Two Types of Salt Problems

Two types of salt problems exist which are very different: those associated with the total salinity and those associated with sodium. Soils may be affected only by salinity or by a combination of both salinity and sodium.

Salinity Hazard

Water with high salinity is toxic to plants and poses a salinity hazard. Soils with high levels of total salinity are call saline soils. High concentrations of salt in the soil can result in a "physiological" drought condition. That is, even though the field appears to have plenty of moisture, the plants wilt because the roots are unable to absorb the water. Water salinity is usually measured by the TDS (total dissolved solids) or the EC (electric conductivity). TDS is sometimes referred to as the total salinity and is measured or expressed in parts per million (ppm) or in the equivalent units of milligrams per liter (mg/L).

EC is actually a measurement of electric current and is reported in one of three possible units as given in Table 2. Subscripts are used with the symbol EC to identify the source of the sample. EC_iw is the electric conductivity of the irrigation water. EC_e is the electric conductivity of the soil as measured in a soil sample (saturated extract) taken from the root zone. EC_d is the soil salinity of the saturated extract taken from below the root zone. EC_d is used to determine the salinity of the drainage water which leaches below the root zone.

Sodium Hazard

Irrigation water containing large amounts of sodium is of special concern due to sodium's effects on the soil and poses a sodium hazard. Sodium hazard is usually expressed in terms of SAR or the sodium adsorption ratio. SAR is calculated from the ratio of sodium to calcium and magnesium. The latter two ions are important since they tend to counter the effects of sodium. For waters containing significant amounts of bicarbonate, the adjusted sodium adsorption ratio (SAR_adj) is sometimes used.

Continued use of water having a high SAR leads to a breakdown in the physical structure of the soil. Sodium is adsorbed and becomes attached to soil particles. The soil then becomes hard and compact when dry and increasingly impervious to water penetration. Fine textured soils, especially those high in clay, are most subject to this action. Certain amendments may be required to maintain soils under high SARs. Calcium and magnesium, if present in the soil in large enough quantities, will counter the effects of the sodium and help maintain good soil properties.

Soluble sodium percent (SSP) is also used to evaluate sodium hazard. SSP is defined as the ratio of sodium in epm (equivalents per million) to the total cation epm multiplied by 100. A water with a SSP greater than 60 percent may result in sodium accumulations that will cause a breakdown in the soil's physical properties.

Table 2. Terms, units, and useful conversions for understanding water quality analysis reports.