Interpreting Irrigation Water Tests

Basic interpretation of how various water quality parameters can influence plant growth during irrigation.
Interpreting Irrigation Water Tests - Articles

Updated: May 31, 2016

Interpreting Irrigation Water Tests

Testing and Parameters

Greenhouse and crop producers across Pennsylvania utilize a variety of sources of water for irrigation. Data from U.S. Geological Survey show that each day over 27 million gallons of water are used for irrigation of about 68,000 acres in the state each day. Over 15,000 acres of irrigation occurs in micro-irrigation systems commonly found in greenhouses. Unfortunately, the quality of these water supplies is often overlooked as a potential source of plant growth issues.

Water quality is most important for crops grown in small amounts of growth media (e.g., plugs, small pots) or hydroponically. Greenhouse and high tunnel growth environments also increase the importance of water quality because irrigation is the only source of water. Crops grown outside are less affected by irrigation water sources if they experience dilution from natural rainfall. In any case, water testing should be the first step when considering the use of irrigation water to ensure that maximum crop yield will be realized and disastrous plant toxicity issues will not develop.

All irrigation water sources should be tested for pH, alkalinity, conductivity, hardness, chloride, and sodium at a minimum since these are common issues in Pennsylvania water supplies. A more thorough test is ideal and should also include total dissolved solids, boron, calcium, magnesium, sodium adsorption ratio (SAR), nitrate-nitrogen, ammonium-nitrogen, phosphorus, potassium, sulfur, iron, manganese, copper, molybdenum, and zinc. Most test results will be expressed as milligrams per Liter (mg/L) which is the same as parts per million (ppm) in aquatic solutions. Water test results should be considered in combination with soil or growth media test results.

The table below includes parameters in the Penn State Agricultural Analytical Services Laboratory irrigation test kit. Approximate levels of concern for each parameter are provided where applicable and discussed further in the text that follows the table. However, some plant species have water quality tolerances that differ from the general levels discussed. Growers are urged to research the specific water quality tolerances of their crops, especially if they are noticing growth or health problems in response to irrigation.

A summary of water quality parameters important for irrigation water sources. Prevalence of problems listed in column 3 is based on 409 irrigation water supplies submitted to the Penn State water testing laboratory between 2007 and 2014.
ParameterLevel of Concern% Exceeding Levels in PANotes
pHBelow 5.0 or above 7.02% are below pH 5.0, 82% are above pH 7.0Must be interpreted along with alkalinity level
Total Alkalinity (as CaCO3)Below 30 mg/L or above 100 mg/L11% are below 30 mg/L, 61% are above 100 mg/LAcid injection used to treat high alkalinity
Hardness (Ca and Mg)Below 50 mg/L or above 150 mg/L13% are below 50 mg/L, 62% are above 150 mg/LEquipment clogging and foliar staining problems at levels above 150 mg/L. Treatment with water softening but may result in increased sodium
Calcium (Ca)Below 40 mg/L (plant deficiency), above 100 (may cause P and Mg deficiency)38% are below 40 mg/L, 19% are above 100 mg/LSee hardness above
Magnesium (Mg)Below 25 mg/L (plant deficiency)62% are below 25 mg/LAdditions may be necessary in fertilizers to prevent deficiency
Electrical Conductivity (EC)Above 1.0 mmhos/cm (plugs), above 1.5 mmhos/cm (other plants)3% exceed 1.0 mmhos/cm, 2% exceed 1.5 mmhos/cmTypically caused by high salts or hardness
Total Dissolved Solids (TDS)Above 640 mg/L (plugs), above 960 mg/L (other plants)Same occurrence as conductivity aboveClosely related to water conductivity
Boron (B)Above 0.5 mg/L (poinsettia), above 1.0 mg/L (some plants), above 2.0 mg/L (most plants)7% above 0.5, 3% above 1.0 and 1% exceed 2.0Anion exchange or reverse osmosis treatment
Chloride (Cl)Above 30 mg/L (sensitive plants), above 100 mg/L most plants40% exceed 30 mg/L, 16% are above 100 mg/LDilution, reverse osmosis or distillation treatment
Sodium (Na)Above 50 mg/L23% are above 50 mg/LSee chloride for treatment options
Sodium Adsorption Ratio (SAR)Above 2.018% are above 2.0Elevated levels are most important if sodium is also elevated above 50 mg/L
Nitrate-Nitrogen (NO3-N)No concern for plant growth. Levels above 5.0 mg/L indicate potential contamination that may affect other water uses23% exceed 5.0 mg/L and 10% exceed the human drinking water standard of 10 mg/LCan vary significantly throughout the year. Nitrates above 5 mg/L may indicate broader contamination problems
Ammonium-Nitrogen (NH4-N)No concern for plant growth. Consider levels in overall fertilization program. Plant problems often related to fertilization rates.24% exceed 1.0 mg/L which may indicate the presence of other contaminants.Toxicity symptoms include stunting, root death, leaf yellowing and distortion of growing points. Correct by switching to nitrate fertilizer
Phosphorus (P)Above 5.0 mg/L may cause nutrient deficiencies. Levels above 1.0 mg/L problematic for discharge water.13% exceed 5.0 mg/L, 21% exceed 1.0 mg/L (most are surface waters)Most often reduced through dilution with other water sources
Potassium (K)No high level of concern for plant growth.76% of samples are below 10 mg/LLow levels or low availability due to high pH may limit production
Sulfur (S)Below 10 mg/L may require addition in fertilizer in rare cases.44% are below 10 mg/L.Rarely a parameter of concern. Addition in fertilizer may be needed for some plants
Iron (Fe)Above 0.30 mg/L for micro-irrigation (clogging), above 1.0 (foliar spotting and clogging), above 5.0 mg/L (toxicity).17% above 0.30 mg/L, 7% above 1.0 mg/L, 2% above 5.0 mg/LLarge scale removal is most efficient using a settling pond. Various oxidizing filters can also be used depending on other chemistry
Manganese (Mn)Above 0.05 mg/L can cause clogging of irrigation equipment, above 2.0 mg/L may be toxic to some sensitive plants.27% above 0.05 mg/L, 2% above 2.0 mg/LSee iron notes above. Manganese removal is more difficult and may require additional pH adjustment
Copper (Cu)Above 0.20 mg/L toxic to some plants5% above 0.20 mg/LCorrosion of pipes is a likely source
Molybdenum (Mo)Above 0.05 mg/L toxic to some plants2% exceed 0.05 mg/LLarge scale removal of molybdenum is generally not cost effective. Use dilution or alternative water supplies
Zinc (Zn)Above 0.30 mg/L9% exceed 0.30 mg/LMost likely from corrosion of galvanized pipe. Plant toxicity most likely where low pH occurs in growth media

pH

The pH of water is measured on a scale of 0 to 14. A pH of 7.0 is neutral while pH levels below 7.0 are acidic and levels above 7.0 are basic. Each whole number difference represents a ten-fold difference in acidity. The pH of water along with alkalinity affects the solubility and availability of nutrients and other chemical characteristics of irrigation water.

In general, most plants prefer slightly acidic conditions in a pH range of 5.0 to 7.0. Problems with low or high pH are exacerbated in plants grown in soil-free or small growing systems since growth media can often act to buffer pH problems. Higher water pH levels can be tolerated if the water alkalinity is not excessive.

High pH (>7.0) may reduce the availability of various metals and micronutrients causing deficiency symptoms. High pH is often accompanied by high alkalinity. High pH problems can be corrected by acid injection or in some cases by using an acid fertilizer. Rainwater in PA is acidic (pH 4.0 to 5.0).

Less commonly, low pH (< 5.0) may result in toxic high levels of metals like iron and manganese; this is usually found in combination with low alkalinity. Low pH problems can be corrected by switching to a basic fertilizer or liming the growing medium.

Total Alkalinity, Bicarbonates, And Carbonates

Perhaps the most important water quality parameter to affect irrigation waters in Pennsylvania is alkalinity. Alkalinity is a measure of the dissolved materials in water that can buffer or neutralize acids. These include carbonates (CO32-), bicarbonates (HCO3-), and hydroxides (OH-, rarely present in that form).

Alkalinity is typically reported as mg/L of calcium carbonate. Alkalinity can originate from carbonates or bicarbonates that dissolve from the rock where the groundwater is stored (e.g., rainwater dissolving limestone). While the separate carbonate and bicarbonate alkalinity test results are helpful in understanding the source of the alkalinity and the potential for other contaminants in the water, from an irrigation perspective the total alkalinity is the most important water test result. The ideal range for total alkalinity is approximately 30 to 100 mg/L but levels up to 150 mg/L may be suitable for many plants.

High alkalinity above 150 mg/L tends to be problematic because it can lead to elevated pH of the growth media which can cause various nutrient problems (e.g., iron and manganese deficiency, calcium and magnesium imbalance).

Low alkalinity (below 30 mg/L) provides no buffering capacity against pH changes. This is especially problematic where acid fertilizers are used. Alkalinity in pond water can vary a great deal throughout the day if photosynthetic algae and plants are present.

Hardness

Hardness is determined by the calcium and magnesium content of water. Since calcium and magnesium are essential plant nutrients, moderate levels of hardness of 100 to 150 mg/L are considered ideal for plant growth. These levels of hardness also inhibit plumbing system corrosion but are not high enough to cause serious clogging from scale formation.

High concentrations of hardness above 150 mg/L will build up on contact surfaces, plug pipes and irrigation lines and damage water heaters. These levels can also cause foliar deposits of scale. Removal of hardness by using a water softener is necessary only if the water is causing problems. Extremely soft water below 50 mg/L may require fertilization with calcium and magnesium as discussed below.

Calcium (Ca)

Calcium concentrations in water are most often a reflection of the type of rock where the water originates. Groundwater and streams in limestone areas will have high calcium levels while water supplies from sandstone or sand/gravel areas of the state will typically have low calcium concentrations.

Calcium levels below 40 mg/L will typically need fertilizer additions of calcium to prevent deficiency while high levels of calcium above 100 mg/L may lead to antagonism and resulting deficiency in phosphorus and or magnesium. High levels of calcium may also lead to clogged irrigation equipment due to scale formation (CaCO3 and other compounds precipitating out of solution).

Water softening (cation exchange) is typically used to reduce calcium levels in water but softening for irrigation should use potassium for regeneration rather than sodium to prevent damage by excess sodium in the softened water.

Magnesium (Mg)

Like calcium, magnesium in water tends to originate from the rock and generally only causes problems when it is below 25 mg/L necessitating the addition of magnesium in fertilizer. Magnesium can also cause scale formation at high concentrations which may require softening.

Electrical Conductivity (EC or Soluble Salts)

Electrical conductivity is a measure of electrical current carried by substances dissolved in water. Conductivity is also often referred to as "soluble salts" or "salinity". As more salts are dissolved, water will better conduct electricity resulting in a higher conductivity reading. Conductivity is usually reported in millimhos per centimeter (mmhos/cm) or milliSiemens per centimeter (mS/cm) which are equivalent units.

Elevated conductivity levels in water can damage growth media and rooting function resulting in nutrient imbalances and water uptake issues. The conductivity of typical clean water is 0 to 0.6 mmhos/cm. Conductivity of fertigation solutions varies with the fertilizer concentration and salt, but generally ranges from 1.5 to 2.5 mmhos/cm. To avoid problems from excessive salts, raw water before fertilizer additions should be below 1 mmhos/cm for plugs and below 1.5 mmhos/cm for other growing conditions. Raw water conductivity above 3 mmhos/cm can be expected to cause severe growth effects on many plants.

While excessive water conductivity is a common problem in the western United States, water supplies in Pennsylvania rarely reach levels of concern unless the same soil or media is irrigated repeatedly without winter exposure to rain and snow. Treating water with high conductivity typically requires either dilution with another lower conductivity water source (e.g, rain) or advanced treatment with reverse osmosis or distillation.

Total Dissolved Solids (TDS)

TDS is a measure of all of the dissolved substances in water. TDS and conductivity levels in water are typically closely correlated and a conversion factor of approximately 640 is often used to predict TDS from conductivity which is easier to measure. The formula is TDS (mg/L) = 640 * EC (mmhos/cm).

Using the conductivity levels of concern above, TDS levels should be below about 640 mg/L to avoid problems in plugs and below about 960 mg/L to avoid problems with other plant growing conditions. TDS levels above about 2,000 mg/L are very likely to cause plant growth problems. As with conductivity issues, high TDS waters will need advanced treatment or dilution to make the water useable for irrigation.

Boron (B)

Boron is a trace mineral that is rarely a problem in Pennsylvania irrigation waters. Boron is a micronutrient needed in small amounts. Boron toxicity may occur if the concentration in irrigation water or fertigation solution exceeds 0.5 to 1.0 mg/L, particularly with long-term slow-growing crops. High boron levels can be treated using anion exchange or reverse osmosis treatment systems but pH adjustment is sometimes needed to improve treatment efficiency.

Chloride (Cl)

Chloride can occur in water supplies naturally or from various activities (road deicing, gas well drilling wastes, etc.). Chloride can damage plants from excessive foliar absorption (sprinkler systems) or excessive root uptake (drip irrigation). Most plants can tolerate chloride up to 100 mg/L although as little as 30 mg/L can be problematic in a few sensitive plants. Chloride is difficult to remove from water so advanced treatment using membranes (reverse osmosis) or distillation is necessary. Dilution with low chloride water can also be used.

Sodium (Na)

Sodium has many sources in water including road salt applications, wastewaters, water softening wastes and naturally high pH waters dominated by sodium bicarbonate. High levels of sodium can damage the growth media and cause various plant growth problems.

If water with excess sodium and low calcium and magnesium is applied frequently to clay soils, the sodium will tend to displace calcium and magnesium on clay particles, resulting in breakdown of structure, precipitation of organic matter, and reduced permeability.

Sodium in excess of 50 mg/L may cause toxicity in sensitive plants, particularly in recirculating irrigation systems. Sodium can be further evaluated based on the sodium adsorption ratio (SAR) which is described below. Sodium is difficult to remove from water requiring reverse osmosis, distillation or dilution.

Sodium Adsorption Ratio (SAR)

SAR is used to assess the relative concentrations of sodium, calcium, and magnesium in irrigation water and provide a useful indicator of its potential damaging effects on soil structure and permeability. Typically a SAR value below 2.0 is considered very safe for plants especially if the sodium concentration is also below 50 mg/L.

Nitrate-Nitrogen (NO3-)

Nitrogen is a critical plant nutrient so nitrate in water can be beneficial for irrigation but should be accounted for in the overall fertilization program. Nitrate-nitrogen in water does represent broader concerns for both human consumption and surface waters.

The drinking water standard for nitrate-nitrogen is 10 mg/L. Typical values for clean water are 0.3 to 5 mg/L. Discharged waste water from greenhouses or nurseries entering surface waters or streams should be lower than 10 mg/L. The acceptable range for fertigation of most crops is 50 to 150 mg/L.

Ammonium-Nitrogen (NH4+)

The ammonium-N concentration in typical clean water ranges from 0 to 2 mg/L. The typical fertigation range is 0 to 75 mg/L. See comments, above, for fertilizer nitrogen. Toxicity in sensitive plants may occur when ammonium is used in fall, winter, or early spring. Toxicity symptoms include stunting, root death, leaf yellowing and distortion of growing points which can be corrected by switching to nitrate fertilizer.

Phosphorus (P)

Phosphorus levels in groundwater and unpolluted surface waters are usually very low (less than 1 mg/L) in Pennsylvania. Higher levels often indicate contamination from fertilizer or manure runoff. Levels above 5 mg/L may cause antagonism and deficiencies in other nutrients. Waste water to be discharged to surface waters should be as low as possible (less than 1 mg/L is desirable) to reduce environmental impact. Phosphorus levels in water need to be considered in the overall fertilization program.

Potassium (K)

High potassium is generally not a concern for plant growth. Levels above 10 mg/L may indicate water contamination from fertilizers or other man-made sources. Water concentrations are useful simply for determining the overall fertilization requirements for plants receiving the irrigation water.

Sulfur (S)

Sulfur is an essential plant nutrient. High concentrations are rarely a concern other than in coal mining regions where extremely high levels are occasionally observed. More often, sulfur levels are tested to determine if sulfur addition is needed in fertilizer. Very low sulfur levels below 10 mg/L are common in most of the state.

Iron (Fe)

Iron can be a complex water quality problem that not only affects plant growth but also can clog irrigation equipment. For micro-irrigation systems, iron levels need to be below 0.3 mg/L to prevent clogging. Levels above 1.0 mg/L can cause foliar spotting in overhead irrigation systems.

Very high iron above 5.0 mg/L can cause severe staining and plant toxicity in sensitive species. Iron toxicity problems are most likely to occur where growth media is acidic (below pH 6.0). Induced iron deficiency can also occur in sensitive species if pH is greater than 7.0 to 7.5.

Iron treatment is most easily accomplished by using a settling pond to aerate and settle the iron sediment before the water is used for irrigation. Various forms of oxidizing filters can also be used to oxidize and filter iron but these can be costly for large volumes of irrigation water. In cases where iron is clogging drip irrigation systems, acidification treatment can be used to keep iron in solution or chlorination/filtration can be used to remove iron and prevent clogging.

Manganese (Mn)

Manganese presents many of the same issues as iron in irrigation water. It can clog irrigation equipment and cause foliar staining. The recommended drinking water standard for manganese is 0.05 mg/L which is also the level where black staining and irrigation clogging may occur.

Concentrations above 2.0 mg/L can be directly toxic to some plant species. Removal of manganese utilizes the same treatment described for iron above, but manganese removal efficiency is generally lower than iron and may require pH adjustment.

Copper (Cu)

Copper in water most often originates from corrosion of copper plumbing lines, especially by acidic, low TDS water. It very rarely occurs in significant concentration in groundwater or surface water. Unfortunately, even low concentration of copper above 0.2 mg/L can be toxic to some plants. If copper is found in irrigation water, corrosion of metal plumbing should be investigated as a cause and replacement with plastic plumbing should be considered. The use of copper algaecides should be avoided in irrigation ponds.

Molybdenum (Mo)

Molybdenum is a trace mineral which can also cause plant toxicity in rare cases. Molybdenum concentrations above 0.05 can be problematic but are very rare in Pennsylvania irrigation water sources. Removal of molybdenum is difficult on a large scale for irrigation.

Zinc (Zn)

Zinc is another trace mineral that rarely occurs in groundwater or surface water. When zinc is found in irrigation water, corrosion from galvanized pipes in the irrigation plumbing should be investigated as a possible source. Mine drainage can also be a source of zinc in western Pennsylvania. Levels above 0.3 mg/L can be toxic to some plants especially in low pH growth media.

References

Reviewed by: William Lamont, Jr., Penn State, Stephen Reiners, Cornell University, Inge Bisconer and Bill Wolfram, Toro Micro-Irrigation

Authors

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