Management of Salt-Affected Soils - Practices of Irrigation and On-farm Water Management - page 295

The combination of high levels of sodium and low total salts tends to disperse soil
particles, making sodic soils of poor tilth. These soils are sticky when wet, nearly
impermeable to water and have a slick look. As they dry, they become dense, hard,
cloddy, and crusty. This dense layer is often impermeable to water and plant roots.
Dispersion occurs when the clay particles swell strongly and separate from each
other on wetting. In addition, scalding can occur when the topsoil is eroded and
sodic subsoil is exposed to the surface, reducing plant available water content and
increasing erodibility.
Crop growth and development problems on sodic soils can be nutritional (sodium
accumulation by plants), associated with poor soil physical conditions, or both.
Plants on sodic soils usually show a burning or drying of tissue at leaf edges, pro-
gressing inward between veins. General stunting is also common. Crops differ in
their ability to tolerate sodic soil, but if sodium levels are high enough, all crops
can be affected. Problems of sodic soil include loss of grazing, salty water supplies,
death of trees, erosion, siltation, and so on.
Sodic soils may impact plant growth by
i. specific toxicity to sodium sensitive plants,
ii. calcium deficiencies or nutrient imbalances caused by excessive exchangeable
sodium
iii. high pH, and
iv. dispersion of soil particles, resulting in poor physical conditions in the soil.
If a soil is highly sodic, a brownish-black crust sometimes forms on the surface
due to dispersion of soil organic matter. By the time darkened crusts are visible
on the soil surface, the problem is severe, and plant growth and soil quality is sig-
nificantly impacted. Dispersion of soil particles often results in crusting and poor
emergence of seed.
8.5 Crop Tolerance to Soil Salinity and Effect of Salinity on Yield
Excessive soil salinity reduces the yield of many crops. This ranges from a slight
crop loss to complete crop failure, depending on the type of crop and the severity
of the salinity problem. Although several treatments and management practices can
reduce salt levels in the soil, there are some situations where it is either impossible or
too costly to attain desirably low soil-salinity levels. In some cases, the only viable
management option is to plant salt tolerant crops.
8.5.1 Factors Influencing Tolerance to Crop
Tolerance of plants to soil salinity is not a fixed characteristic of each species or
a variety, but may vary with the growth stage and environmental conditions. The
factors influencing tolerance to crop are