A Water Quality Toolkit for Greenhouse and Nursery Production

Water quality plays a critical role in determining successful production of nursery and greenhouse crops, and should be considered in fertilization management and disease control.
A Water Quality Toolkit for Greenhouse and Nursery Production - Articles

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Greenhouse and nursery for vegetables and ornamentals at the Southeast Agricultural Research & Extension Center in Landisville, PA

Water quality plays a critical role in determining successful production of nursery and greenhouse crops, and should be considered in fertilization management and disease control. A complete water analysis by an accredited laboratory is recommended prior to any property purchase or the decision to begin a nursery or greenhouse operation and periodically throughout your growing season. In addition to periodic testing at a lab, it is a good idea to have a water quality testing toolkit on hand to test for some parameters on a regular basis, even up to a couple of times per week. Keep records of your test results in a notebook or binder so that you can detect changes over time. Four water test parameters are recommended: pH, alkalinity, hardness and electrical conductivity (EC).

pH

The pH of water is a measure of how acidic or basic the water is. It is measured on a scale from 0-14 in pH units. Pure water has a pH of about 7.0 and is considered neutral. Solutions with a pH less than 7.0 are acidic, and solutions with pH greater than 7.0 are basic (have less acid). In growing media and soil, pH is important to plant growth, because of its impacts on fertilizer and nutrient availability. The pH of your water is important due to effects on pesticides, and its reaction with disinfectants. However, pH of water by itself is often of little consequence during production. It is the relationship between the pH of the water and the alkalinity of the water, primarily high alkalinity, which will have an impact on controlling the pH of soils and other growing media. Water for greenhouse and nursery production should have a pH between 5.4 - 7.0 depending on the crop. Acid solutions can be added to irrigation water to lower the pH (increase acidity) into a better range for the crop. Acidic or basic fertilizers can also be used to influence media pH.

pH Testing Tools

Testing water for pH is very easy and some of the tools to do so can be found at your nearest pet supply, hardware, or pool and spa store. The most basic testing can be done with simple test strips that are dipped into the water sample and then compared with a color chart to determine the pH of the water. The range of the test strips is generally from 0-14. These test strips are relatively inexpensive with the average price for a container of 100 strips around $15. More expensive test strips can give more precise measurements in a smaller range (e.g., 5.0-10.0).

Another option is to use an electronic pH meter that consists of a probe that is placed into the water sample and then provides a digital reading of the pH. Prices for these electronic meters have a wide range with a very basic model costing around $20 and those that can provide more accurate readings ranging from $100-$150. Electronic meters require the purchase and use of standard buffer solutions to calibrate the meter periodically. Buffer solutions of pH 4, pH 7 or pH 10 are used depending on the expected pH of the sample. Be sure to read and save the calibration and storage instructions that come with the pH meter. The pH electrodes used in these meters have a limited lifespan of 1-2 years. You can tell when the electrode no longer works because the meter will no longer be able to be calibrated with a standard solution. Electrodes on some of the more expensive meters can be replaced without replacing the entire meter.

Alkalinity

Alkalinity is a measure of water's ability to neutralize acids or the capacity to change or resist a change in pH of the growing media. Alkalinity establishes the buffering capacity of the water. The term "buffering" refers to how resistant a solution is to changes in pH. Some alkalinity is good because it will buffer plant roots from sudden changes in pH duet to acidic fertilizers and media such as peat moss. Alkalinity in water that is either too high or too low will impact the

media pH which will affect the availability of nutrients. High alkalinity water often has a higher pH. If you are trying to reduce the pH of water by adding acid, more acid will be required if you are starting with high alkalinity water because the acid will react with the calcium or magnesium carbonate or other ions buffering the solution first before the pH decreases. The recommended upper limit for alkalinity for both greenhouse and nursery production is 100 mg/L (100 ppm).

Alkalinity Testing Tools

A very easy way to test for alkalinity is by using test strips. The strip is dipped into the sample and a rough measurement of alkalinity (e.g. 0, 40, 80, 120 etc.) is determined. Twelve test strips can be purchased for about $50.

More accurate testing for alkalinity involves titration, the addition of small, precise quantities of a reagent (sulfuric acid) to a water sample until the sample reaches a certain pH (the endpoint). The amount of acid used corresponds to the total alkalinity of the sample. Kits for testing alkalinity using manual titration are available for around $45. The reading for alkalinity is done by noting a color change resulting from addition of the reagent to determine the alkalinity of the sample.

Using digital titrators are another option. These titrators have counters that display numbers. A plunger is forced into a cartridge containing the reagent by turning a knob on the titrator. As the knob turns, the counter changes in proportion to the amount of reagent used. Alkalinity is then calculated based on the amount used. Digital titrators cost approximately $100; the reagents (chemicals) to conduct total alkalinity tests cost about $36.

Hardness

In terms of drinking water, hardness is a measure of the calcium and magnesium dissolved in water and expressed as though it were all calcium carbonate (CaCO3), one of the major components in limestone or chalk. "Soft" water is low in calcium and magnesium. "Hard" water often also has high alkalinity, though that is not always the case. In the case of irrigation water, if either calcium or magnesium is too high, it can block the plant's ability to take up the other element because the two are chemically similar. Calcium and magnesium levels are seldom high enough to be toxic to plants but very hard water can produce residues on plant leaves, clog irrigation equipment, and cause scaling and buildup on greenhouse boilers. Hardness may be reported in milligrams per liter (mg/L) which is the same as parts per million (ppm) or in a special unit called grains per gallon (gpg). One grain per gallon is equal to about 17 mg/L. Hardness in water for greenhouse and nursery use should not exceed 150 mg/L (CaCO3).

Hardness Testing Tools

Similar to pH, hardness can be tested using simple test strips that will measure hardness in mg/L by dipping the test strip into a water sample and then waiting for a color change to compare to a provided chart. A set of 50 test strips will cost about $15.

Another method is through titration where precise amounts of a reagent is added to the sample until a color change occurs which then gives you a reading of hardness in grains per gallon of CaCO3. This reading can then be converted to parts per million (ppm) of hardness. A test kit using titration is available for around $40 and has enough reagents to perform 50 water sample tests.

Electical Conductivity (EC)

Many substances dissolve releasing charged particles (ions) in water. This allows water to conduct electricity more effectively. We can use this to make a rough estimate of how many ions are dissolved in irrigation water by measuring electrical current. Since, in many cases, the dominant source of ions in irrigation water is the fertilizer we are using, electrical conductivity (EC) becomes a good representation for the amount of fertilizer in the solution. Total dissolved solids (TDS) is measured in the same way as EC but the results are slightly different and are reported in parts per million (ppm). TDS is the measure usually reported for drinking water quality purposes. In the greenhouse and nursery, EC is the preferred measurement scale. Both can be measured in a laboratory or estimated from the electrical current measured by a meter compared to the current in pure water and standard salt solutions. Salinity is a term for the saltiness (the amount of sodium chloride (NaCl)) of water and is usually reported in parts per thousand (ppt). Electrical conductivity is the most practical way to measure salinity or TDS of water. Water high in TDS often contains higher levels of dissolved salts such as sodium chloride. Plant cell membranes are affected by salinity of soil or growth media which can increase over time from repeated irrigation if it is not flushed by rainwater. High salt concentrations can reduce the growth and quality of most crops. Electrical conductivity is reported in millimhos per centimeter (mmhos/cm), millisiemens per centimeter (mS/cm) or decisiemens per meter (dS/M), which are all equivalent units. Electrical conductivity of greater than 1.0 mmhos/cm may indicate a high salinity hazard.

Electrical Conductivity Testing Tools

Electronic meters that measure for TDS/EC are the best tool for measuring this parameter. Basic meters that include TDS, EC and temperature compensation are available starting at around $20 and increase from there. The meter probe is simply dipped into the water sample and a digital reading of either EC or TDS is displayed. Be sure to read and save the calibration and storage instructions that come with the meter.

Purchasing Testing Tools

Many of the meters, probes, test strips and testing kits mentioned in this fact sheet can easily be found by doing an internet search for each and then determining which product will meet your needs both in terms of functionality and cost. As mentioned above, some basic test kits can be found at your local hardware, pool or pet supply store. Specific companies such as those that sell agricultural, forestry and water testing products also sell these water testing tools.

Additional Tips for Your Toolkit

Other items you may want to have on hand include: standard solutions for calibrating equipment, distilled or deionized water for rinsing meter probes, small beakers or cups for holding water samples, and extra batteries for electronic meters. Be sure to store meters and test strips in a safe place protected from sunlight, temperature and humidity extremes, and dust.

Comprehensive Testing

Additional information and more comprehensive testing of greenhouse and nursery irrigation water can be obtained online from Penn State's Agricultural Analytical Services Laboratory (AASL) or by phone at 814-863-0841.

Prepared by Susan Boser, Water Resources Extension Educator and Dana Rizzo, Water Resources Extension Educator

Thank you to Lee Stivers, Extension Educator and Thomas Ford, Extension Educator who contributed content to this publication.

Reviewed by Amy Galford, Water Resources Extension Educator; Bryan Swistock, Water Resources Extension Associate; Dr. Robert Berghage, Associate Professor of Horticulture; and Lee Stivers, Horticulture Extension Educator

Authors