A high SAR will result in the dispersion of soil colloidal particles and can increase the soil pH to values above 8.5. As soil colloidal particles disperse, the soil becomes increasingly resistant to water infiltration and percolation. These sodium-affected soils become hard and compact when dry and very resistant to water penetration due to dispersion and swelling when wet. The reduced permeability is generally a problem of the upper few centimeters of soil. This results in a decreased water supply to the crop just as a salinity problem does, but for a different reason.
Permeability reduces the quantity of water placed in storage.
While salinity reduces the availability of water in storage. High SAR may also cause a toxicity problem to certain crops. This may be just as important in reducing yields as in an actual water shortage to the crop due to poor soil permeability. Permeability refers to the ease with which water enters and percolates down through the soil and is usually measured and reported as an infiltration rate.