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Section 2: Water Quality Issues and Recommendations

pH
The pH of water is the measure of the H⁺ (proton) concentration in the water. It is measured on the pH scale. On this scale, a value of 7.0 is considered neutral while a value below 7.0 is acidic and a value above 7.0 is alkaline or basic. A pH in the range of 5.4 to 7.0 is generally recommended for water to be used to irrigate greenhouse crops.

A major concern with water pH relates to the effect that water will have on the efficacy of pesticides. Many pesticides undergo hydrolysis at a high pH. For example, the half-life of Captan at pH 9.0 is 2 minutes, while it is 10 hours at pH 5.0. The half-life of Lannate at pH 9.0 is 12 hours, but it is 60 hours at pH 5.0.

Alkalinity (bicarbonate equivalent level)
The alkalinity level (also referred to as the bicarbonate or bicarbonate equivalent level) is of interest because it is the best indictor of a how a water source will affect the substrate pH over time. The alkalinity (do not confuse an alkaline pH with the alkalinity level of water) level is essentially a measure of the minerals in the water that act as bases. These bases neutralize acids in the substrate solution and cause the pH of the substrate solution to increase over time. These bases can be thought of as "lime" in the water. Therefore, the alkalinity level can also be thought of as water's acid neutralizing potential. The alkalinity level, not the pH, is the property of water that has the greatest effect on the substrate pH. When considering a water source for greenhouse irrigation purposes, it is very important to determine the alkalinity level of the water in addition to the pH. The pH of water and its alkalinity level are not always proportional. A water source with the highest pH may not necessarily have the highest alkalinity level. The recommended ranges for irrigation water alkalinity level for different container sizes and crop types are listed in Table 1.

It is important to understand the difference between alkaline water, water alkalinity and water hardness. Alkaline (or basic) water is that which has a pH over 7.0. As discussed above, alkalinity refers to the acid neutralizing ability of water (i.e. the carbonates and bicarbonate equivalents in the water) and is not directly related to the water pH. The hardness of water relates to the concentration of Ca and Mg in the water. Although hard water often has a correspondingly high alkalinity level, this is not always true since other sources of Ca and Mg (i.e. calcium chloride and magnesium chloride) can result in “hard” water but not a high water alkalinity level.

Although bicarbonates (HCO₃^-) and carbonates (CO₃^-) are the major contributors to water alkalinity, other materials such as silicates, borates, ammonia and phosphates may contribute to the water alkalinity level. However, most laboratories report the alkalinity level as bicarbonates or carbonates (or bicarbonate equivalents) since these two components account for the majority of the water source’s alkalinity level.

Alkalinity may be expressed using several units. The first is meq^.L^-1 (milliequivalents per liter). The second is parts-per-million (ppm) calcium carbonate equivalents (CaCO₃) or parts-per-million bicarbonate equivalents (HCO₃^-). Greenhouse managers should be careful to pay attention to the specific units that are used by a lab when interpreting water alkalinity test results. Alkalinity expressed in meq^.L^-1 may be converted to ppm CaCO₃ or HCO₃^- using the following conversion factors:

1 meq^.L^-1 = 50 ppm CaCO₃
1 meq^.L^-1 = 61 ppm HCO₃^-

Although minerals in the water that contribute to alkalinity neutralize acids in the substrate solution and tend to cause the pH of the substrate to increase over time, there are many factors that will dictate how much the substrate pH will actually change. These include the water alkalinity level, watering practices, crop duration, fertilization program and the cation exchange capacity (buffering capacity) of the substrate.

The higher the water alkalinity level, the more water applied (the more frequently the crop is irrigated), and the longer the crop duration, the greater the potential substrate pH increase. The use of basic fertilizers such as KNO₃ and Ca(NO₃)₂ will also cause the pH to increase over time while the use of acidic fertilizers such as NH₄NO₃ will tend to cause the pH to go down over time. Using basic fertilizers in combination with high alkalinity water can cause a fairly rapid and large increase in the substrate pH (acting in combination) while acidic fertilizer usage may partially offset the affect of alkalinity in the water (acting in opposition to the water) on substrate pH. The higher the cation exchange capacity of the substrate, the more buffered the substrate and the more resistant the substrate pH will be to change.

The primary problem with irrigation water that has a high alkalinity level is that the use of such water (ignoring all other factors) will tend to cause the substrate pH to increase over time. The higher the alkalinity level, the greater the potential increase in substrate pH. After many weeks, depending upon the actual alkalinity level, the pH may increase to a level at which mineral nutrient deficiencies (i.e. Fe deficiency) begin to occur. However, a certain alkalinity level might be desirable. This is because it provides a source of important mineral elements and can help neutralize the potential acidifying effects of acidic fertilizers and organic acids produced by some substrate components. Some greenhouse growers using very low alkalinity water sources often expirience the problem of decreasing substrute pH over time (especially if using acidic fertilizers). The key is to balance all of these factors to maintain the substrate pH in a desirable range (discussed in more detail in the “Mineral Nutrition” learning unit).

Salinity
Salinity may be measured and reported using two different methods. Soluble salts is a measure of the total dissolved salts (TDS) and is reported as parts-per-million. Thus, soluble salts or TDS is a measure of the concentration of ions (i.e. Cl^-, K⁺, Ca⁺⁺, NH₄⁺, NO₃^-, etc.) in solution. The electrical conductivity (E.C.) is a measure of water’s ability to conduct an electric current and is most often reported as mmho^.cm^-1, mS^.cm^-1 or dS^.m^-1 (with 1 mmho^.cm^-1 = 1 mS^.cm^-1 = 1 dS^.m^-1). The higher the concentration of dissolved ions in the water, the greater is its ability to conduct an electric current. Thus, the higher the ion concentration, the higher the electrical conductivity. Because different ions conduct an electrical current with different efficiencies, the soluble salt level (TDS) and the E.C. are not truly convertible (although 1 mmho^.cm^-1 = 640 ppm TDS has sometimes been used as an average conversion factor for fertilizer and substrate solutions). Sometimes labs will report soluble salts (as S.S. or TDS) with the units in mmho^.cm^-1 or mS^.cm^-1. However, this is an incorrect use of the terms as these units are a measure of E.C. and not soluble salts. Greenhouse managers must therefore be sure to pay attention to the units in which the water salinity is reported.

It is important to understand that both soluble salts and electrical conductivity provide an overall measure of the ions in the water. However, these measurements provide no information as to which specific ions are in the water or their respective concentrations.

A high irrigation water E.C. (or soluble salts) can elevate the E.C. level of the substrate. High substrate E.C. levels can cause root damage, inhibit root growth, make plants more susceptible to disease attack and inhibit seed germination. Recommended E.C. (and soluble salts) levels for irrigation water for greenhouse crops production are listed in Table 1.

Level of potentially phytotoxic ions (B, Cl^-, F^-, Na⁺, and SO₄^2-)
The E.C. of water provides the overall concentration of ions but provides no information as to what ions are in the water or their concentrations. In addition to contributing to the overall E.C. of the water, specific ions may occur in water at levels that are phytotoxic to plants. The ions that are most commonly of concern are B, Cl^-, F^-, Na⁺, and SO₄^2-. Levels of these ions are usually reported as parts-per-million.

High boron concentrations can be phytotoxic to some plants (especially as plugs) and cause shoot tip necrosis (turn brown or black) and abortion. High concentrations of Cl^-, F^- and Na⁺ can cause poor root development and leaf necrosis (leaf scorch). High SO₄^2- concentrations can elevate the E.C. and reduce root growth. Recommended levels for specific mineral element ions in irrigation water are listed in Table 1.

The sodium absorption ratio (SAR) is the ratio of Na⁺ to Ca⁺⁺ and Mg⁺⁺ in the water, and it is sometimes an important water quality factor especially in the production of seedling plugs. The SAR is of interest because excessively high levels of sodium in relation to Ca⁺⁺ and Mg⁺⁺ can result in excessive uptake of Na⁺ by the plant resulting in phytotoxicity (i.e. marginal leaf burn on older foliage). As a general guideline a water SAR of 4 or lower is recommended for irrigation water for most greenhouse crops (2 or lower for plugs). However, since additional Ca⁺⁺ and Mg⁺⁺ are provided in the fertilization program, higher levels may be tolerated in many production situations. Further a maximum Na⁺ concentration in irrigation water of 70 ppm (3 meq^.L^-1) is generally recommended for greenhouse crops (maximum of 40 ppm for plugs). However, depending upon the crops being grown and the fertilization program, higher sodium levels may be tolerated in the irrigation water.

Iron
High levels of Fe in the water can discolor plant foliage and flowers, clog irrigation nozzles, promote the development of certain bacteria and make mineral ions such as Mn⁺, Ca⁺⁺ and Mg⁺⁺ unavailable. High levels of iron in the water can also result in brown or red-brown deposits on surfaces. Desirable iron levels are listed in Table 1.

Bacteria and fungi
Some bacteria and fungi in water may be pathogenic in nature (i.e. Pythium and Phytophthora). Plant pathogenic species of Pythium and Phytophthora have been reported in irrigation water taken from ponds. Other bacteria and fungi, although not plant pathogens, may result in clogging of irrigation lines. This can especially be a problem in subtropical and tropical locations. Iron-fixing bacteria can proliferate in water that is high in iron. These bacteria can clog irrigation systems and also leave a bluish bronze color on plants.

Algae
Algae can result in the clogging of irrigation lines. Additionally, algae growing on walkways or underneath benches serve as a location for fungus gnats to develop. Algae can be a particular problem in ponds and lagoons used as water sources during summer months.