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An irrigation water quality system tries to predict the eventual effect of an irrigation water on the soil and crop being irrigated. It is impossible to predict the actual effect because of differences in irrigation type, crop response, weather, amount of water applied, soil and land characteristics, etc. Irrigation water analysis must be followed up by soil testing to monitor changes and take corrective action.
The ServiTech irrigation water quality interpretations are general guidelines and intended for planning purposes only. These interpretations assume: agronomic crops with moderate salt tolerance and medium-textured soils with mixed clay mineralogy.
The guidelines also assume 75% to 90% irrigation application efficiency; that 10% to 20% of the irrigation water applied leaches through the upper crop root zone.
The irrigation water electrical conductivity (EC) is used to project the soil conductivity (ECe) after longterm irrigation. The salts dissolved in the irrigation water will precipitate or “crystallize” in the soil as the pure water evaporates. This soil conductivity as projected by the water conductivity is used to anticipate which types of crops might be affected by using the irrigation water.
Different plants and different varieties of plant can differ greatly in their relative salt tolerance. Salt tolerance categories and crop selection are discussed in Crop File 4.03.041, "Choosing Crops for Salt Affected Soils".
The adjusted sodium adsorption ratio (SARa) is used to predict the potential effect of the irrigation water sodium on the soil permeability and water infiltration. Over time the irrigation water SARa value and the soil exchangeable sodium percentage (ESP or % Na) are expected to reach similar levels.
Sodium accumulations affect the permeability of finetextured soils more rapidly than coarse-textured soils having lower clay content. The acceptable irrigation water SARa gets lower as soil clay content increases.
Total salinity can have an effect on water infiltration. At higher salinity levels, soil aggregates (or granules) tend to be more stable and better retain the pore spaces necessary for water infiltration. The potential impact of sodium on permeability increases as salinity decreases.
The quality classes (“Excellent” to “Very poor”) are relative rankings that include the simultaneous impact on both the type of soil and the type of crop under irrigation.
Keep in mind that a specific irrigation water source could be classified as “very poor” for a salt sensitive crop, but could be considered “acceptable” for a more salt tolerant crop.
Use the EC and SARa values from the water report to obtain the ratings for salinity hazard, permeability hazard, and general quality from Table 1.
The ratings range from “Very low” to “Extremely high” and from “Excellent” to “Very poor” quality. The tables in Crop File 5.01.012, Irrigation Water Quality: Interpreting Laboratory Results, contains descriptions of these interpretative ratings.
The chloride and boron concentrations in the irrigation water should be used to further modify the quality ranking as given in Tables 2 and 3.
A diagram of relative irrigation water quality rankings is shown in Crop File 5.01.014, Agronomic Irrigation Water Quality: ServiTech Interpretations.
¶ Table 1. General Agronomic Irrigation Water Quality Guidelines |
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| EC = 0 - 250 μmho/cm1 | Salinity Hazard: Very low | ||
| SARa | Permeability Hazard | Quality rating | |
| 0 - 1 | Medium - high | Fair | |
| EC = 250 - 750 μmho/cm | Salinity Hazard: Very low | ||
| SARa | Permeability Hazard | Quality Rating | |
| 1 - 3 | Very low | Excellent | |
| 3 - 6 | Low | Good | |
| 6 - 9 | Medium | Acceptable | |
| 9 - 12 | High | Fair | |
| 12 - 15 | Very high | Poor | |
| Over 15 | Extremely high | Very poor | |
| EC = 750 - 1500 μmho/cm | Salinity Hazard: Low | ||
| SARa | Permeability Hazard | Quality Rating | |
| 0 - 3 | Very low | Good | |
| 3 - 6 | Low | Acceptable | |
| 6 - 9 | Medium | Fair | |
| 9 - 12 | High | Poor | |
| Over 12 | Extremely high | Very poor | |
| EC = 1500- 3000 μmho/cm | Salinity Hazard: Medium | ||
| SARa | Permeability Hazard | Quality Rating | |
| 0 - 4 | Very low | Acceptable | |
| 4 - 6 | Medium | Fair | |
| 9 - 12 | High | Poor | |
| Over 12 | Extremely high | Very poor | |
| EC = 3000 - 5000 μmho/cm | Salinity Hazard: High | ||
| SARa | Permeability Hazard | Quality Rating | |
| 0 - 4 | Low | Fair | |
| 4 - 6 | Medium | Poor | |
| Over 6 | Extremely high | Very poor | |
| EC = 5000 - 7500 μmho/cm | Salinity Hazard: Very high | ||
| SARa | Permeability Hazard | Quality Rating | |
| 0 - 5 | Medium | Poor | |
| Over 5 | High | Very poor | |
| EC = over 7500 μmho/cm | Salinity Hazard: Extremely high | ||
| SARa | Permeability Hazard | Quality Rating | |
| All | Very high | Very poor | |
¶ Table 2. Potential Chloride Hazard for Sprinkler Irrigated Crops |
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| Chloride, mg/l Cl | Hazard Potential |
Crop effect |
| less than 35 | Very low | No foliar injury (leaf burn, spotting, etc.) is expected. |
| 35 - 70 | Low | Foliar injury symptoms not likely to develop. |
| 70 - 150 | Moderate | Acceptable for most crops and plants. Foliar injury may occur when applied to sensitive plant types (e.g., certain tree species, ornamentals, etc.). |
| 150 - 350 | High | Likely to cause foliar injury when applied to moderately sensitive crops. Foliar damage expected with sensitive plant types. |
| 350 - 550 | Very high | May cause foliar injury when applied to moderately tolerant crops. Foliar injury may be severe in sensitive and moderately sensitive plant types. |
| 550 - 700 | Extremely high | May cause foliar injury when applied to tolerant crops. Not advised for use with sensitive or moderately sensitive plant types. Foliar injury may be severe with moderately tolerant plant types. |
| over 700 | Not advised | Likely to cause foliar injury to many crops and commercially grown plants. |
¶ Table 3. Potential Irrigation Water Boron Hazard |
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| Boron, mg/l B | Rating | Crop Effect |
| under 0.5 | Very low | Suitable for all field crops. |
| 0.5 - 0.75 | Low | Suitable for most field crops. |
| 0.75 - 1.0 | Moderate | Slight to moderate hazard for many field crops. |
| 1.0 - 2.0 | High | Moderate to severe hazard for many field crops. |
| 2.0 - 6.0 | Very high | Severe hazard for many field crops. |
| over 6.0 | Extremely high | Severe hazard for most crops. |
11000 :mho/cm = 1.0 mmho/cm . 1.0 dS/m