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Water analysis results mean nothing unless they can help predict the suitability of a water for irrigation. The most important criteria for judging water quality are the electrical conductivity, adjusted sodium adsorption ratio, chloride, and boron content. Irrigation water use guidelines are given in Crop File 5.01.013, "Irrigation Water Quality: Guidelines".
Electrical conductivity (EC) is an estimate of the total amount of ions present in a water sample. Two electrodes are immersed in the water sample to measure the amount of electrical resistance (often expressed as micromhos per centimeter or µmho/cm1). An electric current cannot pass between electrodes in distilled water. As ion content (i.e., "salt content") increases, more current passes between the electrodes, and electrical conductivity rises.
The irrigation water conductivity helps predict the potential salt buildup in a particular soil. Salts dissolved in irrigation water are left behind as an irrigated soil dries. They accumulate in the crop root zone if not leached downward by rainfall or excess irrigation water. High salt levels interfere with the ability of roots to extract soil water. Crop growth and yields drop off as salt levels increase, depending on the relative salt tolerance of the particular crop.
Table 1 lists relative salinity hazard ratings and expected crop response.
¶ Table 1. Salinity Hazard Ratings |
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| Very low: | Should not affect even very salt sensitive crops. |
| Low: | May affect growth of very salt sensitive crops (includes many fruit, vegetable. and ornamental crops). |
| Medium: | May affect growth of moderately sensitive crops (like alfalfa, com, or peanuts) . Occasional leaching my be needed to prevent salt accumulation. |
| High: | May affect growth of moderately tolerant crops (like sorghum or wheat). Leaching will be needed to prevent salt accumulations or to grow less salt tolerant crops. |
| Very high: | May affect growth of salt tolerant crops (like barley cotton, or sugarbeets). Heavy leaching will be needed to prevent salt accumulations or to produce less salt tolerant crops. |
| Extremely high: | Plant salt tolerant crops. Heavy leaching and specialized management practices are needed to prevent salt accumulations. |
The unadjusted sodium adsorption ratio (SAR) expresses the relationship of the calcium (Ca), magnesium (Mg), and sodium (Na) ions in the water sample. This helps predict potential impact of the water on the soil structure. A high SAR means a high proportion of sodium to calcium-plus-magnesium and a greater hazard of reduced soil permeability.
Dissolved bicarbonate (HCO3-) ions combine with calcium and magnesium ions as an irrigated soil approaches dryness, then may precipitate as insoluble carbonates (CaCO3, MgCO3). This effectively removes calcium and magnesium from both the soil solution and exchange surfaces of the clay particles. Sodium ions in solution can then fill the clay surface sites previously occupied by calcium and magnesium ions. If exchangeable sodium accumulates to excess levels, the soil structure may deteriorate.
The adjusted sodium adsorption ration (SARa) includes Ca, Mg, Na, and HCO3 concentrations. In high bicarbonate waters, the SARa is often two to four times higher than the unadjusted SAR. We find the SARa predicts the impact of irrigation water sodium on the soil more accurately than the unadjusted SAR.
Table 2 lists relative salinity hazard ratings and expected crop response.
¶ Table 2. Permeability Hazard Ratings |
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| Very low: | No use restrictions. |
| Low: | Use with caution on fine-textured soils. Routine applications of gypsum may be needed to prevent surface crusting. |
| Medium: | Use with caution on fine-textured or medium-textured soils. Routine applications of gypsum and moderate leaching may be needed to maintain permeability. |
| High: | Use with caution on coarse-textured or medium-textured soils with good internal drainage. Unrestricted use on fine-textured soils is likely to result in severe damage to soil structure. Gypsum and leaching will be needed to maintain permeability. |
| Very high: | Use with caution on coarse-textured soils with good internal drainage. Unrestricted use on fine-textured or medium-textured soils is likely to result in severe damage to soil structure. Gypsum applications, leaching, and other management practices will be needed to maintain permeability. |
| Extremely high: | Specialized soil and water management practices are needed to maintain soil permeability. |
Chloride and boron may be toxic to certain plants if they accumulate in the soil to excessive levels. High boron water occurs in a few isolated areas of the High Plains, so boron toxicity is relatively uncommon. High chloride water is often a problem in areas with extensive oil well drilling and exploration.
Chloride that accumulates in the soil from irrigation can be taken up by the roots and translocated to the foliage. Chloride toxicity symptoms appear as leaf burning, usually starting at the margins and spreading over the whole leaf. Certain crops and varieties have internal mechanisms to prevent soil chloride toxicity by blocking chloride movement from the roots to the foliage, so may not be affected by soil chloride accumulations.
Foliar toxicity can occur under sprinkler irrigation that results in droplets of high chloride water accumulating on the leaf surface. The pure water evaporates from the droplets, causing the chloride levels to concentrate in those droplets. Chloride can then be absorbed through the leaf cuticle and can accumulate in leaf tissue to toxic levels.
Table 3 gives relative foliar chloride tolerances for common sprinkler-irrigated crops. There is little research comparing foliar chloride tolerance of different crops, but foliar tolerance can differ from soil tolerance.
¶ Table 3. Relative Foliar Chloride Tolerance Under Sprinkler Irrigation |
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| Very sensitive | Sensitive | Semi-tolerant | Tolerant |
| Potato | Alfalfa | Barley | Cotton |
| Tomato | Sesame | Corn | Sugarbeet |
| Fruit crops | Soybeans | Safflower | Sunflower |
| Citrus crops | Sorghum | ||