WATER

Revised by Rosa Raudales, PhD

Not all water is created equal; the chemistry of water differs by source. Three key measures of water chemistry that affect plants are alkalinity, pH, and the concentration of dissolved minerals. Mineral content is reported as the concentration of individual elements such as calcium and sodium or total salts. The total concentration of salts is reported as electrical conductivity (EC, measured in millisiemens per centimeter, or mS/cm) or total dissolved salts (TDS, measured in ppm).

Alkalinity is the ability of the water to buffer acids. When water contains components that behave similar to limestone such as carbonates (H₂CO₃), or bicarbonates (HCO), then water pH is not affected as much by the addition of acidic substances such as a water-soluble fertilizer with high ammonium or urea. Water alkalinity reacts with acids to prevent dramatic changes in pH. Water with low or zero alkalinity will result in fast and dramatic drops of pH because there is nothing in the water to react with or buffer the acid in the solution.

Water with no dissolved minerals and no alkalinity is considered “pure water” or “soft water.” The electrical conductivity and alkalinity of pure water are zero. The benefit of this water is that it is a clean slate to develop a nutrient program. The grower has full control; it is easy to add elements. The disadvantage is that with no alkalinity, there is no buffering ability, so the pH fluctuates.

Adding a small amount of an acidic substance has extreme effects on the pH of pure water; however, alkalinity can also be added in a measured and controlled way to pure water. An ideal water alkalinity is around 125 to 150 ppm of calcium carbonate equivalents because it represents some buffering capacity, but not so much that large amounts of acid have to be added to adjust the pH. Water alkalinity can be added by injecting soluble limestone that contains carbonates or oxides, which are typically associated with calcium or magnesium. Cal-Mag can be used to adjust the water.

Waters from different areas vary dramatically in quality. Even within a close distance, two water sources might differ. For example, well water near coastal areas tends to have high calcium content due to shells, surface water near agricultural fields might have a high concentration of phosphorus or nitrogen and a low pH caused by eutrophication (algae blooms, for instance), deep wells may have low oxygen and high iron and manganese, and some water sources in northern areas may have contamination from road salt.

Water with a high concentration of one element or total dissolved salts takes a large effort to adjust to nutrient programs, which is costly because it requires blending the water with a purer water source, or removal of these elements by treating water with reverse osmosis, deionization, or demineralization.

Water districts and companies continuously test the water they supply. The test results are public records and are available from the water district or company. Results may be posted on the internet. In addition to measures of alkalinity, dissolved solids, and pH, reports show some contaminants. While these tests are useful to track changes over time, they may be insufficient for agricultural purposes and not always timely to develop nutrient programs, prevent nutrient disorders, or chlorine toxicity. In some situations it is advisable to send water samples to a horticultural laboratory for testing two or three times a year or more frequently for closed-loop production systems.

Water Quality

Water is a chemical molecule composed of two atoms of hydrogen (H) and one of oxygen (O). H₂O has a relatively simple chemistry, but its role in the world cannot be underestimated: it is essential for life. Among many functions, water is a key element for plant growth; it is involved in photosynthesis, movement of solutes across cell membranes, nutrient uptake and redistribution, regulation of plant temperature, and flower and fruit development.

While the quality of the molecule itself does not change, water has the ability to dissolve and mix with many molecules. Therefore, water quality refers to chemical, microbial, and physical parameters that water carries and may affect its suitability for a specific use.

The quality of irrigation water strongly affects plant health and the efficacy of irrigation systems.

Water chemistry factors to consider when developing fertilizer programs are alkalinity, the concentration of dissolved ions or salts, and pH.

Alkalinity

Alkalinity refers to the buffering capacity of a solution, or its capacity to neutralize acids. Alkalinity is determined primarily by the total concentration of carbonates and bicarbonates in the water. These bi/carbonates associate with elements such as calcium (Ca), magnesium (Mg), and sodium (Na). Other compounds such as hydroxides, sulfides, phosphates, silicates, and borates may also contribute to alkalinity, but their concentration is usually low in irrigation water. Alkalinity determines how solutions respond to acid.

Water alkalinity should not be confused with alkaline pH (pH > 7.0). A simple way to understand alkalinity is that alkalinity behaves in the same way as dissolved limestone in the water. The following chart illustrates how alkalinity works and why decisions should be made based on the water analysis and not just following the recommendations from other operations.

Water alkalinity is typically reported as equivalents of calcium carbonate (CaCO₃) or calcium bicarbonate (Ca(HCO₃)₂) in water analysis. Water alkalinity greater than 150 ppm CaCO₃ is considered high and increases the pH of the substrate over time.

Alkalinity Units and Conversions

Alkalinity can be managed by the type of fertilizer used. Alkalinity between 150 and 250 ppm can be managed by using fertilizers that result in an acid reaction in the substrate. Fertilizers result in acidic reactions when they have less than 20% or more of the total nitrogen in the form of ammonium (NH₄+) or urea. The higher the proportion of NH₄+ and urea in the fertilizer, the more acidic the reaction. These fertilizers must be used with caution, because in addition to being strongly acidic, high concentrations of ammonium can be toxic to crops.

In contrast, fertilizers with all the nitrogen in the form of nitrate have a mildly neutral reaction. Nitrate is not toxic to crops. Therefore, when the alkalinity is above 250 ppm, it is necessary to first inject acids to bring down the alkalinity to approximately 100 ppm CaCO₃ or 2 mEq/L and then use fertilizers with low ammonium/urea concentration. Never rely solely on fertilizers to manage extremely high alkalinity to prevent crop toxicity.

Use digital calculators available online to estimate the amount of acid to inject to lower alkalinity. An alkalinity between 60 and 150 ppm is an ideal range and should be matched with fertilizers with a more neutral reaction (20-30% NH₄+/urea of the total N). Water alkalinity under 60 ppm CaCO₃ is considered low and can be managed by using fertilizers with mostly nitrate (less than 10% of the total nitrogen from ammonium or urea). In addition, limestone can be incorporated in the substrate, or flowable limestone can be sporadically injected in the nutrient solution to increase the buffering capacity of substrates.

Water alkalinity is a standard measurement in the analysis of irrigation suitability in horticultural crops, especially in protected agriculture. Industrial or home water tests do not typically include water alkalinity. Instead they measure water hardness, which is also presented in CaCO₃ equivalents. However, water hardness and water alkalinity are not related and should not be confused despite the complicated use of the same units.

Water Hardness

Water hardness is the concentration of dissolved calcium, magnesium, and other cations in water. Water hardness can be carbonated (e.g., CaCO₃) or noncarbonated (e.g., CaCl₂). Water hardness is relevant to domestic and industrial use because high values reduce the solubility of soap and can result in scaling of water heaters. However, when it comes to irrigation water, the actual values of individual elements such as calcium, chloride, or carbonates are more informative to select nutrient programs and water treatments.

The concentration of calcium and magnesium is not a major concern for agriculture because these elements are essential for plant growth at high concentrations and are added in with fertilizers. Therefore, water hardness by itself is not a major concern in horticulture production.

Water alkalinity can be measured in-house by using kits that promote a reaction, measuring the pH after the reaction, and then matching the results to a conversion table. The alkalinity of water sources does not change frequently, so there is no need to measure it often or in-house. There isn’t a direct meter that can perform the test; a pH kit must be used.

pH

Water pH refers to the concentration of hydrogen ions (H+) in relation to hydroxide ions (OH-) and is an indicator of the acidity of a solution. The pH scale ranges from 0 to 14—where less than 7 is acid, 7 is neutral, and greater than 7 is basic, also known as alkaline. The ideal starting pH for irrigation water is between 5 and 7, because it is close to the pH required by most crops (5.8-6.2).

Water pH affects the availability of iron, manganese, boron, zinc, and copper. As pH increases, the solubility of these elements decreases, and vice versa. The solubility of some agrochemicals such as fertilizers, fungicides, insecticides, and plant growth regulators is also affected by water pH.

For example, iron is very soluble and available for plants at low pH. While the total concentration of iron does not change when the pH increases, iron precipitates and becomes a solid, rust-colored precipitate that accumulates on surfaces and irrigation systems and is unavailable for plant uptake. The pH of the water can also affect the efficacy of agrochemicals. Growers should pay close attention to the directions on agricultural additive labels and adjust the pH to the recommended level to maximize the efficacy of products.

Growers using organic-based substrates in containers, such as peat or coconut coir, should focus on managing the pH of the substrate by selecting fertilizers that are compatible with water alkalinity and crop needs (see Nutrients & Fertilizers), instead of adjusting the water pH.

In contrast, growers using free-floating solutions, such as deep-water culture or nutrient film technique, or inorganic substrates such as rockwool or perlite, should focus on the pH of the solution, since this is where roots take up nutrients. If the pH is outside the recommended range, adjust the pH first and add the fertilizers afterward to prevent excessive nutrient fallout. With mineral fertilizers, two signs of nutrient fallout are cloudiness in the water or precipitate at the bottom of the tank. Check the pH again after fertilizers are added and adjust as needed. Remember the pH is being adjusted to increase nutrient availability.

The pH of water can be adjusted by adding acids or bases to the solution. Nitric, phosphoric, sulfuric, and citric acids are options to lower water pH. All these acids, except citric, are mineral acids that add elements (N, P, or S) to the solution. Citric acid is an organic acid that does not add additional elements to the solution and is considered a milder and “safer” acid compared with mineral acids. From a technical perspective, any acid will achieve pH reduction, for example, pH Down and pH Up contain acids such as citric acid, phosphoric acid, or nitric acid, or bases such as potassium hydroxide, as their active ingredients.

Select an acid based on (1) the elements (N, P, or S) that may be convenient to add to the solution, (2) organic versus mineral sources, (3) worker safety, of which citric is milder, nitric is the most hazardous to handle, and phosphoric and sulfuric are in the middle, and (4) cost.

To properly estimate how much acid to add to achieve a target pH, the water alkalinity must first be known. Online tools help calculate how much acid to use to reach a target pH. Many growers inject acids until they reach the target pH without using a calculator. While this may work, it can also lead to a lot of guessing that results in adding too many additional nutrients.

To increase the water pH, use flowable limestone or hydroxides. Flowable or soluble limestone dissolves well in water and often includes magnesium carbonate, and calcium or magnesium oxide. Depending on the composition of the product, the label will make recommendations of how much to add to increase a given pH by a specific amount. All acids and bases should be handled with caution and adequate personal protection equipment.

Be patient when adjusting the pH of the solution and test frequently. Mix the solution well and allow the acid or base to react before measuring the pH and adding more product. Water pH can be easily monitored and adjusted in-house with hand-held or inline meters. It is inexpensive to get an inline automatic injector system that adjusts the pH of solutions. However, these systems can only be as accurate as the maintenance they receive. Test and calibrate the meters frequently. The temperature of the water also affects pH. Meters are usually calibrated at 68°F (20°C). Temperature compensating meters are preferred.

The pH & Alkalinity Relationship

There is a connection between pH and alkalinity; however, pH does not equate to alkalinity because they are two different measurements. Water alkalinity refers to the concentration of carbonates and bicarbonates, while pH refers to the concentration of hydrogen ions.

Alkalinity in the water reacts with hydrogen ions to increase the pH (as in this equation: H+ + HCO₃– → H₂O + CO₂). Typically, water with high alkalinity will have a high pH. However, water with high pH does not necessarily have high alkalinity. Water with a pH of 4.5 has zero alkalinity.

Water alkalinity is a stronger base than water pH; the pH of the substrate is affected by water alkalinity more than water pH. For example, water alkalinity of 50 ppm CaCO₃ (very low) would have the same effect in the growing media as water with a pH of 11 (very high). Water with a pH of 8.0 would have the same effect in the medium as alkalinity of 0.05 ppm CaCO₃.

Although the rhizosphere is temporarily affected while watering, the water pH has only a little effect on the substrate pH over time. Water alkalinity, which behaves like limestone, significantly increases the pH over time.

Dissolved Ions

Dissolved ions in a solution can be measured by estimating the concentration of total salts or individual elements. Both affect how water can be managed and used. Extremely high levels of dissolved elements affect the suitability of a water source.

Electrical conductivity (EC) is the ability of a solution to carry electrical current and an indicator of the total concentration of salts in a solution. Water EC indicates whether the salts, in general, are high or low, but it does not point to which specific elements are abundant. Adding fertilizer to water that already has a high EC (greater than 1mS/cm) can result in burnt roots and shoots. Therefore, water with low EC is desirable to give growers room to determine the amount of nutrients that can be used before harming the crop.

Electrical conductivity (EC) units:

EC can be easily measured, and the value can be used to track changes over time. Source water with an EC less than 0.5 mS/cm is ideal because it indicates there is a low concentration of dissolved salts. Water sources with levels above 0.5 mS/cm should be sent to an analytical laboratory for a complete nutrient analysis.

The goal of a complete nutrient analysis is to identify if any element is at a level that can be phytotoxic to crops or if any element that is essential for plant growth is high enough that fertilizer programs need adjustment. Water with an EC higher than 0.7 mS/cm can result in salt accumulation in substrates or nutrient solutions, hard and stunted growth, and lead to damage on the foliage in propagation.

High EC is problematic because it leaves little room to provide nutrients through fertilizers. Therefore, any water source with an EC greater than 1.0 should be treated by blending the water with a purer water source such as rainwater or reverse osmosis (RO) treated water.

The treatment choice will depend on a combination of factors specific to an operation, including cost and access to alternative water sources.

RO treatment can be expensive and time-consuming, and creates toxic wastewater, but in many circumstances it is the only option. Sometimes switching to a different water source might be the best option—from a technical and financial perspective.

Water Testing

Growers should send samples of the water for complete nutrient analysis at least twice per year. Growers who see nutrient disorder problems should test water quality, substrates, and nutrient solutions.

East Bay Municipal Utility District Annual Report 2020

Specific Elements

Water often contains elements essential to plant growth as well as nonessential elements. Elements essential for plant growth are nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), boron (B), chlorine (Cl), iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), and molybdenum (Mo). Silicon (Si) is not considered essential but increases plant vigor and resistance to stress. Ca, Mg, S (in the form of sulfates, SO₄-), B, Cl, Fe, Mn, Zn, Cu, carbonates, and sodium are often found in natural water sources.

Management of macronutrients (N, P, K, Ca, Mg, and S) in the water is relatively easy because they are needed in large amounts by plants and very rarely, with the exception of calcium, are they present in amounts larger than what the plants need.

Shallow wells in agricultural areas are often contaminated by agricultural runoff. Water from deep wells occasionally contains a high concentration of iron but is rarely contaminated by agricultural runoff.

The quality of surface water such as ponds, lakes, and rivers depends on the surrounding areas. The only way to know a water is to have it analyzed. It will prevent risks from unhealthy chemicals and help to determine how it needs to be treated for agricultural purposes.

N, P, and K in water indicates contamination from agricultural runoff because these elements are not found in high concentrations naturally. This may not be a significant issue from a nutrient management perspective, but there is a greater possibility of contamination with plant growth regulators, pathogens, pesticides, herbicides, and/or fungicides.

Any essential nutrient in the water that is below the level required for plant growth does not require removal (with reverse osmosis or blending with pure water sources). To properly use this water source, fertilizer levels can be decreased to accommodate the nutrients already present. For example, if there are 20 mg/L of N in the water source and the plant requirement is 100 mg/L N, then the amount of N is reduced from fertilizers and only 80 mg/L is applied.

Micronutrients (B, Cl, Fe, Mn, Zn, Cu, and Mo) and nonessential elements (sodium and fluoride) in water sources are more difficult to manage because even small concentrations can cause plant phytotoxicity or affect the efficacy of irrigation systems.

The specific characteristics of water sources are difficult to predict. However, there are some common trends.

Rainwater and drinking water treated by cities are usually low in dissolved elements.

The type of dissolved elements and their quantity in groundwater is highly dependent on regional geological characteristics and the minerals that weathered into water sources. For example, high concentrations of sodium and calcium are common in coastal areas.

Reverse Osmosis (RO)

RO is a filtration system that uses a pressurized, ultrafine, semipermeable membrane to remove salts and other particles from water. Osmosis is a process in which a solution moves from a low concentration of solutes to a higher concentration. In reverse osmosis the pressure and the charge of the membrane forces the inverse process, moving the solution from high to low concentration and therefore removing impurities from the water.

Reverse osmosis can be used to remove excessive amounts of salts from irrigation water that contains excess sodium, chloride, or other micronutrients. Boron and some agricultural chemicals are difficult to remove with RO systems.

Water flow slows when RO systems are in line because the filter pore size is small. RO can easily be clogged by suspended solids such as sand, silt, organic matter, chemical precipitates (Fe, Mn, carbonates, pesticides), or bacteria and algae, so prefiltration of suspended solids is essential before membrane filtration to prevent clogging or damage.

Many argue that starting with RO-treated water is essential to develop nutrient programs. However, water with an EC of less than 0.5 mS/cm does not need treatment.

If the target element is a micronutrient that is above the toxicity threshold or a salt that cannot be managed with the nutrient program and no alternative water source is available to blend the water, RO treatment should be considered.

RO treatment is an effective technology and in many situations is the only option available to deal with high concentration of salts. However, its environmental footprint is a concern that must be considered. Brine is a solution with high concentration of salts and a by-product of RO treatment.

Brine discharge is regulated because the high concentration of salts may affect the ecosystem. There is ongoing research to identify applications for brine; until then, most of it is discharged back into water sources where it disrupts the balance of elements in water bodies and affects wildlife.

Water Quality beyond Nutrient Programs

The quality of water affects plant production in other ways than just nutrient programs. Water can be a source or dispersal mechanism of plant pathogens or a carrier of elements and particles that clog the irrigation system or contain chemicals that are detrimental to crop quality.

Chemical Parameters

Chlorine

Drinking-water treatment facilities apply up to 4 ppm fluoride and chlorine (the sanitizer, not to be confused with the chloride ion) in the water. These levels are within the US Environmental Protection Agency standards for drinking water. It is rarely a problem for annual crops, and can be rinsed from the medium with flushing.

Chlorine is an essential micronutrient for plants, but plants’ needs for chlorine are rather low. Most of the time water treatment facilities use a lower range, and chlorine dissipates as it moves in the water distribution systems, but the grower has little control or knowledge on when these changes might happen. Chlorine and fluoride can be easily removed by having an activated carbon filter. Chlorine can be monitored in-house by using colorimetric kits or having an inline meter.

Agrichemical Residues

Agricultural runoff can include agrichemicals such as herbicides, pesticides, fungicides, plant growth regulators, nutrients, and plant pathogens. Agricultural runoff can be present in surface water or groundwater where growers have very little control. Agricultural runoff increases in the water source when it is connected to the runoff or if nutrient solution is recirculated.

Think about water treatment as a risk management strategy, and implement an approach that has a continuous treatment point in place as a preventative mechanism. Carbon filtration is a good option. The goal is to have a proactive rather than reactive approach. Carbon filters also remove most agrichemicals. However, it is not known if they affect plant pathogen survival.

Chemical Precipitates

At high concentrations, greater than 4 mg/L, iron can precipitate, accumulating and clogging irrigation systems and staining surfaces of plant tissue or walls.

Iron is in solution in its dissolved form or when the pH is less than 6.0. As the pH increases, iron precipitates, forming into a solid form of rust. Precipitated iron accumulates in irrigation lines and can clog irrigation emitters and main pipes. Additionally, it stains the leaves with overhead irrigation.

Iron also precipitates when it goes from low to high oxygen levels, which happens when it is pulled from deep wells. Iron is typically a problem when using deep wells. Although not all wells have high iron concentration, problems with high iron are mostly observed in deep wells. Iron in water can be managed using a system that includes oxidation, which promotes precipitation, followed by filtration.

An effective method is injecting potassium permanganate (at 1:1 ratio) as an oxidizer, followed by filters with glauconite greensand. Aeration or other types of chemical oxidations (chlorine) can also be used. However, potassium permanganate is ideal because it injects K as a by-product, which is an essential macronutrient. Aeration can be slow, and chlorine can be phytotoxic.

Calcium in the water reacts with bicarbonates (HCO₃-) or sulfates (SO₄^–2), and precipitates. This reaction happens when both elements are at high concentrations (Ca greater than 150 mg/L and bicarbonates greater than 180 mg/L, sulfates greater than 120 mg/L). This is why calcium is separated from other elements in stock solutions.

A quick fix to this problem is to inject acid to dissociate the bicarbonate ions. A more expensive solution is to use membrane filtration (e.g., reverse osmosis) to remove salts. Membrane filtration should only be used if the levels are extreme and cannot be managed with acid.

Dissolved Organics

Dissolved organics include agrochemicals and humic acids. Carbon filtration, reverse osmosis, and ozone remove a vast amount of agrochemicals from water.

Biological Parameters

Waterborne microbes that may affect crop production include plant pathogens, algae, and bacteria associated with clogging. Water sources vary in the risk that they pose by carrying detrimental microorganisms. Surface water tends to harbor microorganisms including plant pathogens and biofilm-forming bacteria at a larger concentration and diversity than any other water source.

Growers should be aware of the different risks they face when using different water sources. Deep wells, rainwater, and drinking water tend to have very low concentration of microorganisms. Iron and manganese oxidizing bacteria can be found in wells both deep and shallow. Any water source with agricultural runoff, including recirculated water within an operation, is likely to have plant pathogens.

Plant Pathogens

Irrigation water can be an inoculum or dispersal mechanism of plant pathogens. Water sourced from the ground, rain, or cities does not have plant pathogens. In rare cases groundwater can be contaminated from agricultural runoff.

Surface water bodies such as ponds and lakes and recirculated, but not precipitated, water can have a high concentration and diversity of plant pathogens. Subirrigation systems, regardless of the source, are at high risk of dispersing pathogens from container to container. This problem is exacerbated because the solution containing pathogens moves from one container or zone to another and also because organic debris such as plant matter or planting medium on floors or benches protects pathogens from desiccation or dehydration.

Waterborne plant pathogens include bacteria, oomycetes, and fungi. Oomycetes, also known as “water molds,” include organisms that cause common diseases such as seedling blight, damping-off, and root and stem rot. They persist really well in water and produce swimming spores called zoospores that actively swim toward root exudates.

Two common oomycete pathogens found in water that affect cannabis are several species of Pythium and Phytophthora. Fungi such as Fusarium and Rhizoctonia are also present in the water. Recent research from the University of California has also found that sand filters can remove oomycetes from irrigation water (Lee and Oki 2013).

Unlike oomycetes, fungi do not swim freely; instead, they are carried in organic debris or overwintering structures. This is one reason, among others stated below, why removing organic debris is an important step in water sanitation.

Biofilms

Biofilms are a group of microorganisms, mostly bacteria, that form a polysaccharide coating that protects them from desiccation. Think of them as slime. In irrigation systems when water-soluble fertilizers are used, biofilms also harbor algae. Biofilms frequently accumulate in fine emitters, filters, or main irrigation pipes and clog the system. Biofilms are not well understood in irrigation. There is no one silver bullet to combat biofilms because they are diverse in composition. However, a general approach is recommended to prevent and remove them. First, follow the sanitation recommendations below. (See Water Sanitation.)

Flush the system with water at high pressure to remove the amount of organic and mineral residues that may be sitting on the system and physically (partially) destroy the biofilm. Use highly acidic water to dissolve some of the slime. After flushing the system, inject chemical sanitizers such as chlorine dioxide (ClO₂), hypochlorous acid (HOCl or HClO), peroxyacetic/peracetic acid (C₂H₄O₃), or hydrogen peroxide (H₂O₂) at high concentration—this is known as shocking the system.

The concentration used to shock the irrigation system is higher than the doses recommended for use during production and are toxic to crops. The system should be shocked only when the space is empty. If possible, the solution should be allowed to stay in the irrigation system overnight and be rinsed with abundant water at high pressure to push out the slime.

Algae

Algae are omnipresent. No matter how clean an operation, when there is light reaching a solution, it ends up accumulating algae. Prevent algae by stopping light from reaching the nutrient solution and clean up and dry out wet spots.

Water Sanitation

Water treatment options include sanitizers that kill organisms, chlorination, activated peroxygens, quaternary ammonium, ozone, UV light, heat treatment, chlorine dioxide, hypochlorous acid (HOCl or HClO), hydrogen peroxide (H₂O₂), peroxyacetic/peracetic acid (C₂H₄O₃), copper, biofungicides, and more. Little is known about the tolerance of cannabis to some of these sanitizers, so it is best to start with a low dose. Regardless of the target organism, here are some general rules to make sanitation effective:

Select a sanitizer. Sanitizers differ in their residual activity, or how much control they provide throughout the irrigation. Point treatments such as ultraviolet (UV) light and heat treatment can control organisms when the solutions are in direct contact with the treatment system, and they leave no residues in the solution.
In contrast, injectable sanitizers continue to react throughout the irrigation. The residual activity of the injectable products varies by their inherent properties and the concentration applied. The benefit of residual activity is sustained control throughout the system; the disadvantage is the risk of phytotoxicity when the product is applied at high doses or if it accumulates in closed-loop irrigation systems.

Sanitizers vary in their efficacy to control specific organisms. However, in irrigation sanitation the goal is not to sterilize the solution but to reduce the risk caused by microbes without causing plant phytotoxicity. Water sanitation is only one part of the whole integrated plant disease management.
Filter first, sanitize later. Sanitizers are nonspecific and react with any organic matter in the solution. To achieve a higher degree of sanitation, it is essential to remove non target organic matter or debris such as dead roots, leaves, and substrates from the solution via filtration and then apply the sanitation step.
Extend contact time. When possible, apply the sanitizer and allow it to have a prolonged contact time with the solution before irrigating crops. Storage tanks are an essential component of water management and sanitation because they provide the ability to slow water flow and reduce the size of the equipment or dose needed to treat water. The longer the water is in contact with a sanitizer product or treatment, the higher the microbial mortality.
Filter multiple times. Multiple-stage filtration is a must for any water distribution system. Install filters in series, from coarse to fine pores, to improve filtration efficiency and reduce the need of cleaning filters (use filters with automatic backwash). Filtration will help remove the organic debris discussed above. Having multiple stages will prevent the system from clogging too frequently.
Monitor the efficacy of the system. All water sanitation systems require constant monitoring to ensure proper sanitation and prevent phytotoxicity. A system will only be effective if it is functioning properly. Include sampling valves to take samples or inline meters to measure active ingredients. In commercial operations, staff must be trained on how to monitor and adjust the system.
Match the system to the culture of the operation. While one technology might be “better” than others, at the end of the day the treatment option that will work best is the one that is well understood and maintained. Choose a technology that fits the operation’s culture. Many operations like high-tech options, but many prefer simplicity. All options can work if there is someone who stands behind it and maintains it properly. Speak with other growers about how effective the systems are, the service the companies provide, and how easy the system is to manage.
Think beyond water treatments. Water sanitation is only one part of integrated plant disease management to prevent waterborne microbial problems. Use resistant cultivars when available.

The entry of pathogens from other sources include:

Infected cuttings and liners
Uncleaned tools
Workers bringing in infections from outdoors
Workers moving infections from one section to another
Overwatering, which promotes bacteria and other root diseases

Physical Parameters

Physical parameters of water quality include any particles in suspension (not dissolved) that accumulate and clog the irrigation systems. They can be organic or inorganic.

Organic particles include debris such as plant parts, particles from media substrates, and even algae. Sand filters remove microbial loads and large organic particles. Screen and media filters are also effective in removing large organic debris and weeds. Media filters can easily clog if the debris is too coarse, so they must be monitored and cleaned.

Inorganic particles and debris include fine granular minerals such as sand, clay, and silt. These contaminants can be physically removed with “paper,” sock, screen, or disc filters. Reverse osmosis or other membrane filtration should not be used to remove suspended inorganic particles and debris because they can physically damage the membranes.

Water treatment needs to be an integrated system that should be designed to target the combination of potential risks in an operation. Therefore, before installing a system, it is essential to understand the problem, match the treatment option to the problem, and develop a management plant to monitor proper functioning.

Water Treatment Options by Target Problem

Temperature, Oxygen & Carbon Dioxide

The temperature of the water affects plant growth, microbial activity, and the ability of water to hold oxygen. The optimum root zone temperature for oxygenated hydroponic solutions is 68°F (20°C). When the water temperature in the root zone is lower, plants are less efficient using energy, their metabolisms slow down, nutrient uptake reduces, and therefore yields are affected. When water is too warm, it holds less oxygen and can be cooled using water chillers or by blending with lower temperature water. When roots have access to enough oxygen, planting media temperature can be higher.

Water is an excellent conductor of temperature, and most greenhouses use hot water to increase root zone temperatures. Water at 100°F (38°C) is passed in pipe loops under the production benches or concrete floor to increase the root zone temperature during winter months, thus avoiding the need and cost of heating the whole facility.

Root respiration (intake of oxygen) is required for root growth and nutrient mobilization. Oxygen is available to the roots in air spaces in growing media or as dissolved oxygen (DO) in hydroponic solutions. DO is a measure of the amount of gaseous oxygen in water.

A DO of 6 ppm or more is ideal for hydroponic solutions. DO under 4 ppm has negative effects on plant growth, including stress and reduced growth and nutrient movement. Increasing the DO above 6 ppm has little to no effect on plant health, quality, or yields in hydroponically grown crops. Maintaining an optimum level can be achieved by promoting water movement or injecting air from the environment and avoiding stagnation in storage or production tanks. Here are some methods:

Air pumps with bubblers
Pumping a cascade or shower
Injecting nano oxygen bubbles
Adding H₂O₂
Electrolysis

Water in deep wells may have low oxygen; however, as soon as the water reaches the ground-surface, enough oxygen dissolves into the water from the air.

Water in ponds or storage tanks with no aeration can accumulate anaerobic bacteria, which may smell bad or clog the system. A simple system (e.g., a pump) that promotes water movement is sufficient to prevent water stagnation. DO can be easily monitored with meters, very similar to a pH meter.

Temperature and DO have an inverse relationship. As water temperature rises, oxygen solubility goes down. For example, water at 50°F (10°C) can hold 11.3 ppm DO, whereas 77°F (25°C) can only hold 8.6 ppm DO. This relationship is especially important when producing in deep-water culture, where the roots are only exposed to the solution.

In container production the oxygen is stored in the pockets of air in the substrate. In other hydroponic systems the oxygen is obtained from the DO in the nutrient solution as it moves.

Carbon dioxide (CO₂) is very soluble in water. Most of the CO₂ in water comes as a by-product of algae, root, and microbial respiration (use oxygen and extrude CO₂). Ponds with a lot of algae (or plants) tend to have large pH fluctuations during the day.

Algae and plants use CO₂ and release oxygen when photosynthesizing during the sunny portions of the day. When light is not present (as is expected in the root zone for most plants), plant roots release CO₂ as they metabolize sugars. CO₂ reacts with water and forms carbonic acid, which lowers the pH of the water. This reaction is similar to what happens in hydroponic solutions, and that is why the pH of nutrient solutions tends to go down as plant biomass increases. Prevent algae accumulation by avoiding nutrient runoff into the pond and by having a good aeration system.

Electrolyzed Water

Electrolyzed oxidizing water (EOW), or electrolyzed water, is a potent disinfectant and has been shown to be effective in sanitizing food crops (Siddiqui 2018). The use of EOW for sanitation purposes is gaining traction in the cannabis industry as well. The disinfecting solution is nothing more than hypochlorous acid that can be created by using the process of electrolysis with a chloride-containing salt (NaCl or KCl) dissolved in water. Electrolysis is the process where water molecules are split into their atomic components through a direct electrical current:

2 H₂O → 2 H₂ + O₂

During this process, hydrogen gas accumulates around the negative electrode (cathode), and oxygen gas forms around the positive electrode (anode). With dilute NaCl dissolved in the water, the electrolysis process results in two solutions: one with sodium hydroxide (NaOH) and one with hypochlorous acid (HOCl). Hypochlorous acid is a weak acid that has strong oxidizing powers that can neutralize colonies of Escherichia coli, Salmonella typhimurium, and Listeria monocytogenes (Park et al. 2009).

Similar to using ozonation for sanitation, electrolyzed water can be created on demand with few inputs. Ozonation requires an ozone generator; likewise, electrolyzed water requires an EOW generator. The EOW generator only requires water, salt, and access to electricity to create a sanitizing solution on demand. Electrolyzed water can also be purchased in a stabilized form as a disinfectant spray.

There is not much research identifying the benefits of using electrolyzed water on the growth of cannabis; however, it is known that when hypochlorous acid comes in contact with fertilizers in nutrient solution, especially nitrates, it quickly degrades into chloramines. Chloramines are a significantly weaker group of oxidizing agents and are not as effective as hypochlorous acid when it comes to antimicrobial activity.

EOW is an effective biofilm eliminator when it is used in pure water. Again, coming in contact with fertilizers, the acid quickly produces chloramines, which are not effective in treating biofilms in irrigation lines.

Direct application of electrolyzed water to the root zone of cannabis plants can run the risk of phytotoxicity. It is widely known that excessive use of hypochlorous acid in the root zone can harm the growth of plants. In low concentrations that are usually seen as safe for direct application to plant roots (<2.5mg/L), hypochlorous acid successfully inactivated a pathogen commonly found to cause root rot in cannabis, Rhizoctonia solani (Serge et al. 2019).

Electrolyzed water is a potent antimicrobial agent, so direct application to a living soil has a detrimental effect on the beneficial microorganisms that provide benefit to the plants’ productivity. A healthy and diverse microbiome in the root zone provides protection from pathogens that may take advantage of a decimated microbial community.

Electrolyzed water’s safest application is cleaning and disinfecting equipment, grow rooms, tables, pots, and irrigation systems. The effectiveness of EOW in combination with its ease of access from on-demand production makes it a very attractive product for sanitation purposes; however, the effects of direct use on plants is unknown.