Soil and Plant Testing Laboratory
573-882-0623

Interpretation guide for irrigation water

Irrigating with poor quality water usually doesn't have an immediate deleterious effect on plants. Rather, it results in a long-term hazard in which salts or sodium in the water accumulate in the soil and eventually decrease soil productivity. Because constituents in water are deposited onto the soil, interpreting water analysis is inextricably tied to soil properties. Sandy soils are less likely to accumulate salts or sodium than finely textured soils, and they can be more easily leached to remove salts or sodium. Soils with a high water table or poor drainage are more susceptible to salt or sodium accumulation.

The following list describes different water analyses and their relevance to irrigation water.

The hazard of irrigating with saline water

Low

0 to 0.25 mmhos/cm electrical conductivity
Less than 175 ppm total dissolved salts
Acceptable water for all crops on all soils.

Medium

0.25 to 0.75 mmhos/cm electrical conductivity
175 to 500 ppm total dissolved salts
Will be detrimental to salt sensitive crops, field beans, peanuts, peaches. Leaching will be required to reduce salt accumulations in soil.

High

0.75 to 2.25 mmhos/cm electrical conductivity
500 to 1500 ppm total dissolved salts
Will have adverse effects on moderately sensitive crops, most grain, forage and vegetable crops. Should not be used on soils with restricted drainage. Special soil and water management required.

Very high

More than 2.25 mmhos/cm electrical conductivity
More than 1500 ppm total dissolved salts
Should be used only for salt tolerant crops, cotton and barley, on sandy, permeable soils with careful soil and water management. Test soil annually for salt accumulation.

Total dissolved solids
Total dissolved solids (effectively dissolved salts) is a measure of salinity. Dissolved salts conduct electricity in relation to their concentration, so electrical conductivity is another measure of salinity. Water salinity is derived primarily from the ions of calcium, magnesium, sodium, chloride and bicarbonates. Saline water induces a physiological drought in plants. Furthermore, salts applied in irrigation water are left behind in the soil following evapotranspiration, which leads to soil degradation. If saline water is to be used, it should be generously applied in order to leach salts and prevent salt accumulation.

Guidelines regarding the hazard of irrigating with water of varying SAR and EC values relative to the development of soil permeability problems

0 to 3 sodium adsorption ratio

No restriction
Electrical conductivity less than or equal to 0.7
Slight to moderate restriction
Electrical conductivity 0.7 to 0.2
Severe restriction
Electrical conductivity m ore than 0.2

3 to 6 sodium adsorption ratio

No restriction
Electrical conductivity less than or equal to 1.2
Slight to moderate restriction
Electrical conductivity 1.2 to 0.3
Severe restriction
Electrical conductivity more than 0.3

6 to 12 sodium adsorption ratio

No restriction
Electrical conductivity less than or equal to 1.9
Slight to moderate restriction
Electrical conductivity 1.9 to 0.5
Severe restriction
Electrical conductivity more than 0.5

SAR
The sodium adsorption ratio (SAR) expresses the sodium hazard of water. It is calculated from sodium, calcium and magnesium concentrations in water. Calcium and magnesium counter sodium’s effect on soil. Sodium in irrigation water can accumulate in soil and result in undesirable physical soil characteristics. When wet, soil with high sodium levels has reduced water permeability and when dry soil becomes very hard. Sodium can also accumulate in soil to sufficiently large amounts such that plant uptake of sodium becomes toxic to the plant. Fine textured soils under low leaching conditions are most susceptible to degradation from irrigating with water that has moderate SAR values (3 to 6). From the perspective of inducing soil permeability problems, SAR and electrical conductivity both need to be considered. Low salinity water (usually low in calcium and magnesium) increases the deleterious effect of sodium in water.
Sodium
Medium to high levels of sodium in water with low levels of calcium and magnesium can result in toxicity of some sensitive plants such as fruit trees and woody ornamentals. Annual crops are usually not affected except for sodium's affect on salinity and sodium buildup in soil.
Chloride
Although an essential nutrient to crop growth, toxic levels of it in water can restrict plant growth. Water chloride concentrations up to 70 ppm are safe for all plants. From 70 to 140 ppm chloride, sensitive plants may incur some injury. From 140 to 350 ppm chloride moderately tolerant plants will likely incur injury. Severe problems can be expected at concentrations above 350 ppm chloride. Woody and vine plants and stone fruits are susceptible to chloride toxicity.
Calcium and magnesium
These minerals exist as positively charged ions in water, and they counteract the deleterious effect of sodium. Their concentrations are used in the calculation of SAR.
Carbonates and bicarbonates
High levels of bicarbonate and carbonate in water increase the sodium hazard of water to a level greater than that indicated by SAR. Bicarbonates and carbonates combine with calcium and magnesium and precipitate from the soil solution as a whitish residue. This increases the concentration of sodium. The presence of carbonate versus bicarbonate is an indication of pH. Carbonate is present in water at a pH greater than 8.0.
pH
A general water quality indicator, pH indicates whether water is acid or alkaline. Values less than 5.5 or greater than 8.5 should be investigated as to the source of the abnormal values. Acid water with a pH less than 6.0 will be corrosive to pipes.
Nitrate
Nitrate in irrigation water is plant available. When concentrations are large enough, the nitrate in water can supplement the nitrogen applied in a regular fertilization program. In each acre-foot of one ppm NO₃-N water, there is 2.72 lb of nitrogen. At concentrations greater than 30 ppm NO₃-N, toxicity problems can be expected.
Sulfate
Sulfate exists in water as a negatively charged ion. It contributes to the total salt content.
Potassium
Potassium behaves much like sodium, but it is usually found in only small amounts in water.

Updated 4/26/05