Irrigation Water Quality: How to Read EC, SAR and Sodium on Your Water Test
Soil salinity problems often start with the water. Every irrigation adds dissolved salts to the root zone, and if those salts are not managed they accumulate season after season. Testing your irrigation water is one of the most valuable and least expensive steps a grower can take. This guide explains how to collect a sample and how to read the numbers that matter most on a water quality report.
Why test irrigation water
Water from wells, canals, ponds and reclaimed sources can carry very different amounts of salts, sodium and specific elements. Some wells change over time, especially near coasts or during drought when groundwater levels drop. A water test helps you:
- Estimate how much salt you are adding to your soil each season
- Identify sodium problems that can damage soil structure and reduce infiltration
- Spot elements such as chloride or boron that can injure sensitive crops
- Plan leaching, amendments and crop choices
Water testing pairs with soil testing. Our earlier article on soil salinity signs, testing and management covers the soil side.
How to collect a good sample
- Ask your laboratory for its sample bottle and instructions; many provide clean bottles at no charge.
- Run the well or pump long enough to flush standing water from the system before sampling.
- Rinse the bottle with the water being sampled unless the lab says otherwise, then fill it, leaving little or no air space.
- Label the bottle with the source, date and time, keep it cool and deliver it to the lab promptly.
- Sample each source separately, and consider testing at different times of year if quality may change seasonally.
Ask for an irrigation suitability analysis, which typically includes EC, pH, major cations and anions, SAR, boron and bicarbonate.
Reading the key numbers
Electrical conductivity (ECw)
EC measures total dissolved salts by how well the water conducts electricity. It is usually reported in deciSiemens per meter (dS/m), which is numerically equal to millimhos per centimeter (mmhos/cm). Widely used guidelines from the Food and Agriculture Organization (FAO Irrigation and Drainage Paper 29) treat water below about 0.7 dS/m as having no restriction on use for salinity, 0.7 to 3.0 dS/m as slight to moderate restriction, and above 3.0 dS/m as severe restriction. How much this matters depends on crop tolerance, soil texture, drainage and rainfall.
Total dissolved solids (TDS)
TDS expresses salt content in milligrams per liter (mg/L), roughly the same as parts per million. Labs often estimate TDS from EC; a commonly used approximation is about 640 mg/L per 1 dS/m, though the exact factor varies with the salts present.
Sodium adsorption ratio (SAR)
SAR compares sodium to calcium and magnesium in the water. It is calculated as sodium divided by the square root of half the sum of calcium plus magnesium, with each expressed in milliequivalents per liter. High SAR water can cause soil particles to disperse, sealing the surface and slowing water infiltration. Importantly, the effect of SAR depends on EC: water with low salinity and moderate SAR can cause more infiltration problems than saltier water with the same SAR. Your lab or adviser should interpret SAR and EC together.
Chloride and sodium (specific ion toxicity)
Some crops, including many tree crops, berries and avocados, are sensitive to chloride and sodium taken up by roots. Sprinkler irrigation can also cause leaf burn when these ions are absorbed directly through leaves, especially in hot, dry, windy conditions.
Boron
Boron is a micronutrient in small amounts but toxic to sensitive crops at relatively low concentrations in irrigation water. Compare your result to the tolerance of the specific crops you grow.
Bicarbonate and pH
High bicarbonate can cause calcium to precipitate in soil, effectively raising the sodium hazard over time, and can leave white deposits on leaves with overhead irrigation. A pH outside the normal range for irrigation water is often a signal to look more closely at the water's chemistry.
What to do with the results
- Manage the leaching fraction. Applying enough water to move salts below the root zone, with adequate drainage, is the foundation of salinity management.
- Use calcium where sodium is the problem. On sodium-affected soils, calcium amendments such as gypsum can replace sodium on soil particles so it can be leached away.
- Blend or switch sources. If you have more than one water source, blending or reserving the better water for sensitive crops and growth stages can help.
- Match crops to conditions. Crops differ widely in salt tolerance; choose varieties suited to your water.
- Adjust irrigation methods. Drip irrigation can reduce leaf injury compared with overhead sprinklers when water contains sodium or chloride.
- Re-test. Test water at least annually, and more often if the source is changing.
Questions growers often ask
How often should I test irrigation water?
At least once a year for each source, and more often if you notice changes in water level, taste, deposits or crop response, or after drought or storm surge.
Can rain fix a salinity problem?
Significant rainfall can leach salts from the root zone, which helps, but it does not change the quality of your irrigation water, so salts return with each irrigation season.
Talk with a specialist
Water and soil reports are most useful when interpreted together and in light of your crop, climate and irrigation system. University extension services and certified crop advisers are excellent resources. Universal Agriculture offers soil salinity assessment, custom soil amendment solutions including our proprietary Salt Fighter blend, and sustainable farming consultation; see our services or contact us to discuss your water and soil results.
When a healthier field produces a crop worth selling, growers can list it free on CropListing, a farmer-to-buyer crop marketplace in our group.
This article is general educational information. Thresholds vary by crop, soil and climate; consult your laboratory, extension office or a certified crop adviser before making management decisions.