and an ESP greater than 15 (SAR>13). Under these conditions, osmotic stress can occur, but issues with infiltration are often moderated by the presence of salts which can flocculate soil clays. If treated incorrectly, these soils can become sodic, and problems will likely worsen. High concentrations of sodium can also become toxic to some crops.
Classified as soils with high total salt concentrations but low sodium levels. Saline soils can be identified through soil testing.
Saline soils are defined as soils with a pH of less than 8.5, an electrical conductivity, or EC, greater than 2.0 dS/m, and an ESP less than 15 (SAR<13). Non-tolerant plants growing on saline soils will suffer from osmotic or water stress.
Soil salinity is a condition in which water-soluble salts accumulate in soil profile or crop root zone in amounts sufficient to adversely affect crops. Soil salinity can impact crop yield, plant growth, and water and nutrient uptake. It can also cause drainage problems, short- and long-term salinity stress, ion toxicities, and soil degradation. Issues with soil salinity are most often observed in low-lying areas of a field, but they may also appear in mid-slopes where soil water is seeping to the surface. Growers can use soil testing to identify types and amounts of soil salts.