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Editor's Note: The gray information boxes throughout this article present target ranges of elements for growing cannabis, growth concerns, information on how to interpret test results and suggestions for correcting irrigation water problems.

Water is the single largest input for growing cannabis and therefore an essential resource for growing healthy plants. Plants, by weight, are comprised of 90% to 95% water. Elements in irrigation water can affect plant growth, especially in container-grown plants, due to those plants’ restricted root masses and the high potential for change within the soilless substrate because of its relatively low buffering capacity, or the ability to regulate pH changes. Because of this, it is important to regularly monitor your irrigation water to be sure it is not creating nutrient imbalances and inhibiting plant growth.

Water Sources

Water quality varies depending on the source. The three sources of water growers most commonly use are well water, municipal water, and surface water.

Well Water

In many areas of North America, well water frequently contains high levels of dissolved elements, especially bicarbonates (HCO3–), calcium (Ca), and magnesium (Mg), which can lead to elevated substrate pH levels and tie up iron (Fe) availability to the plant. These elements are commonly found in areas with limestone bedrock, such as the Great Plains or the Canadian prairie provinces. The chemical composition (pH, electrical conductivity (EC), alkalinity, and dissolved nutrients) of well water also varies with well depth, due to the water being pumped from different aquifers, and can even vary seasonally due to changes in the water table. (More information about alkalinity can be found in the article “How to Control Alkalinity in Greenhouse-Grown Cannabis” in the October 2019 issue of Cannabis Business Times.)

Municipal Water

Municipal water obtained from rivers or lakes generally has a lower level of dissolved elements than well water. Because municipalities treat water with chloride (Cl) or fluoride (F) to meet drinking water quality standards, excessively high levels of Cl and F may be present, and can cause leaf margin necrosis (browning and/or death) in susceptible plant species. By law, water treatment plants must monitor the chemical quality of their water, and you can contact your local water treatment plant to obtain test results.

Surface Water

Surface or pond water usually contains lower levels of dissolved elements. Growers should be aware of the chemical composition of their surface water to ensure that they are providing adequate levels of Ca and Mg. In addition, caution should be used to protect the water source from herbicide runoff or pollution, which can be detrimental to plant growth.

Elemental Components

Container-grown plants are most frequently limited by imbalances in EC, alkalinity, sodium (Na), and boron (B). High EC levels inhibit seed germination and root growth of both cuttings and established crops. Alkalinity directly influences the pH of the root substrate; as irrigation water alkalinity increases, so does root substrate pH. High levels of Na can antagonize (reduce) the uptake of potassium (K), Ca, and Mg. Leaf necrosis occurs when high levels of B are present in irrigation water. Levels of nitrate-nitrogen (NO3-N), phosphorus (P), K, Ca, Mg, sulfur (S), copper (Cu), iron (Fe), manganese (Mn), Cl, F, and zinc (Zn) are rarely a problem in irrigation water.

Water Testing

Testing your water source’s chemical composition annually is a good production practice for cannabis growers to consider. Because of the potential for water quality to change with the seasons, also consider sampling quarterly for the first one to two years to establish a baseline of how your values change over time, especially if you are irrigating with well water.

Irrigation Water Sampling Procedure

To obtain a water sample for evaluation, follow this general procedure (illustrated above):

  1. Run your irrigation line for about 5 minutes to clear the line of impurities.
  2. Label a 500 mL container, such as a rinsed plastic water bottle or lab-issued sampling bottle, with your name and/or operation name, address, water source, and analysis requested.
  3. Rinse the bottle two or three times with the irrigation water to be sampled.
  4. Fill the plastic water bottle, cap it, and then seal the cap in place with tape.
  5. Provide all requested information on lab-issued documents, and submit it to a commercial laboratory for analysis within 24 hours.

Testing labs may have their own submission procedures, so it is important to ask about their practices before submitting the sample.

Evaluation

The target ranges of elements for growing cannabis, growth concerns, information on how to interpret these test results and suggestions for correcting irrigation water problems are presented in the gray columns throughout this article.

With water being the single largest input used for growing a cannabis crop, knowing the quality of that input is important to ensure your nutritional program is on track.

Dr. Brian E. Whipker is a professor of floriculture at North Carolina State University specializing in plant nutrition, plant growth regulators and diagnostics. During the past two years, he co-authored eight scientific journal articles on the impact of fertilization with greenhouse species and three disorder diagnostic guides. Dr. Whipker has more than 28 years of greenhouse experience working with growers.

Paul Cockson is a graduate research and teaching assistant at North Carolina State University. He has a degree in plant and soil sciences with a concentration in agroecology. For the past few years, he has worked in the plant nutrition lab at NCSU with Dr. Brian Whipker.

Patrick Veazie is an undergraduate researcher at North Carolina State University.

David Logan is an undergraduate research assistant at North Carolina State University.

Dr. W. Garrett Owen is an assistant professor and extension specialist of floriculture, greenhouse, and controlled-environment crop production in the Department of Horticulture at the University of Kentucky.