By: Mark McFarland, Robert Lemon and Charles Stichler
Why well water can be salty
Irrigation water quality is determined by the total amounts of salts and the types of salts present in the water. A salt is a combination of two elements or ions. One has a positive charge (for example, sodium), and the other has a negative charge (such as chloride).
Water may contain a variety of salts including sodium chloride (table salt), sodium sulfate, calcium chloride, calcium sulfate (gypsum), magnesium chloride, etc. The types and amounts of salts in water, and thus the salinity of that water, depend on the source.
The quality of well water depends on the composition of the underground formations from which the water is pumped. When these are “marine” (ocean) formations, they usually will have higher salt levels and produce water that is more salty. The quality of surface water depends largely on the source of runoff. Drainage water from irrigated land, saline seeps, oil fields, and city and industrial wastewaters generally has higher salt levels.
What problems can salty water cause?
Salty irrigation water can cause two major problems in crop production—salinity hazard, and sodium hazard. When irrigation water is used by plants or evaporates from the soil surface, salts contained in the water are left behind and can accumulate in the soil. These salts create a salinity hazard because they compete with plants for water. Even if a saline soil is water saturated, plant roots may be unable to absorb the water, and plants will show signs of drought stress. Foliar applications of salty water often cause marginal leaf burn and, in severe cases, can lead to defoliation and significant yield loss. Sodium hazard is caused by high levels of sodium, which can be toxic to plants and damage medium and fine-textured soils. When the sodium level in a soil becomes high, the soil will lose its structure, become dense and form hard crusts on the surface.
What tests should be done on irrigation water?
To evaluate a salt hazard, a water sample should be analyzed for three major factors:
- Total soluble salts.
- Sodium hazard (SAR).
- Toxic ions.
Total soluble salts measures the salinity hazard by estimating the combined effects of all the different salts that may be in the water. It is measured as the electrical conductivity (EC) of the water. Salty water carries an electrical current better than pure water, and EC rises as the amount of salt increases. Many people make the mistake of testing only for chlorides, but chlorides are only one part of the salts and do not determine the entire problem.
Sodium hazard is based on a calculation of the sodium adsorption ratio (SAR). This measurement determines if sodium levels are high enough to damage the soil or if the concentration is great enough to reduce plant growth. Sometimes a factor called the exchangeable sodium percentage (ESP) may be listed or discussed on a water test; however, this is actually a measurement of soil salinity, not water quality.
Toxic ions include elements like chloride, sulfate, sodium and boron. Sometimes, even though the salt level is not excessive, one or more of these elements may become toxic to plants. Many plants are particularly sensitive to boron. In general, it is best to request a water analysis that lists the concentrations of all major cations (calcium, magnesium, sodium, potassium) and anions (chloride, sulfate, nitrate, boron) so that the levels of all elements can be evaluated.
What are the critical levels?
Agricultural crops differ greatly in their ability to tolerate salts. Some crops have special methods for managing high salt levels inside the plant that allow them to continue to grow and produce. In most cases, critical levels have been established for each crop and each type of salt test or problem. One of the most confusing factors is that there can be many different units of measurement for the same test. That is, the numbers have the same relative meaning, but the units of measurement used to express the value are different (much like saying 12 inches or 1 foot).
The Texas Cooperative Extension Soil, Water and Forage Testing Laboratory uses standard units of micromhos per centimeter (umhos/cm) for total soluble salts and parts per million (ppm) for individual ions. Other laboratories may use different units of measure that can be calculated by making simple conversions. Table 1 lists the different tests and corresponding critical values for different units of measurement. These values represent the maximum salt level in irrigation water that can be used without reducing crop yield. Keep in mind that these values are estimates. Actual crop response may vary depending on soil type, rainfall, irrigation frequency and weather conditions. Note cotton’s ability to tolerate higher levels of salt than other common Texas crops.
Irrigation water with a salt level near the critical value is referred to as “marginal” quality water. In some cases, marginal quality water can be used to produce a crop, recognizing that some loss in yield (10 percent to 75 percent) may occur. Plants can continue to grow in the presence of low salts, but the yield potential will not be maximized. Plants grown in salty soils or irrigated with salty water are stressed condition.
Management systems for marginal quality water must be carefully designed. Major factors that must be considered include soil type, internal drainage, irrigation system and methods (rates, frequency) and cropping systems. Growers should consult an experienced agronomist or irrigation specialist for assistance in planning a management strategy for using marginal quality irrigation water.
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