CANNABIS CULTURE – High-quality crops require high-quality water.
Water is an essential component for all plant growth. It delivers essential nutrients for growth and development, translocation of photosynthates, evaporative cooling of the plants, and to maintain structural support through turgor pressure. Water is considered the universal solvent as it can dissolve more substances than any other liquid, allowing for the delivery of those essential elements, but also some potentially deleterious materials. Therefore, the quality of irrigation water provided to the plants can set the plants up for healthy production or hinder it.
Of course, “high quality” has different meanings based on the types of crops grown and the method in which they are grown. Cannabis, for example, generally requires elevated levels of nitrogen, potassium, and calcium; while leafy greens may require much lower rates. Additionally, throughout the cannabis life cycle the needs of the plant change requiring changes in the available fertility profile; higher relative proportions of nitrogen during vegetative growth followed by elevated phosphorous levels during flower development. And recirculating water cultures (ie. NFT, aeroponics) can accumulate sodium and chloride, whereas these elements will typically be leached out of top-watered drain-to-waste irrigation systems. In hydroponic systems, the plants receive all of their required nutrition for growth and development from the nutrient solution, or water, further demanding high quality water.
To evaluate quality, we need to identify the characteristics that are important for the plants we are growing and the way that we are growing them. And we need to establish targets and thresholds to meet our goals. The quality of water you end up with will be a factor of available water sources, your plant’s needs, and the most cost-effective way of aligning those two. Like all plants, cannabis production requires a water source which has a relatively low electrical conductivity (EC), has a pH range to maintain nutrient uptake, and is free of pathogens.
Start at the source
Whether investigating a new production site or maintaining an existing facility, understanding your options and expectations will help influence management decisions that impact plant production and costs. Thorough evaluation of the source of your irrigation water is a critical first step in developing a production plan.
The primary characteristics of the source irrigation water that define quality and ultimately impact plant growth are:
- Other minerals and pathogens
These characteristics are largely influenced by the water source used: municipal, surface or groundwater. Each has its advantages and disadvantages.
In urban areas, municipal water is often the only available source of water. One of the primary benefits of municipal water is that it is generally consistent regarding quantity and output, meaning that the flow rate and availability are relatively predictable and reliable.
Municipal water sources all go through some degree of pre-treatment to ensure potability. Additionally, municipal water treatment facilities add compounds to the pretreated water for residual water sanitation (chlorine) or for potential health benefits (fluoride). Though these elements are intended to ensure a safe and potable source of water for human consumption, they may also provide a benefit to plants. Low concentrations of free chlorine can serve as a safe oxidizer to reduce microbial growth that may impact plant growth directly (pathogens) or impact the irrigation system (biofilms).
Municipalities generally offer water quality reports, free of charge, which can provide guidance on whether additional treatment is required for use as a suitable irrigation water. Since these analyses are generally directed towards potability and residential use, growers should follow up with a more thorough analysis for irrigation suitability. Additionally, municipal water supplies often blend multiple sources of water, so inconsistencies in quality may exist, therefore periodic testing and analysis is recommended.
The greatest downside of using municipal water is the cost. While municipal water is often too expensive for large fields, it may be a suitable option for high-value crops and small areas.
Groundwater is subsurface water that occurs below the water table within the pore spaces of geologic formations and is accessed and supplied by drilled wells. Groundwater is a common source of water in rural or nonurban areas and for agricultural use. These subsurface aquifers become saturated by precipitation and store and transmit water in usable quantities.
Because groundwater reserves are recharged through precipitation events infiltrating through the soil profile, chemical contaminants can be present in the water. Shallow wells are often more susceptible to contamination from surface sources, but overall quality of the water in a well is influenced by land use activities, environmental conditions, and subsurface geologic features.
Surface water sources include natural features such as rivers, streams, and lakes; and man-made features such as farm ponds, ditches, and canals. Surface water is generally the least expensive source of water for irrigation but is also the least dependable.
Surface water sources are not only dependent upon runoff from adjacent land and upstream features, but from upstream use. Therefore, the quantity of water available from surface sources can vary especially on a seasonal basis. Since surface water sources are charged from a wide range of sources and geographic area, largely from runoff, the quality can also vary. Surface water is subject to many sediment, biological, and chemical contaminants which will influence quality for irrigation and potability.
Surface water sources also require water rights, so it is important to not assume that if water exists on the property that access is automatic. Contact your local Extension office, water board, or DNRC for more information to determine water rights.
Evaluating water quality
Without reliable data, growing results may be unpredictable. There are two primary options for determining water quality: simple in-house testing or through comprehensive analysis from analytical laboratories.
In-house water testing is an inexpensive option that provides instant results, though available tools and associated costs limit the extent to what can be monitored. Most often, growers are monitoring electrical conductivity (EC) and pH using handheld sensors, and colorimetric kits are available for approximating mineral content. Colorimetric kits can be purchased at most garden centers and allow growers to gain a ballpark value of specific nutrient content (typically macronutrients like nitrogen, phosphorous, and potassium) by adding a reagent to the solution and matching the resulting color with a provided table.
- Instantaneous results
- Key long-term monitoring tool
- Generally limited to measuring pH and EC
- Frequent calibration is required
In many cases, especially when investigating a new site, a more comprehensive (and costly) water analysis than can be achieved through handheld sensors is recommended. In these instances, an analytical lab is required to gather the necessary results and interpretation.
Water is typically analyzed for irrigation suitability, nutrient solution composition, and potability. Individual components including chemical and physical properties as well as biological and chemical contaminants can be tested for as well. Typically, irrigation suitability tests provide a comprehensive evaluation to evaluate the potential use of a water source and whether pretreatment would be required.
- Comprehensive results on
- Irrigation suitability
- Nutrient solution
- Sampling protocol can target specific data sought
- Can be costly
- Results are not instantaneous
Why does it all matter?
Water quality is a key driver of plant growth, and understanding what’s in your water—and what’s not—is important to making the right decisions to ensure optimal production.
EC—Electrical conductivity is the measurement of the concentration of ions in a solution. The greater the ion concentration, the greater the conductivity of the solution or higher the EC. Generally, growers seek source irrigation water with a low EC, or low measured ion concentration. This allows the grower to add additional fertility (which are salts or ions) without negatively impacting plant growth. If the source water has a high EC, then the grower may be limited to the amount of fertility that can be added and the growth of the plants may be limited. During production EC is often used to monitor the nutrient concentration of the fertigation water.
pH—pH of a solution is the measure of the concentration of hydrogen (H+) ions. It’s important to plant growth in that it affects the relative solubility and, therefore, availability of nutrients. Solution pH above 7.0 can reduce solubility of phosphorous and micronutrients, potentially causing nutrient deficiencies and reduced plant growth.
Alkalinity—Alkalinity is the measure of the amount of carbonates and bicarbonates present. In practical terms, it measures how resistant the solution is to pH change. Two solutions may have the same measured pH, but the one with greater alkalinity will require more acid to neutralize the pH.
Other minerals and pathogens—Excessive concentrations of minerals such as sodium, chloride, and boron can harm plant growth, as can waterborne pathogens like Phytophthora and Pythium.
If you’re using really clean tap water, you may not need any pre-treatment at all. But regardless of your source, you’ll want to know what contaminants, if any, are in your water. Then you can decide the purity and volume desired, and the cost of achieving your target.
The three primary types of treatment are water softening, deionization and reverse osmosis.
- Water softening is inexpensive. It effectively treats hardwater issues, which can lead to clogs in our equipment, but it leaves behind sodium ions that can be harmful. Water softening is not recommended for water that is directly used for irrigation but can be an effective tool to precondition water for further treatment.
- Deionization effectively uses resins to remove ions from a solution using an ion exchange process. No harmful elements are introduced to the water. Deionization is very effective but costs up to six times the cost of our final option, reverse osmosis.
- Reverse osmosis is very effective. It applies pressure to force water (and solutes) across a membrane, leaving behind the potentially harmful salts. Reverse osmosis systems vary in efficiencies and require regular maintenance.
Treatment systems require regular maintenance to ensure water quality remains high. Options include deploying filtration, ultraviolet light, hydrogen peroxide or chlorination.
Technology is our friend
One of the key advantages indoor growing offers is the near-total control over variables that enables predictable, repeatable results. Technology can help determine and improve water quality, and it can help maintain it. Real-time tracking of physical and chemical properties of water is essential in maintaining its quality, and there are now systems available with sensors that provide real-time feedback and automated controls to deliver a more stable and consistent nutrient solution and grow environment.
For more detail on the subject, download an AEssenseGrows white paper titled, “How to Test and Evaluate Water Quality” or view an on-demand webinar titled, “You Are What Your Drink!—The Importance of Water Quality in Plant Production.”