How to interpret a water test report
Irrigation water quality can make or break a garden farm. But how to interpret a water test report? This page will help you to understand the test report and will help you to flag potential problems.
“Salts” are soluble cations (negatively charged ions) and anions (positively charged ions). Here we will concern ourselves with salts that are present in irrigation water, either from natural sources or added.
- Note that “salts” in the context of irrigation water are not just sodium or sodium chloride, table salt. They include all of the soluble cations and anions.
- If irrigation water is high in salts, they will build up in the soil through repeated irrigations. The only way to remove them is by periodically irrigating enough to move the dissolved salts downward through the soil profile below the root zone. This is called “leaching”. Some amount of salts will also be removed with the crop.
- With the exception of sodium, carbonates and bicarbonates, all cations and anions in the table below are necessary for plant growth.
- Yields may increase with increasing EC, up to a point.
- High soluble salts damage plants by osmotic effect. Water will move from an area of lower salt concentration (the root) to an area of higher salt concentration (the soil). This causes water stress even though there is plenty of water in the soil.
- Soluble salts can burn leaves when applied to foliage.
- Salt deposits can cause leaf and fruit discoloration, reducing crop quality and market value.
- If irrigation water supplies nitrogen in excess of the crop need, yield and quality may be reduced.
- High SAR (sodium adsorption ratio) may clog the soil pore structure and limit infiltration.
- Salts can accumulate in the soil, changing the nutrient balance of the soil.
- Tolerance of vegetables, fruits and herbaceous plants to salinity may be found at this FAO link.
Cations (positively charges ions) and anions (negatively charged ions) are referred to as “salts” when dissolved in water. In a water test, cation and anion concentrations in may be reported with units of ppm, mg/L or meq/L. 1 mg/L = 1 ppm. The conversion to and from meq/L is given below.
1. To convert from meq/L to mg/L or ppm, multiply by the conversion factor, e.g. 5 meq/L Ca = 5 * 20 = 100 ppm Ca. To convert from ppm or mg/L to meq/L, divide by the conversion factor.
2. Boron can also be in several anionic and neutral forms in water.
3. To convert to the amount of nitrogen nutrient in nitrate, use a conversion factor of 14.
4. To convert to the amount of sulfur nutrient in sulfate, use a conversion factor of 32.1.
- A measure of the balance of acidic hydrogen ions and alkaline hydroxide ions.
- pH of neutral, balanced water is 7.0. pH ranges from 14 (very alkaline) to 0 (very acid).
- Water pH has an effect on availability of plant nutrients.
- Units are no units. An increase or decrease in pH by 1 is a factor of 10x difference in hydrogen ion or hydroxide ion concentration.
- Typically irrigation water should be in the range of 5.0 to 7.0 pH, depending on the crop.
- A measure of the ability of the soluble compounds in the water to neutralize acids.
- Bicarbonates and carbonates in the water have the greatest influence on alkalinity.
- Units are ppm of calcium carbonate (CaCO3) equivalent. It may also be reported as meq/L (1 meq/L CaCO3 = 50 ppm CaCO3) or bicarbonate (HCO3) equivalent (1 ppm CaCO3 = 1.22 ppm HCO3).
- Alkalinity in the range 30 – 150 ppm is good for orchids. Levels of 30-60 ppm are mentioned for greenhouse cannabis growers.
- This link shows reduced tolerance for high alkalinity in plants grown in small containers. This is likely because small volumes of soil are poorly buffered to pH changes.
- The sum of the molar concentration of Ca2+ and Mg2+. When expressed in ppm or mg/L, it is the equivalent mass of calcium carbonate (CaCO3) that, when dissolved in a unit volume of pure water, would result in the same total molar concentration of Ca2+ and Mg2+.
- Hardness is what produces scale in pipes, white spots on glassware and soap scum.
- Units are ppm (CaCO3 equivalent), mg/L (1 ppm = 1 mg/L), grains per gallon or gpg (1 ppm = 0.05842 gpg).
Classification of hardness by the USGS:
|Classification||Hardness in ppm|
- A measure of the bulk salinity of water.
- EC measures the ability of water to conduct electricity. This is affected by the amount of dissolved salts or impurities.
- Units are micro-Siemens per cm (µS/cm) or milli-Siemens/cm (mS/cm). 1 mS/cm = 1000 µS/cm. Units may also be written as mmho/cm (1 mmho/cm = 1 mS/cm) and deciSiemens/meter or dS/m (1 dS/m = 1 mS/cm).
- Seawater will have an EC of approximately 50 mS/cm while rain water will be near zero.
- EC is related to TDS.
Total Dissolved Solids (TDS) or Soluble Salts (Logan Labs saturated paste test) or Salt Concentration (Logan Labs water test)
- A measure of the amount of substances that have been dissolved in the water.
- A true measurement involves evaporating the water and measuring the weight of the remaining solids. However, a measurement of EC and a conversion factor will give a good approximation. There are some dissolved solids that don’t contribute to EC but these are usually a small percentage.
- Units are parts per million (ppm) or milligrams per liter (mg/L). 1 ppm = 1 mg/L.
- TDS (ppm) = EC (dS/m) * 640, for EC between 0.1 and 5.0 dS/m. TDS = EC (dS/m) * 800, for EC > 5.0 dS/m.
- Good quality water will be in the range of 0 to 600 ppm. Readings over 1000 – 1200 ppm are unsatisfactory.
|Irrigation Salinity||EC (mS/cm)||TDS (ppm)|
|No effects usually noticed||0.75||500|
|Can have detrimental effects on sensitive crops||0.75 - 1.5||500 - 1000|
|Can have adverse effects on many crops||1.5 - 3.0||1000 - 2000|
|Can be used for tolerant plants (on permeable soils)||3.0 - 7.5||2000 - 5000|
- Salinity tolerance of vegetables, fruits and herbaceous plants may be found at this FAO link.
- Ratio of the concentration of sodium to the sum of calcium and magnesium in water.
- Sodium affects the ability of the soil to form clay aggregates and therefore affects the structure and permeability of the soil by clogging the soil’s pore structure. High sodium in the water can create crusts on soil which reduce crop yield.
- The equation for SAR is below. It is calculated using the concentration of Na, Ca and Mg in meq/L. A conversion from ppm to meq/L is given under the discussion of cations and anions above.
- Units – SAR does not have units.
- Same as SAR except the calcium concentration is adjusted for the level of bicarbonate and EC of the water.
- An adjusted SAR should be calculated when levels of bicarbonate are greater than 1 meq/L.
- The adjusted Sodium Adsorption Ratio (adj SAR) can be calculated from the procedure given in Ayers and Westcot (1976): adj SAR = SAR [1 + (8.4 – pHc)]
- pHc is the theoretical pH that water could have if in equilibrium with CaCO3.
A relatively high SAR compared to a relatively low EC will give an irrigated soil a tendency to reduce the rate of water infiltration due to dispersion of clay particles in the soil. The table below shows the levels of SAR and EC(irrigation water) where restrictions on use may occur.
|SAR||None||Slight to Moderate||Severe|
|0 - 3||>0.7||0.7 - 0.2||less than 0.2|
|3 - 6||>1.2||1.2 - 0.3||less than 0.3|
|6 - 12||>1.9||1.9 - 0.5||less than 0.5|
|12 - 20||>2.9||2.9 - 1.3||less than 1.3|
|20 - 40||>5.0||5.0 - 2.9||less than 2.9|
This number should be between 0.8 and 1.2 as an indication of an accurate water analysis.