Water Quality in Greenhouses

Good water quality is one of the most important factors in growing quality greenhouse crops, yet it is often taken for granted, and it is often assumed that all water is compatible for plants.

There are many factors which determine water quality. Among the most important are soluble salts, alkalinity, and the sodium adsorption ratio. But there are several other factors to consider, such as whether hard water salts such as calcium and magnesium, heavy metals or individual toxic ions are present. In order to determine this, water must be tested at a laboratory that is equipped to test water for irrigation purposes.

Poor quality water can be responsible for slow growth, poor aesthetic quality of the crop and, in some cases, can result in the gradual death of the plants. High soluble salts can directly injure roots, interfering with water and nutrient uptake. Salts can accumulate in plant leaf margins, causing burning of the edges. Water with high alkalinity can adversely affect the pH of the growing medium, interfering with nutrient uptake and causing nutrient deficiencies which reduce plant health.

Soluble Salts/Salinity

Salinity is measured as electrical conductivity (EC). It is a relative measure of the total quantity of salts dissolved in the water. Pure water is a poor conductor, whereas saline water is a good conductor. EC is reported as deciSiemens per meter (dS/m), microSiemens per centimetre (uS/cm) or milliSiemens per centimetre (mS/cm). Crops vary in their sensitivity to salinity according to their kind, stage of growth, and container size. In general, mature plants are the most tolerant, while plant plugs are the most sensitive.

The most common salts in irrigation water are:

• sodium (Na+)
• calcium (Ca++)
• magnesium (Mg++)
• iron (Fe++ or Fe+++)
• bicarbonate (HC0₃-)
• carbonate (C0₃-)
• sulphate (S0₄-)
• chloride (Cl-)

Sodium and chloride easily damage plants in excess amounts, while calcium, magnesium, bicarbonate and carbonate are somewhat less direct in damaging plant growth. Fertilizers are a source of salts and add to the total salts plants receive. Monitoring of soluble salts in the growing medium is very important, as there are often accumulations with time.

Soluble salts are routinely measured by water quality laboratories, but portable, battery-powered meters are available to growers from greenhouse supply companies. Water with an EC of more than 1.0 mS/cm is considered marginal for greenhouses. If it is over 2.2 mS/cm, it is not recommended for bedding plants. The optimum EC range is 0.3 - 1.0 mS/cm; 0.5 mS/cm is particularly recommended for plugs and seedlings. This is important, as some water sources in Saskatchewan have tested 10.0 mS/cm or higher.

Alkalinity

Alkalinity is the most important factor influencing root media pH. This is a measure of the ability of the water to neutralize acids which may be added. Water with high alkalinity may adversely affect media pH over time. While a change in pH has minimal effect on potassium and nitrogen uptake, phosphorus, magnesium and manganese uptake are strongly affected. Calcium bicarbonate or calcium carbonate equivalent levels give a measure of alkalinity. Undesirably high bicarbonate levels can be neutralized by injecting acid into the water supply or, to some extent, with the use of ammonium containing fertilizers. The latter needs to be used carefully, as some crops have low tolerance of ammonium, especially if grown cool.

While there is no one best alkalinity level, in general alkalinity levels of 120 mg/l (ppm) can be a problem, particularly for long term crops. Levels of 140-200 mg/l may be acceptable for short term crops grown in an acidic media and fertilized with acidic fertilizer. For plugs, levels below 80 mg/l are desirable because there is little buffer capacity in the small amount of root media in each plug cell.

pH

pH measures the concentration of hydrogen ions in relation to hydroxide ions. In pure water, the concentration of each is equal, and its pH is considered neutral, and arbitrarily set at seven. Values below seven indicate an acidic condition; those above seven are basic or alkaline. Water pH (acidity) is not a good indicator of the water's capacity to modify the pH of the growing mix. It can affect the stability of pesticides and growth regulators and often acidification is desirable.

Acceptable Ranges for Greenhouse Irrigation Water

Common Nutrient Problems with Greenhouse Water Supplies

• Calcium and Magnesium - main contributors to making water hard.
• Bicarbonate - common in water supplies. High levels contribute to high alkalinity. In combination with calcium and magnesium, precipitation occurs, leading to unsightly residues on foliage and containers, and blocking of irrigation drip system emitters. For levels >50 mg/l, acidification is desirable.
• Iron - when levels are over 0.25 mg/l (ppm), precipitation can be a problem, blocking emitters.
• Boron - very toxic; levels should be monitored. In sensitive crops like poinsettias, a fertilizer containing little or no boron may be desirable.
• Zinc - high levels may be found in water that has been in contact with galvanized metal.
• Copper - may be higher than desirable in organic media.
• Fluoride - not commonly found at high levels, but when added to water supplies, may contribute to marginal leaf or tip burn on certain plants, particularly Easter lilies, dracaenas, spathiphyllum, parlour palms, prayer plants, freesias and spider plants. The problem is usually greater when the medium pH is under 6.0, and when prelate or superphosphate is present.
• pH - it should be noted that the availability of most nutrients are dependent on the pH. High pH water is mainly of concern when alkalinity is also high.
• Sodium adsorption ratio - this refers to the tendency of sodium to be dominant at the nutrient exchange sites in the medium, hindering uptake of important nutrients such as calcium, magnesium and potassium. Over long periods of time, sodium adversely affects mineral soil structure, leading to poor aeration and drainage. The effects on soilless media are less serious.

Managing Poor Quality Water

Salinity - If levels are higher than desirable, there are several practices that will be of benefit. It is important to maintain a higher moisture level than normal in the rooting medium at all times, to reduce salt damage to roots. Good drainage is important, with peat moss making up at least 50 per cent of the medium. Fertilizers with a low salt index should be used. Some growers have used sugar solutions at a rate of 1 kg/50 l; 10 l/m² - this is intended to feed microbial populations which tie up salts. For all crops, thorough leaching at every watering is important. Reverse-osmosis units, although costly, can also be used. It may be necessary to mix treated and untreated water, as treating the entire water supply may be impractical. Where necessary, avoid crops with long production cycles or those particularly sensitive to salts, such as African violets, azaleas, calceolaria, geraniums and petunias. Collected rainwater may be a source of low salinity water.

High Sodium - periodic leaching with calcium has been used to manage sodium if it is below 100 mg/l (ppm). Above that rate, reverse-osmosis may be required. Plants can absorb sodium from overhead irrigation as well as through roots. Water from water softeners is high in sodium and should not be used.

High Alkalinity - acidification of water is commonly practiced, usually with phosphoric (the safest to handle), or nitric acid. If alkalinity is over 250 mg/l, sulfuric acid is recommended. These acids will also provide nutrients. Other options include using more acidic fertilizers, or starting with a more acidic growing medium. Feeding on a constant basis also helps. An undesirable effect of acidification with both phosphoric and sulfuric acid is undesirable residues on foliage when the treated water is used for misting. Corrosiveness of the acids on pumps and water lines should also be considered before acids are used. Rainwater can be used to dilute the alkalinity level.

High Iron - derivatives of sodium metaphosphate can be used to prevent precipitation. Specialized equipment can be purchased for iron removal. The iron is oxidized, and then filtered out through a resin bed.

High Fluoride - when growing sensitive crops, use a medium pH over 6.5, and keep up calcium levels. This causes fluoride to precipitate, making it unavailable for plant uptake.

Algae - common in greenhouse water supplies, causes blockage of emitters and unsightly growth on pots, media, benches and walkways. The presence of algae provides a food source for fungus, gnats and shore flies. Water stored in containers should be shielded from light to discourage algae growth. Where practical, commercial products containing bromine can be constantly injected into the water line to provide good algae control. Chlorine, ozone and ultraviolet light have also been used as controls.

Water Testing

Water quality fluctuates throughout the year. It is recommended to do complete water analyses at least four times a year; more often if new wells are being used, and constant testing of EC and pH levels. A pre-season laboratory analysis is strongly recommended. The water should be sent to a laboratory to analyze the water for irrigation suitability (as opposed to human consumption). In Saskatchewan, Enviro-Test Laboratories, Saskatoon can provide these services. Other laboratories in Canada and the United States are also used by some growers. When collecting water for samples, allow the water to run for 20 minutes and then collect it in a new plastic bottle. Most laboratories can supply bottles. The bottle should be completely filled and should not have a metallic lid.

Meters to test pH and salinity (conductivity) are available from greenhouse supply companies.

Water Treatment

Reverse Osmosis - units have a membrane which allows only water to pass through, thereby leaving dissolved salts and solids behind, along with a certain amount of waste water. These units are costly, but are one of the most widely used methods for purifying greenhouse water.

De-ionizers - use specialized exchange resins to trap dissolved salts and replace them with hydrogen and hydroxyl ions, constituents of pure water. The exchange resins must be periodically replaced or flushed with acids or alkalis.

Filtration systems - utilizing sand, cellulose or ceramic sieves are also used to some extent to remove undesirable minerals.

Acidification - involves the use of acids, usually nitric or phosphoric, to reduce alkalinity. To reduce a bicarbonate level from 150 to 50 mg/l would require approximately 13.6 ml of nitric acid (62 per cent) per 1,000 l water, or 16.8 ml of phosphoric acid (75 per cent) per 1,000 l water. A pH meter should be used to check the final pH of the water, and a laboratory test would be recommended before use.

Ultraviolet light - is used to kill harmful organisms, but is not too effective unless the water is relatively clear. If water is turbid, filtration may be necessary beforehand.

Heat treatments (to about 95-97 degrees C) - are used to destroy pathogenic bacteria, fungi and viruses. Filtration of the solution prior to treatment is recommended.

Distillation - is rarely practical because of the cost of operation.

Leaching - while not a treatment, can be practiced if poor quality water must be used. Guidelines are given in the following table:

Leaching Guidelines