Chapter 11

Irrigation Water for Greenhouses

Water Quality for Greenhouse Crops

Irrigation water quality is a critical factor for production of greenhouse crops. There are many factors which determine water quality. Among the most important are alkalinity, pH and soluble salts. But there are several other factors to consider, such as whether hard water salts such as calcium and magnesium or heavy metals that can clog irrigation systems or individual toxic ions are present. 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.

Water Quality Target Parameters

Water quality target parameters include physical, biological, and chemical properties. Physical properties include suspended solids such as soil particles. Suspended solids such as soil particles are potential problems since these particulates can clog irrigation nozzles and cause abrasion of irrigation equipment. Important biological properties include iron fixing bacteria and plant pathogens or algae. Bacteria and algae are a concern since they may cause clogging of irrigation system components. Chemical properties are typically given the most focus when dealing with irrigation water. From the grower’s standpoint, the most critical chemical water quality parameters are soluble salts, hardness, sodium and chloride concentration and pH.

pH

In general terms, pH is a measure of the hydrogen ion concentration. The pH can vary on a scale from 0-14 with a pH of 7 being neutral, less than 7 considered acid and above 7 called basic. Sometimes the term “alkaline” is used instead of “basic” and often “alkaline” is confused with “alkalinity.” Irrigation water with a pH of 4 might be termed very acid and water with a pH of 8.5 very alkaline. The main effect of water pH on plant growth is through control of nutrient availability (See Figure 11.1). A low pH may be responsible for excess iron and manganese availability leading to toxicity, or calcium and magnesium deficiencies. A high pH may cause iron and manganese and other minor nutrients to become unavailable to plants, leading to deficiencies. High pH water can also cause salts to precipitate out of fertilizer stock tanks and can also reduce the efficacy of pesticides.

Alkalinity

Alkalinity is the concentration of soluble compounds in the water that have the ability to resist or neutralize the effects of acids, including those found in fertilizers and growing media. Alkalinity acts as a buffer to acidic materials. The higher the alkalinity, the greater the amounts of acid that will be required to produce a desired pH drop. The major chemicals responsible for alkalinity in water are bicarbonate ions (HCO3-) from dissolved salts such as calcium bicarbonate (Ca(HCO3)2), sodium bicarbonate (NaHCO3), and magnesium bicarbonate Mg(HCO3)2); and to a lesser extent carbonate ions (CO32-) from dissolved salts such as calcium carbonate (CaCO3) and magnesium carbonate (MgCO3). Hydroxide ions (OH-) are a minor contributor in most cases. Ammonia, borates, organic bases, phosphates, and silicates can also be minor contributors to alkalinity. These dissolved carbonate and bicarbonate ions neutralize hydrogen (H+) ions, which increases the pH of the media solution.

Water Hardness

Water hardness is a measure of the combined content of calcium and magnesium in water, while alkalinity is a measure of all chemical bases in the water (bicarbonates and carbonates).  The level of hardness (See Table 10.2) is expressed as the total amount of CaCO3 in milligrams per liter of water (mg/L) or ppm. As in the case of alkalinity, 1 meq/L CaCO3 = 50 mg/L CaCO3 (Note: 1 mg/L = 1 ppm). Water with hardness in the range of 100 to 150 mg CaCO3/L is considered desirable for plant growth. Plants tolerate high levels of these elements, so toxicity is not normally a problem. However, excessive hardness may cause foliar deposits of calcium or magnesium carbonate under overhead irrigation.

Electrical Conductivity

Electrical conductivity (EC, also known as conductivity or soluble salts) is a term used to measure the total concentration of salts in the water. A salt is a combination of positively charged elements (cations) and negatively charged elements (anions). The most common cations of interest in water are calcium (Ca+2), magnesium (Mg+2) and sodium (Na+); the most common anions are bicarbonate (HCO3-), chloride (Cl-) and sulfate (SO4-). Fertilizer, fungicidal drenches, and breakdown of organic substrate components can also contribute additional salts. The ability of water to conduct an electrical current is directly related to the concentration of salts present in solution. Thus, the higher the ability of water to conduct electricity, the higher its salt content, and the less desirable it becomes for plant growth. Irrigation water is classified by its salt hazard in Table 10.3.

Salinity Affects on Plants

Plants are adversely affected by salinity in several ways. The most important of these is the osmotic effect, which limits the ability of plants to take up water. Plants absorb water from the soil (or growing medium) through their roots. A key process that allows the roots to do this is osmosis. Dissolved substances within the roots such as salts and sugars attract water through the root membrane, from where it moves to the rest of the plant. This process continues as long as the concentration of dissolved substances inside the roots is higher than that of the soil water available to the plant. If the soil water salt concentration is too high the plant will not be able to absorb water: it will wilt and begin to die. The point at which this happens depends on the type of plant, the salt concentration of the water, and the type of salts in the soil water.

Monitoring Container Leachate for EC

Monitoring container leachates of the substrate can help growers take preventive steps to reduce possible damage to roots due to high electrical conductivity before visible symptoms occur. Some greenhouses monitor ECís on a weekly basis, to determine how they will irrigate or fertigate each zone the following week. If the leachate EC is elevated during crop production, more water will be applied the following week to leach out salts or fertilizer rates can be reduced.

Sodium Adsorption Ratio or SAR

Sodium is another salinity factor which, if found in high levels, could reduce water movement into the plant and retard growth. The sodium adsorption ratio (SAR) is a general water quality index that indicates the percentage of sodium in the water and function of the ratio of sodium to calcium and magnesium. The SAR values help to determine the potential for permeability problems in the growing media, as well as the possibility for plant sodium toxicity after long-term use of the water.

Adjusted Sodium Adsorption Ratio

The adjusted sodium adsorption ratio (sometimes symbolized as SARadj) is a modification of the original SAR calculation. It serves the same purposes, but is modified to include the effects of bicarbonates (HCO3-) and carbonates (CO32-), in addition to calcium (Ca2?) and magnesium (Mg2+). Carbonate and bicarbonate can aggravate a sodium hazard by combining with and removing from the soil solution some of the exchangeable calcium and magnesium, which increases the concentration of sodium.

Plant Macronutrients

The macro elements Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg), and Sulfur (S) are elements essential for plant growth, and at moderate levels will not cause production problems. However, content of these elements should be evaluated as an indicator of potential contamination of the water (for N, P, and K) as well as an indicator of fertilizer requirements (for Ca, Mg, and S).

Nitrogen

The presence of nitrogen in substantial quantities will not harm plants but can be indicative of a strong possibility that the water has been contaminated with a fertilizer or other contaminant. In such cases additional testing should be done to determine if pathogens are present.

Calcium and Magnesium

The calcium and magnesium ratio in the media solution (and in the irrigation water) should be 3 Ca to 1 Mg if expressed as meq/L or 5 Ca to 1 Mg if expressed as ppm Ca and Mg. If the ratio is significantly different, a deficiency may occur in the nutrient that is undesirably low.

Sulfur

Sulfur concentrations in irrigation water are usually less than 25 ppm, and excessive sulfur is not normally a problem, except where acidification poses problems. It is not usually necessary to add sulfur to reach these recommended levels, as sulfur is a common contaminant of many fertilizer sources.

Plant Micronutrients

Waters can contain small concentrations of Aluminum, (Al), Boron, (B), Copper, (Cu), Fluoride, (F), Iron, (Fe), Manganese, (Mn), Molybdenum, (Mo), Sodium (Na), and Zinc (Zn). With the exception of aluminum and fluoride, these elements are essential to plant growth and are required in small quantities. Micronutrient toxicities are more common when substrate pH is low, which increases the availability of most of these nutrients.

Boron

Among the plant micronutrients found in water, boron can be particularly troublesome. Like other nutrients, greenhouse crop sensitivity to boron varies so certain species may be susceptible to damage at a concentration of 0.5 ppm while others tolerate levels up to 4 ppm. Plant age also influences susceptibility or degree of problem. Seedlings will generally be more susceptible than mature plants of the same species.

Chlorine

Though not usually considered an essential micronutrient, chlorine (as chloride) is needed in small quantities by plants. However, chloride levels greater than 70 ppm (2 meq/L) can become a production problem. The principal effect of too much chloride (Cl-) is to increase the osmotic potential of the substrate solution, which reduces the availability of water to plants and can lead to wilting. High chloride levels can also lead to chloride toxicity symptoms in container production.

Fluoride

Fluoride is often added to municipal water at a concentration of 1 ppm to prevent tooth decay. This level is safe for most crops but not for members of the lily family such as the genera Chamaedorea, Chlorophytum, Ctenanthe, Dracaena, Marantha, Spathiphyllum and a few other plants. Toxic levels of fluoride can scorch of the tips of older leaves.

Iron

Overhead irrigation with water containing 0.5 ppm or more of iron frequently results the formation of a bluish-bronze sheen as well as brown deposits on plant foliage and brown staining of greenhouse glass or plastic.

Sodium

High sodium is a concern to growers since it can contribute to salinity problems, interfere with magnesium and calcium availability in the substrate and cause foliar burns. Water with sodium levels greater than 3 meq/L should not be used for overhead irrigation of ornamentals and greenhouse plants as foliar absorption of sodium can lead to sodium toxicity in sensitive species.

Zinc or Copper

Problems associated with zinc or copper toxicities are generally not associated with water systems; however, in a few cases acid water transported through galvanized or copper pipes may result in toxic levels of these elements in the irrigation water. These elements are a concern if using an acid growing media.

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