Section 1: Water Management

Water is a major factor in the production of greenhouse crops, and greenhouses use large volumes of water for irrigation (estimated to be 0.5 gallons per square foot per day). Proper water management requires that the appropriate volume of water (or liquid fertilizer solution) be applied with each irrigation. This amount is typically a volume sufficient to thoroughly wet the substrate in the containers but not to result in significant leaching (water flowing out of the bottom of the container). Irrigation frequency depends on the substrate, temperature, relative humidity and plant growth as all of these factors affect the rate of water loss from the substrate. Typically, the surface of the root substrate should be allowed to dry slightly between irrigations. Over-watering of greenhouse crops is generally a result of irrigating too frequently. Water quality must also be considered. This is because water quality or chemical characteristics (i.e. pH, alkalinity, mineral element content, etc.) of water used for irrigation and fertilization will significantly affect the chemistry of the substrate.

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 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 affect 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 bicarbonates 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. 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 (potassium nitrate) and Ca(NO₃)₂ (calcium nitrate) will also cause the pH to increase over time while the use of acidic fertilizers such as NH₄NO₃ (ammonium nitrate) 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 opposed to the water) on substrate pH. The higher the C.E.C. 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 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. 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). 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 or meq^.L^-1.

High boron concentrations can be phytotoxic to some plants (especially as plugs) and cause shoot tip necrosis 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 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 results 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.

Section 3: Correcting Water Quality Problems

pH and Alkalinity
Acid injection may be used to adjust the pH of the water as well as the alkalinity level. Enough acid may be added to either lower the alkalinity down to the desired level (with little impact on pH since the carbonates and bicarbonates neutralize the acid and buffer the water pH) or enough may be added to neutralize the alkalinity and lower the pH to a desired level.

Acidification reduces the alkalinity level of water by neutralizing the bicarbonates and carbonates (as well as other bases) in the water. Acids most commonly used to neutralize alkalinity in water to be used for greenhouse crops production include citric acid (either 99.5% granular or 50% liquid), nitric acid (61.4% or 67% liquid), phosphoric acid (75% or 85% liquid), and sulfuric acid (35% or 93% liquid).

Each of these acids has advantages and disadvantages and the most appropriate one to use depends upon the actual water alkalinity level, the volume of water that needs to be adjusted and the mineral nutrition program being used.

Citric acid is effective at reducing water alkalinity but it is expensive. However, citric acid does not provide mineral nutrients and does not interact with other fertilizer salts. Therefore, it is most useful where the alkalinity or pH of a fertilizer stock solution needs to be adjusted or where water pH needs to be adjusted for mixing of pesticides. Although it can cause minor skin and eye irritation, citric acid is relatively safe and easy to use.

Nitric acid is very effective at reducing water alkalinity but it can be dangerous and difficult to use. Nitric acid should not come in contact with the skin or eyes and care should be taken not to breathe the fumes. Appropriate protective gear should be worn when dealing with nitric acid. Nitric acid also provides nitrogen to the irrigation water (and thus to the mineral nutrition program). As is shown in Table 2, a fluid ounce (1.0 fl. oz.) of 67% nitric acid in 1000 gallons of water will provide 1.64 ppm nitrogen.

Sulfuric acid is very commonly used to reduce water alkalinity or to adjust water pH. It is relatively low cost. Although it can cause skin and eye irritation, it is less dangerous and less caustic than nitric acid. Sulfuric acid also provides sulfur to the irrigation water. As shown in Table 2, a fluid ounce (1.0 fl. oz.) of 35% sulfuric acid in 1000 gallons of water will provide 1.14 ppm sulfur. Excessive levels of sulfur from the use of too much sulfuric acid can result in the tie-up (make unavailable for uptake by the plant) of calcium in the substrate (insoluble calcium sulfate, or gypsum, forms). Appropriate protective gear should be worn when dealing with sulfuric acid.

Phosphoric acid is also commonly used to adjust water alkalinity and pH. However, it is a weaker acid than nitric and sulfuric acid.  It is caustic and can cause skin and eye irritation. Phosphoric acid provides phosphorus to the irrigation water. As is shown in Table 2, a fluid ounce (1.0 fl. oz.) of 75% phosphoric acid in 1000 gallons of water will provide 2.88 ppm phosphorus. In cases where the water alkalinity is very high, a large amount of phosphoric acid would be required to adjust the water alkalinity to the desired level. This would result in the supply of a large amount of phosphorus to the irrigation water. In such a case, the level of phosphorus in the irrigation water might exceed the recommended level. High levels of phosphorus can cause stretching or elongation of young plants and very high phosphorus levels can cause the tie-up of iron. Therefore, where large water alkalinity level corrections are required, it may be preferable to use sulfuric or nitric acid to adjust the water alkalinity level.

The amount of acid to be injected depends upon the acid, the water alkalinity and the target alkalinity level (or the targeted pH). Table 2 lists the amount of various acids to add to 1000 gallons of water for each meq^.L^-1 of alkalinity in the water to adjust the final water pH to 5.8 (alkalinity is neutralized and the pH lowered). These values can be used a general recommendation but since the starting pH of the water is not included in the calculations of these values, the exact amount of acid required may vary slightly and greenhouse managers should conduct trials to determine the exact response of their water to acid injection.

In many cases, a greenhouse manager will only want to reduce the water alkalinity to an acceptable level but not neutralize all of the carbonates and bicarbonates (and other bases) in the water as shown in the above example. In this case, the desired water alkalinity (expressed as meq^.L^-1) is subtracted from the actual water alkalinity. This gives the amount of alkalinity that must be neutralized. For every 1.0 meq^.L^-1 that needs to be neutralized, approximately 7.0 fl. oz of 85% phosphoric acid or 3.5 fl. oz of 93% sulfuric acid are required per 1000 gallons of water. For example, if the water has an alkalinity level of 2.8 meq^.L^-1 and the desired level is 1.0, then 1.8 meq^.L^-1 must be neutralized. To achieve this, 12.6 fl. oz. of 85% phosphoric acid or 6.3 fl. oz of 93% sulfuric acid would need to be added per 1000 gallons of water.

The above method does not account for the pH of the water and differences in the actual alkalinity values in water and thus provides only a general recommendation. Another method (and more exact) to determine how much acid is required to reach a target alkalinity level or pH is to use the North Carolina State University Alkalinity calculator. This software allows the greenhouse manager to enter the pH and alkalinity of the water source and the desired water alkalinity or water pH. The software calculates the amount of each type of acid required, the amount of mineral nutrients provided and a cost estimate.

Many commercial soil testing laboratories will also test water quality and will provide greenhouse managers with recommendations regarding acid injection for any corrections to alkalinity or pH required.

Caution should always be taken when using acids. Small amounts of acid should be added to larger volumes of water (NOT water to acid). As discussed below under injectors, injectors used for acid injection must be acid compatible. Acid should be injected into the water prior to the injection of fertilizers.

In addition to acid injection, waster alkalinity may be reduced by using blended water. In this case low alkalinity water (such as rainwater) is blended with or added to a water source that has an undesirably high alkalinity level. The two water sources are blended together at a ratio that results in water with a desirable alkalinity level.

In cases where water alkalinity is only slightly higher than the desired level, acidic fertilizers might be used to offset or counter the increase in pH caused by the alkalinity of the water. This subject is discussed in more detail in the “Mineral Nutrition” learning unit.

Salinity (E.C.)
High water E.C. levels are a common problem in many areas of the U.S. The first possible method of correcting a high E.C. level in irrigation water is to develop an alternative source of irrigation water that has an acceptable E.C. If a reliable source of acceptable E.C. water is available, it should be used for irrigation. If only small amounts (insufficient to meet the entire need of the greenhouse operation) of low E.C. water are available, the high and low E.C. water sources may be blended to achieve an acceptable E.C. level. Some greenhouse managers collect rainwater to blend with other sources of water that may have a high E.C. In a some cases, greenhouse managers have used reverse osmosis (R.O.) units to produce "pure water" that can be blended with other water sources. These units are expensive to operate, but they may be an option depending upon the operation and the volume of water required. Never use a water softener for greenhouse irrigation water. These systems replace calcium and magnesium compounds with sodium and a high sodium level in the water can cause serious problems during crop production.

Phytotoxic ions
The major method of correcting the problem of having one or more specific ions in the water at a higher than acceptable level is to select a water source low in potentially phytotoxic ions or to blend water as described under the salinity section.

If high iron is a problem, the water may need to be aerated to cause iron to precipitate out of the water, the water may need to be filtered, or sequestering agents (i.e. Aqua-Solv) may be added to the water to precipitate out the iron.

Bacteria, fungi and algae
Greenhouse mangers may need to disinfest (kill microorganisms such as bacteria and fungi) water used for greenhouse irrigation. This may be because fresh water sources such as ground water or water from ponds or lagoons may contain algae and plants pathogens (i.e. fungi, bacteria or viruses) or because recollected and recirculated water or fertilizer solutions (i.e used in ebb-and-flow benches, Dutch trays or flood floors) are being used which might spread potential disease causing organisms around the greenhouse operation. There are numerous methods for disinfesting greenhouse irrigation water and recirculated fertilizer solutions and these are discussed below.

Ultraviolet
Ultraviolet light (UV) is a common method used by greenhouse managers to disinfest irrigation water and recirculated fertilizer solutions. Ultraviolet light treatment has proven to be effective at controlling many plant pathogens including Pythium, Fusarium, Phytothera, and Alternaria. In this system water is passed over UV light-generating tubes which generate UV-C (typically around 254 nm wavelength) light. The UV light kills algae and plant pathogens. A limitation with using UV light is that although it can penetrate approximately 25 cm of clean water, it may penetrate low quality water only a few millimeters since particles in the water block the UV light. Additionally, debris or organic material in the water will block the UV light. If the UV light is blocked, some pathogens will not be killed since they are protected in the shadow of the material blocking the UV light. Therefore, water quality is an important issue when using UV light to disinfest water or fertilizer solutions. When using UV light, water or fertilizer solution is usually passed through filters to remove any debris before the solution is passed through the UV-light unit. Ultraviolet light can affect chelating agents used in fertilizer solutions. If chelating agents are to be used in a fertilizer solution that will be treated with UV light, consult with the manufacturer regarding compatibility.

The effectiveness of UV-C light for disinfestation is dependent on duration and intensity of the UV light. UV-C radiation intensity for greenhouse irrigation water is typically 100 mJ/cm² for selective disinfection (killing most fungi and bacteria) or 250 mJ/cm² for total disinfestation including viruses. The use of UV-C radiation is an effective treatment for disinfesting greenhouse irrigation water, but since there is no residual activity, steps must be taken to keep the water clean after treatment.

Slow Sand Filtration (Biofiltration)
Slow sand filtration relies on both physical and biological activity in controlling plant pathogens. A slow sand filter has a high surface area, which can be colonized by suppressive microorganisms. This sand medium (typically in layers of different grain sizes) also presents a physical barrier to the passage of spores of fungal plant pathogens and nematodes but not to bacteria or viruses. The effectiveness of slow sand filtration is debated, and it is most commonly recommended as a pre-treatment for the other sanitation technologies such as UV light, ozonation and chlorination since particulates reduce the efficiency of these treatments.

Heat Treatment
Heat treatment is commonly used in Europe to disinfest greenhouse water. Water temperature should reach 95°C (203°F) for at least 30 seconds for effective control of pathogens. A major disadvantage associated with this method is the high cost of heating the water.

Ultra-Filtration
Ultra-filtration relies on fine membrane filters that contain small pores which physically filter the suspended solids as well as fungi, bacteria and viruses. This kind of filtration completely eliminates the water pathogens, but can be very expensive. With this method, pre-filtration (i.e. through sand columns) is required and problems may arise from the filter pores becoming blocked over time. Filters with different pore sizes have been tried (0.05 μm -0.25μm). However, cost and clogging issues have limited the use of this technology in most greenhouse applications.

Ozone
Ozone treatment is more expensive in capital and operating costs than other technologies. Ozone (O₃) occurs naturally during thunderstorms where oxygen (O₂) comes into contact with the strong electrical field caused by lightening.  Ozone used to disinfest greenhouse irrigation water is produced by ozone generators that also use a strong electrical field to generate ozone. The ozone is then added to the water by bubbling it in the water or more often by injecting it under pressure into the water in a pressurized tank (this results in more ozone being absorbed by the water). The ozone rapidly breaks down into dissolved oxygen and hydroxyl ions (OH^-) as it reacts with impurities in the water. The capacity of ozone to disinfest water is affected by organic matter present, water pH, water E.C., and amount and type of iron chelates in the water. The rate of ozone breakdown increases at high water pH and its effectiveness is reduced by organic matter and mineral salts in the water. Water temperature also affects the half-life of ozone with the half-life decreasing as water temperature increases. Ozone has been shown to reduce the concentration of available iron in the water or fertilizer solution.

Recommended ozone concentrations vary depending upon the source of the information. One recommendation was for ozone to be injected into greenhouse irrigation water at 0.5 to 1.5 ppm. However, another recommendation was for residual levels of ozone not to exceed 1.0 ppm to avoid phytotoxicity. The difference may be that the first recommendation is the initial injection rate where the later is the ozone concentration after the injected ozone destroys pathogens, algae and other organic matter in the water. It also has been noted in published reports that 0.2 ppm ozone exposure for 30 minutes will destroy biofilms (organic layer build up of bacteria and algae) and that a residual concentration of 0.01 to 0.05 ppm ozone will control algae.

For optimum results, the pH of the water should be approximately 4.0 with a contact time of one hour. Because of the required exposure time, ozone is typically injected into a water storage system and should be maintained under pressure to prevent off-gassing of the ozone. In clean water, dissolved ozone can be measured with by the water's oxidation-reduction potential (ORP), but, with the strong oxidizing power of ozone, water with moderate turbidity can result in ORP values far below expected levels. One of the primary advantages of ozone disinfestation is that no corrosive chemicals are involved. Yet, there is little residual disinfesting potential after treatment. Also, ozone may react with some fertilizers oxidizing iron, manganese and sulfides.

Chlorination
Chlorination is effective at killing a range of plant pathogens. It works rapidly, and is relatively inexpensive to utilize. However, it is active over a smaller pH range than some other methods. Chlorination is usually carried out by injecting metered amounts of sodium hypochlorite solution, calcium hypochlorite solution or chlorine gas. The difficulty with effective chlorination is that the amount of chlorine required depends on the impurities, primarily organic matter, in the water. To determine whether chlorination is effective, the residual free chlorine needs to be monitored regularly.

Chlorine activity may be measured and reported as free residual chlorine or total chlorine in ppm. A typical recommended rate for disinfesting greenhouse irrigation water has been to inject enough chlorine to have 5.0 ppm at the well head with a residual level (maintained in the water after initial oxidation of organic matter in the water) of 0.5 to 2.0 ppm (for continuous application). However, chlorine exists in water as HOCl (hypochlorous acid) or OCl^-1 (hypochlorite). Hypochlorous acid is a strong, fast acting oxidizer, whereas, hypochlorite is much weaker. As the pH of the solution increases, HOCl converts to its ionic form OCl^-1. Therefore, measuring the total chlorine of a solution does not necessarily indicate the oxidizing strength of a solution (the actual disinfesting strength depends on the concentration of chlorine and the form). In order to maintain free chlorine in its most active form, the solution pH should be maintained between 7.4 and 7.6 A better measurement of the disinfesting potential of chlorine in water (as well as other oxidizing agents such as fluorine, bromine and ozone), is the water’s oxidation-reduction potential (ORP with units of milliVolts). Data from many sources has established that 650-700 milliVolts ORP as the minimum threshold for typical anti-bacterial activity. Numerous meters are available that measure both free chlorine in water as well as the ORP of water. Some of these meters are designed to be placed in the irrigation line while others are simple hand-held devices.

Chlorine for disinfesting irrigation water is available in four common forms including chlorine gas, sodium hypochlorite, calcium hypochlorite and chlorine dioxide (discussed below). Each has advantages and disadvantages. Chlorine gas is probably the cheapest form of chlorine available but it requires on-site injection equipment. It is most active between the pH 6.0 to 7.5. Chlorine gas requires specific pH and concentration control, and it is highly corrosive if improperly handled. Chlorine gas is also dangerous and requires alarms and scrubbing technology in the event of a spill. Sodium hypochlorite is the active ingredient in standard household bleach. Household bleach typically contains 3% to 6% NaOCl, while industrial bleaches are typically 10% to 12% NaOCl. Sodium hypochlorite dissociates in water to release hypochlorous acid and there is a concomitant formation of sodium hydroxide. Sodium hydroxide is corrosive to metals so specialized equipment and pumps are required with its use. Because of its corrosiveness, sodium hypochlorite is not often used for greenhouse irrigation water disinfestation. Calcium hypochlorite is a dry form of chlorine and when dissolved in water, is an effective disinfestant that eliminates bacteria, algae, slime, fungi and other microorganisms. Calcium hypochlorite has many advantages over chlorine gas and sodium hypochlorite. Calcium hypochlorite is much safer to handle compared to both chlorine gas and sodium hypochlorite. It is easier to store, and it is not as corrosive and is less harsh on equipment. Calcium hypochlorite is typically sold in tablets and some form of tank must be used for the tablets to dissolve into water. The tanks are designed so that the tablets break down at a consistent rate, releasing a controlled amount of chlorine into the water flow (i.e. Accu-Tab System).

There is a danger of phytotoxicity to plants if the residual free chlorine levels rise too high. Health hazards are a concern and safety precautions must be taken when handling the chlorination chemicals and also when storing the products.

Chlorine dioxide
Chlorine dioxide can be used to kill algae, bacteria, viruses and fungi in greenhouse irrigation systems. Chlorine dioxide is a yellowish gas which is produced on site by combining hydrochloric acid and sodium chlorite. Chlorine dioxide is 25 times more effective than chlorine gas as a disinfectant and is a synthetic yellowish-green gas with chlorine like odor. Chlorine dioxide is unstable as a gas, but is stable and soluble in water. The instability of chlorine dioxide requires that it must be produced and used at the same location requiring specialized equipment. Yet, chlorine dioxide is an extremely effective biocide, disinfectant agent and oxidizer even in the presence of high organic load conditions common to recirculated greenhouse irrigation water. Compared to calcium hypochlorite, chlorine dioxide is effective over a broader pH range with maximum efficacy at pH of 8.5. Disadvantages include the cost and complexity of the system, residual build up in substrate over time and the need for appropriate storage facilities for the chemicals involved.

Bromine
Bromine is similar in action to chlorine but it is more active across a wider pH range. Bromine is created similarly to chlorine, by adding sodium bromide, a natural compound found in sea water, to sodium hypochlorite. A lower concentration of bromine is needed to effectively kill pathogens as compared to chlorine.

Iodine
Iodine is a strong fungicide and bactericide. Iodine is effective even with high organic loads in the water (dirty water) and is not affected by pH. It remains active in water with a pH between 3.0 and 8.5. Iodine has no impact on nutrients in the fertilizer solution and therefore, fertigation and sanitation can be used in combination. Monitoring of iodine levels is very important to reduce the potential for phytotoxicity to plants.

Copper ionization
Copper coils may be placed in the water or fertilizer storage tanks. An electric current is passed through the coil which results in copper ions being given off from the coil into the water. The copper acts as an effective biocide and kills most bacteria and fungi. Copper concentrations of 1.0 to 1.6 ppm are typically used and a concentration of 1.15 ppm was found effective against Pythium.

Section 4: Common Irrigation Systems Used in Greenhouses

Water is a very important issue in greenhouse crops production. Concerns over water availability have resulted in water use restrictions in some parts of the U.S. (i.e. California, Florida, and Arizona). In many other locations, concerns over runoff of water containing fertilizers and pesticides have resulted in regulations requiring that runoff from greenhouse operations be contained and not allowed to flow into ground water, streams, or lakes. These types of concerns over water use and runoff have changed the way that water and fertilizer solutions are provided to greenhouse crops.

There is an expression that goes something like this: "The person at the end of the hose controls your profits". Although this is an exaggeration, it demonstrates the importance of proper water management in greenhouse crops production. Proper irrigation is important because it impacts so many aspects of production. Either too much or too little water can be detrimental to the development of a crop. Over watering may result in elevated pH, increased disease incidence, poor root and plant growth and leaching of mineral nutrients from the root substrate. Under watering can result in poor root and plant growth, flower and leaf abscission, increased disease incidence and high E.C. levels. In large production situations, it is difficult to provide water to plants only when needed. Natural variations in the rate of water loss due to evaporation and transpiration (evapotranspiration) require irrigation of some plants and not others. However, it is not practical or economical to hand water large numbers of plants so maintaining uniform and optimal moisture levels across a crop can be difficult.

There are many ways in which water (and thus mineral nutrients when a liquid fertilization program is being used) may be applied to a crop. Each of these methods or systems has advantages and disadvantages. It is common for greenhouse operations to utilize more than one type of irrigation system depending on the crops being grown. Various types of irrigation systems are discussed below.
 
Hand watering
Irrigating crops by hand using a hose with some type of water breaker is a time-consuming and costly process. However, irrigating by hand might be required at times when uneven drying of substrates occurs and "spot" watering needs to be conducted.

Boom irrigation systems can be controlled using various types of clocks, timers or computer controls. Many booms also have a programmable control unit attached to the boom that may be used to control when the boom operates and the speed at which it moves. Additionally, many booms are designed so that magnets may be placed on the track to differentiate zones. A sensor can detect the location of the magnets as it moves along the track. The control unit can be programmed to water these zones differently (i.e. different frequencies, speeds, etc.).

As with overhead emitters, irrigation booms spray the water or fertilizer solution into the air before it lands on plants or in the container. Some of the solution lands on plant foliage and is lost to evaporation. However, because the boom arms are typically closer to the plants than most overhead emitters and because, if designed and used correctly, water is typically not sprayed on sidewalks, irrigation booms are more water efficient than overhead emitters. Irrigation booms still apply water and fertilizer solution in such a way that leaching and runoff may occur. Irrigation booms are typically more expensive than simple overhead emitters. However, if the boom arms only cover the area where plants are grown, plants are spaced close together (or emitters adjust to match plant spacing) and the appropriate volume of solution applied during each irrigation pass, the amount of water and fertilizer solution wasted or lost through runoff can be minimized. Reducing water and fertilizer usage and runoff reduces production costs and reduces a greenhouse operation’s potential impact on the surrounding environment.

Drip tubes (Chapin or spaghetti tubes)
Drip tube irrigation systems supply water through individual tubes placed on the substrate surface of each container. The water or fertilizer solution trickles from the tube and spreads through the substrate. Large containers may require more than one drip tube to provide for even distribution of solution over the substrate surface. Water or fertilizer solution is pumped through supply lines and to each individual container. The supply line(s) to each container terminate with some type of emitter. The emitter is designed to keep the supply line in the container and to help spread the water or fertilizer solution over the substrate surface.

Various types of emitters may be used with drip tube irrigation systems. Some are small lead weights that hold the supply line in the container and spread the solution out in two or three directions over the surface of the substrate. Other types of emitters are made of plastic. The bottom end of the emitter has a small spike that is inserted into the substrate. The spike holds the emitter in the container. The top of the emitter forces the solution against a small plastic plate which causes the solution to spray out over the surface of the substrate. The amount of water supplied to the containers is a function of the flow rate of the supply lines (gallons per minute supplied as a function of the size of the supply lines and the water pressure), the delivery rate of the attached emitters (listed on emitters as gallons per minute) and the time that the system is operated

Drip tubes are a relatively low-cost and simple irrigation system. Although this is a top-watering method and leaching can occur, if the volume of water applied is only that which is needed to maintain a moist substrate, leaching can be greatly minimized. Because water or fertilizer is being applied directly to the substrate, this system can save significant amounts of water and fertilizer as compared to overhead emitters. However, since every container must have at least one drip tube, benches can become covered with drip tubes and keeping track of a potentially large numbers of drip lines and emitters can be a challenge. Further, if the number of containers on a bench changes, the number of tubes must change (tubes must be added, removed or plugged if not being used).

Additionally, greenhouse managers must watch for clogged drip tubes. If a tube becomes clogged, the container it serves will not receive water or fertilizer solution. Often greenhouse managers will periodically inject acid through the drip tubes (without plants being present) to dissolve any build up of fertilizer salts that are clogging the drip lines or emitters.

Capillary mat systems
This type of irrigation system uses a lint mat covered with a perforated black plastic. Drip lines (drip tape) are placed on the lint mat and underneath the perforated black plastic. Water or fertilizer solution is supplied to the lint mat through the drip lines. The black plastic is used to exclude light from the lint mat and thus reduce algae growth on the mat. Containers are placed on top of the perforated black plastic. Often when the containers are initially placed on the perforated black plastic, they are top watered (allowing a small amount of leaching from the bottom of the container) to establish a capillary connection with the wet lint mat.

As the substrate in the containers lose water through evapotranspiration, water or fertilizer solution moves from the lint mat into the substrate through the holes in the bottom of the container by capillary action. The mat is kept constantly wet through periodic application of water or fertilizer solution through the drip lines. If the containers are picked up, knocked over, or if the lint mat or substrate is allowed to dry out, the capillary connection between the lint mat and the substrate inside of the container will be broken and water or fertilizer solution will not be able to move from the lint mat into the substrate. If the capillary connection is broken, the containers should be replaced on the wet mat and overhead watered (allowing some leaching from the bottom of the container) once to reestablish the capillary connection.

Capillary mat irrigation systems are typically simple and low cost irrigation systems. They may be placed on many types of benches or on floors. However, if the surface is not level or has low spots, irrigation may be uneven. In some cases greenhouse managers will place capillary mats on a bench with a slight gradient. This helps minimize wet or dry spots because the solution in the wet lint mat tends to flow to the low end of the bench (i.e. low spots drain and potential dry spots have water flowing through). The excess water or fertilizer solution is captured in a PVC trough at the low end of the bench and reused.

Capillary mats provide a low-cost irrigation system that reduces water and fertilizer use (as compared to over head emitters) and minimizes runoff. However, irrigation can be uneven and disease organisms spread easily using capillary mat irrigation. Fertilizer Salts can also build up in the mat over time. Additionally, capillary mat irrigation is an example of a subirrigation system and special concerns regarding fertilization apply (see "Special Considerations When Using Subirrigation and Recirulating Irrigation Systems").

Ebb-and-flow benches
Ebb-and-flow benches (also called ebb-and-flood benches) combine an elevated benching system with a closed recirculating irrigation system. Ebb-and-flow benches may be designed as stationary benches or as rolling benches.

The primary characteristic of an ebb-and flow irrigation system is the tray that makes up the bench surface. The tray may be made of hardened plastic or aluminum. The length and width of the tray varies depending upon desired dimensions. The tray has side walls that are generally 4 to 6 inches tall.  On the bottom inside of the tray there are two layers of channels with one being about ½ inch deep and running the length of the tray and the next set of channels being about ¼ inch deep and running perpendicular to the length of the tray.

At one end of the tray there is an inlet and an outlet. The inlet allows fertilizer solution (or water) to be pumped into the tray. As fertilizer solution is pumped into the tray, it first floods the deepest of the channels. When the deepest channels are flooded, the solution floods the next level of channels. After both sets of channels are flooded the solution continues to rise above the bottom surface of the bench. In a production situation, containers with plants would be placed on the tray surface. As the solution floods the tray and rises up around the containers, the solution comes into contact with the root substrate inside of the containers and the fertilizer solution moves up and into the substrate in the container by capillary action. Thus, rather than top watering, the crop is subirrigated (water and/or fertilized from the bottom). When flooding, the depth of the solution is maintained so that it does not extend more than approximately ¼ to ½ of an inch up the height of the container (the container is sitting in a fertilizer solution that is ¼ to ½ inch deep).  For a 4 or 4.5-inch containers (or smaller) the flooding depth is typically about ¼ inch while it may be ½ inch for larger containers.

The depth of the solution and the exposure time are controlled through several possible methods. In the first, the fertilizer solution is pumped into the bench through one of two pipes. At the same time, fertilizer solution flows out of the tray through an opening with a metal screen. The screen, however, allows the fertilizer solution to flow out at a slower rate than it flows in and the net effect is that the bench floods. The depth of the flooding is dictated by how long the fertilizer solution flows into the bench (inflow rate and screen size may be adjusted to change the flooding rate and level in the tray). A more well controlled method is one in which the fertilizer solution is pumped into the tray. The tray has an elevated outlet so that solution flows out only when it reaches a certain depth (or height). This prevents the solution from rising above a certain height. The inflow of solution can continue for as long as flooding is desired. When the desired flooding time is reached, the pump is turned off and the solution can flow back to the holding tank through the same plumbing that the solution was pumped into the flood tray. Other systems may have the outlet on a timer. The bench is flooded and after a prescribed period of time, the outlet is electronically opened so the fertilizer solution can flow back to the storage tanks. Typically, ebb-and-flow trays are flooded for 10 minutes (total contact time with the water or fertilizer solution) and then the trays drained. This time may vary depending on the size of the container and the substrate being used.

All ebb-and-flow benches have several critical support components. There must be large storage tanks that hold the water or fertilizer solution(s) used to flood the ebb-and-flow trays. The storage tank (holding tanks) must have at least enough capacity to fill the trays being flooded at a given time. Additional storage tanks that contain concentrated fertilizer solution and plain water may be included. The system requires a pump to force the fertilizer solution from the storage tanks to the ebb-and-flow trays in the greenhouse. The capacity of the pump should be such that the zone being irrigated can be completely flooded in 5 minutes or less. Typically, some type of filter (i.e. sand or screen) is placed between the ebb-and-flood trays and the storage tank in the return flow line so that soil or plant debris is screened out. Timers or computer controls may be used to automate the irrigation process.

Probes (sensors) may be placed in the line flowing from the storage tank to the trays to monitor electrical conductivity (E.C.) and pH. If the E.C. is too low (due to plants removing fertilizer elements from the solution), concentrated fertilizer stock solution may be added to the fertilizer solution to increase the E.C.  If the E.C. is too high, plain water may be added to lower the E.C. If the pH is not within an acceptable range, an acid such as phosphoric acid, sulfuric, or nitric acid may be added to decrease the pH or a base such as potassium hydroxide may be added to increase the pH.

There is a risk of spreading soil-borne disease-causing organisms such as Pythium and Phytophthora species in recirculated fertilization solutions. Therefore, after the solution is drained from the bench and filtered, it may be treated with U.V. light or ozone to kill any organisms present. Additionally, chloride, fluoride or copper may be added to the fertilizer solution to kill any disease-causing organisms present. More detailed information regarding disinfestation of water and fertilizer solutions is presented under the “Disinfesting Recirculated Water” in this learning unit.

Although more expensive than overhead emitter, booms or capillary mats, ebb-and-flow irrigation systems provide an efficient and automated irrigation and fertilization system. In addition to preventing runoff of water and fertilizers, ebb-and-flow systems reduce water and fertilizer usage. Additionally, an ebb-and-flow irrigation system is an example of a subirrigation system and special concerns regarding fertilization apply (see “Special Considerations When Using Subirrigation and Recirulating Irrigation Systems”).

Dutch trays
Dutch trays are very similar to ebb-and-flow benches and they function and are managed in a manner similar to ebb-and-flow benches. However, Dutch trays are usually aluminum. Additionally, where ebb-and-flow systems can be designed as rolling benches, they are usually limited to simply rolling from side-to-side to minimize the number of aisles required and thus increase space usage efficiency. Dutch trays are placed on steel tracks that serve to not only support the trays, but the trays (which have small wheel-like structures underneath) roll on the metal support structure. This allows trays to be rolled together to maximize space usage efficiency, but the track system also serves as a transportation system around the greenhouse facility.

The tray units for a Dutch tray system may be rolled down the length of the greenhouse and spaced tray to tray. When the plant material needs to be moved to another location in the facility, the trays may be rolled on the steel tracks to a major aisle or walkway. When a change in direction is needed, the tray may be offloaded from tracks going in one direction and onto tracks going in a perpendicular direction using pneumatic lifts that raise and lower sections of track.

Because of their degree of mobility, Dutch trays cannot be physically connected to the fertilizer solution supply line or the drain line (otherwise all the tubes and supply and drainage lines would have to move with the trays). This problem is solved by having stand-alone supply lines and a drain line that runs underneath the benches but does not connect directly to the trays.

Although a Dutch tray system provides an automated transportation system, a closed recirculating irrigation system and maximizes space usage efficiency, it does limit access to crop. For example, if crops that need to shipped or handled are in the middle of a greenhouse, many trays might need to be moved to gain access to the trays with the desired plants. One solution to this problem has been to utilize various types of crane systems that can move over top of the trays, reach down, pick up a desired tray and bring the tray to the end of the greenhouse and place it on an open track.

Although more expensive than overhead emitter, booms or capillary mats, Dutch tray irrigation systems provide an efficient and automated irrigation and fertilization system. In addition to preventing runoff of water and fertilizers, Dutch tray irrigation systems reduce water and fertilizer usage. A Dutch tray irrigation system is an example of a subirrigation system and special concerns regarding fertilization apply (see “Special Considerations When Using Subirrigation and Recirulating Irrigation Systems”).

Troughs
Troughs are narrow linear structures typically made from aluminum, plastic or PVC. There are several distinct types of troughs but they all are generally designed to hold a single row of containers or plants. Troughs may be open faced and designed to have containers placed on them or they may be a closed faced (or partially closed faced) and designed to have plants placed directly into the trough without a container. In all cases, the trough is mounted on some type of support structure with a slight grade (2% to 3%) from one end to the other to facilitate the flow of water or fertilizer solution. 

One type of trough system is designed to support plants in containers. In this case, the trough is open faced and often has a shallow channel in the trough where the container is placed. The trough is positioned on a slight grade to facilitate the flow of the water or fertilizer solution. At the high end of the trough, a supply tube discharges water or fertilizer solution into the trough. The solution flows down the length of the trough in a shallow stream. As the solution comes into contact with the containers, and thus the substrate inside of the containers, solution is taken up into the substrate through capillary action. Excess solution is recaptured at the opposite end in a collection tray or manifold. The excess solution is typically filtered and returned to storage tanks. The recollected solution may be disinfested and monitored in the same manner as described for ebb-and-flow trays.

Another type of trough system is commonly used for the hydroponic production of greenhouse-grown vegetables and herbs. These are typically closed-faced troughs or partially closed-faced troughs made from plastic or food-grade PVC. The trough is designed so that seedlings or young plants may be pushed or slid into the top of the trough. The seedlings may have been germinated in Oasis® foam, an Ellipot® or in some other type of substrate. The root ball and substrate are held in place by the trough and thus the trough provides physical support to the plant. The shoots of the plants grow above the trough while the roots grow inside of the trough. As with the previous type of trough system, a continuous stream of water or fertilizer solution flows inside the trough from one end to the opposite end. The flowing solution is typically a thin film or layer of solution that baths the roots (but typically does not totally cover them) as it flows down the length of the trough. In this way, the plants have access to water and mineral nutrients, but are not totally submerged in water. This method of supplying water and nutrients is sometimes referred to as the nutrient film technique (NFT). The trough used in this type of system may have varying designs and dimensions, may be supported on some type of elevated benching structure or may be close to the ground.

Another type of trough system is typically used for propagation. This system uses PVC pipes with a notch removed (partially open faced) along the length of the top of the pipe. Oasis® foam or trays with some type of solid substrate may be slid into the trough. Vegetative cuttings are stuck into the foam or substrate. The cuttings are misted using an overhead mist system until roots begin to develop. As with other trough systems, water or fertilizer solution flows down the length of the inside of the trough. While roots are developing (and plants are being misted), water is used in the trough to keep the substrate moist. After roots begin to develop, a fertilizer solution may be used to provide both water and mineral nutrients to the developing cutting (and mist may be terminated). When the cuttings are ready for shipping, the foam cubes or cutting trays are slid out of the trough and packaged for shipping.

When troughs are used, the same types of support components such as storage tanks, pumps, supply lines, recollection lines, filters, solution disinfesting systems and monitoring probes may be used as for ebb-and flow bences. The primary difference in a trough system and an ebb-and-flow system is that in the ebb-and-flow system, the fertilizer solution is gradually and evenly raised up around the containers and then drained away. In a trough, plant roots are continuously exposed to a thin flowing stream of water or nutrients.

Flood floors
A flood floor is essentially a hybrid of a concrete floor and an ebb-and-flow irrigation system. The greenhouse floor is poured concrete with raised edges or curbs that allow the floor to be flooded and drained. As with ebb-and-flow benches, water or fertilizer solution moves into the substrate through capillary action.

There are two basic flood floor designs. The first is known as a traditional flood floor. In this type of flood floor, the concrete is poured and a laser is used to level the floor with a slight grade from the sides towards the center (creating a very shallow “V”). The center is usually 1 to 1 ½ inches lower than the high points along the curbs or edges. Water or fertilizer solution is pumped into the flood floor. In most cases, the inlets are located in the low center portion of the floor. The pump capacity is usually designed so that the flood floor can be filled to a depth of ¼ to ½ inch in approximately 5 minutes. The floor remains flooded for up to 10 minutes and is then drained back through the same supply line used to pump the solution into the flood floor. As with ebb-and-flow benches, the solution may be pumped through a filter before it is returned to the storage tank. The solution may also be treated with ultraviolet light or ozone to kill any potential plant pathogens.

Another basic type of flood floor is referred to as a cascading flood floor. This type of production surface combines the concept of food floors with troughs, and they are best suited for small containers (less than 6-inch containers). Cascading flood floors are concrete flood floors that are sloped from one side to the other with a drop of approximately ¼ to ⅜ inch from the high side to the low side. The water or fertilizer solution is pumped into the flood floor on the high side, flows down the slope and is recollected on the low side of the floor. The solution is pumped into the flood floor at a rate such that 30 to 45 seconds are required for the solution to cross the flood floor and a solution depth of approximately ¼ inch is maintained. The flooding process continues for approximately 10 minutes and is then terminated. The recollected water is pumped back to storage tanks. As with traditional flood floors and ebb-and-flood benches, the recollected solution is typically filtered and may be treated for disease-causing pathogens.

Regardless of the exact design, the advantages of flood floors include reduced water use, elimination of runoff, and high space usage efficiency. The primary disadvantage is the initial cost since they can be expensive to install. Flood floors are an example of a subirrigation system and special concerns regarding fertilization apply (see “Special Considerations When Using Subirrigation and Recirulating Irrigation Systems”).

Floating Beds (Deepflow technique)
Floating beds are most commonly used in hydroponic production of vegetables such as lettuce (see "Hydroponics"). Plants are floated on a fertilizer solution using Styrofoam sheets or plug trays. Initially, fertilizer solution moves into the substrate in the trays through capillary action. Later as the plants develop, roots may grow out of the tray and into the fertilizer solution. The plant roots grow into the fertilizer solution. Since the plant is taking nutrients form the solution and excreting other components, the solution needs to be monitored and adjusted as required.
  
Root misting (aeroponics)
This is another method used in hydroponic systems. The plants are suspended in the air (typically under a bench and in the dark or under very low light) and the roots are periodically misted with water or fertilizer solution (see "Hydroponics").

Hanging basket systems
Hanging baskets may be irrigated using several methods. Hanging baskets may be placed on traditional benches where they are watered by hand or with drip tubes. Hanging baskets may also be placed on flood floors (with the drainage trays temporarily removed). They may be hung in the air and irrigated using one of several types of elevated systems. The simplest elevated system for hanging baskets is one in which baskets are hung using simple hooks. A drip irrigation line is often placed in the hanging basket to provide water or fertilizer solution. Another system is referred to as an ECHO system. In this system, baskets are hung on hooks that are part of a rotating line. The line may be rotated when irrigation is required. As the containers pass one end of the loop, they pass under an irrigation nozzle. Finally, the cage system is one in which baskets are hung from a suspended cage or metal structure. This structure is usually automated so that it can be moved from one end of the greenhouse to the other. This allows material to be moved so that plant material under the baskets can receive sunlight for at least part of the day.

Section 5: Common Supporting Components of Irrigation Systems

Injectors
Fertilizer injectors are a critical component of irrigation/fertilization systems as they are used to blend a concentrated fertilizer stock solution with water to deliver a fertilizer solution of the desired concentration to the crop. If concentrated stock solutions and injectors are not used, extremely large volumes of the final fertilizer solution (a thus very large storage tanks) will need to be prepared (such as are needed with ebb-and-flow benches, troughs, and flood floors). Fertilizer injectors may be purchased that blend the concentrated stock solution with water at different proportions but 1:16, 1:100 and 1:200 are common ratios. The ratio listed indicates the amount of fertilizer concentrate taken up from the stock tank and the amount of final fertilizer solution that is made (after dilution). Therefore, for a 1:100 injection ration, 1 gallon (or 1 liter) of concentrated fertilizer stock solution is blended with water to produce 100 gallons (or 100 liters) of final fertilizer solution (1 part fertilizer concentrate + 99 parts water = 100 parts of diluted fertilizer).

There are various types of injectors. Some allow the greenhouse manager to set the injection ratio (i.e. Dosatrons) of concentrated fertilizer stock solution to water. Others injectors allow for a certain E.C. to be set and the appropriate amount of fertilizer stock solution mixed with water to achieve the desired E.C.

Hozon and Syfonex (sometimes referred to as Venturi injectors) injectors are small, brass units that screw onto a faucet at one end and onto a hose at the other end. When water passes through a restricted opening in the brass unit, a pressure differential between the waterline and the stock tank occurs. The suction pulls concentrated fertilizer solution from a tank or bucket into the brass unit from the suction line and mixes it with the clear water coming in through the faucet (thus creating a larger volume of diluted fertilizer flowing from the hose). These types of injectors are simple, inexpensive and can be easily attached to any faucet. They typically are designed to inject concentrated fertilizer at a 1:16 ratio. However, they do not provide constant control over the concentration being blended. This is because as fluctuations in water pressure occur, the suction in the brass unit changes and the amount of concentrated fertilizer taken up changes. A minimum of 35 pounds per square inch of constant water pressure has been recommended for these injectors to function properly. Additionally, back pressure (i.e. stepping on the hose or having a “kink” in the hose) can cause the unit to stop siphoning. Hozon and Syfonex injectors should not be more than 50 feet from the hose outlet (watering end of hose). Longer hose lengths can cause significant back pressure and affect the amount of concentrated fertilizer stock being injected. If more than 50 feet of hose is required, the injector (and buckets of concentrated fertilizer solution) may be connected between two sections of hose. Besides fertilizers, these types of injectors can be used to dispense insecticides, fungicides, and other water-soluble chemicals through a hose. However, care should be taken to insure that the mixing ratio that is occurring is correct

Dosatron, DosMatic, Anderson, Smith, and Gewa injectors (also referred to as positive displacement injectors), provide consistent injector ratios over variations in water pressure as long as the specified water flow rate for the specific device is maintained. A specific amount of fertilizer stock solution, determined by filling a specifically sized chamber, is injected into the incoming clear water, which also is measured by the unit. Thus, as with hozon and syfonex injectors, a specific amount of concentrated fertilizer solution is mixed with a specific amount of water to create a dilute fertilizer solution. The major concern with these types of injectors is that a minimum and maximum water flow rate, as specified for each device and model, must be maintained.

Dosatron injectors operate using water pressure as the power source. They are installed directly into the water supply line. Water drives the injector, which takes up the required amount of fertilizer concentrate directly from a stock tank. Inside the injector, the concentrated fertilizer solution is blended with the clear water and the diluted fertilizer solution then moves out and down through the hose or supply line to be applied to the crop. The amount of concentrate that is taken up and blended with water is controlled by and proportional to the volume of water entering the injector irrespective of variations in water pressure. There are various models of Dosatron injectors with required flow rates ranging from 7 to 264 gallons per minute and with injection ratios of 1:50 to 1:3000. Dosatron units may also be used to inject chemicals, including acids so long as the concentration of the acid does not exceed 5%. Higher concentrations of acids may be injected if the units have the proper internal components. The greenhouse manager should be sure to read the product information carefully to determine proper uses of each model of injector.

As in the case of Dosatron injectors, DosMatic injectors operate without electricity and use water pressure as their power source and are installed directly in the irrigation line. Also like a Dosatron injector, water drives the uptake of concentrated fertilizer stock solution and the concentrate is mixed with clear water to form a dilute fertilizer solution before it moves downstream to the crop. The amount of concentrate taken up and mixed with water is directly proportional to the volume of water entering the injector despite variations in water pressure. Maximum flow rates for DosMatic injectors range from less than 1 to 100 gallons per minute with injector ratios from 1:10 to 1:4000 depending on the model. The injection ratio can be adjusted while in use. Many DosMatic injectors allow for injection of most acids and disinfectant chemicals. Some models are specially designed for irrigation water pH adjustment. The greenhouse manager should be sure to read the product information carefully to determine proper uses of each model of injector.

Anderson injectors use positive displacement to inject the concentrated fertilizer solution into the waterline. The injector measures the water flow rate and injects the desired volume of concentrated chemical into the water line. The volume is proportioned by a flow-metered pump. Most of these injectors require electricity, but can operate using rechargeable battery packs or solar cells. However, the S-series Anderson injector does not require electricity. Injection accuracy is maintained over a wide range of water flow rates and pressure ranges. Injector ratios can be set from 1:80 to over than 1:200 depending on the specific model. The injector ratio can also be adjusted while in operation. Maximum water flow rates for this type of injector range from 0.75 to 3,000 gallons per minute at pressures ranging from 15 to 125 pounds per square inch. Anderson injectors can have from one to many separate injection heads that allow for the injection of multiple concentrated stock solutions. All models can handle a wide range of chemicals including acids. Models are available for stationary and portable uses. Various monitors and probes can be installed in-line on the outlet side of the injector for monitoring of pH and electrical conductivity of the fertilizer or chemical solution being supplied to the crops.

Smith injectors require no electricity to operate as they rely on water passing through a water motor to run the injector pump. The pump meters the volume of water running through the unit. The water causes a pump to revolve and a prescribed amount of fertilizer is injected for each revolution of the pump (for each stroke of the pump). The concentrated fertilizer solution to clear water remains the same regardless of changes in flow rates or water pressure. Smith injectors may be used to inject a variety of fertilizers and chemicals. Models are available that are specifically designed for acid injection and algaecide injection. Different models are designed to operate at different water flow rates and have different injection ratios that are factory set (cannot be adjusted by the greenhouse manager).

The Gewa injector is unique in that is has no suction or pumping device. Suspended inside of a steel tank is a plastic membrane or bladder. The concentrated fertilizer solution is placed inside of the bladder. When the water is turned on, it surrounds the membrane and applies pressure to the bladder which in turn forces a calibrated amount of concentrated fertilizer solution into the waterline. During operation, the bladder inside of the tank folds down on itself. Because the concentrated fertilizer solution has a higher specific weight than the clear water, folding-in takes place from the top down (fertilizer solution settles to bottom). Be sure the center tube inside of the bladder is in place during operation to prevent the membrane from sealing off the flow of solution. The only moving part in a Gewa injector is a float valve. This bronze, spring-loaded float value measures water flow on the inlet side of the valve and allows a specific amount of concentrated fertilizer solution to be injected. A sudden decrease in water pressure or water flow rate does not affect the proportioning of the valve. Various models of Gewa injectors are available and they come in sizes from 4- to 26-gallon capacity. Injection ratios range from 1:20 up to 1:300 and are accurate to within 4 percent. The thick steel tank makes up the bulk (and weight) of the Gewa injector. A water flow rate of 1.6 to 88 gallons per minute and water pressures up to 125 pounds per square inch are accommodated by this type of injector. Periodically, the inner membrane must be replaced, which requires disassembling the unit. Gewa injectors are most commonly used a portable injectors that may be placed on hand carts and moved around the greenhouse operation.

When choosing an injector, numerous factors should be considered. These factors include the size of the greenhouse operation, types of fertilizers or chemicals (including acids) that need to be injected, water flow rates available, water pressure available, minimum and maximum size of areas to be fertilized at a time, amount of fertilizer solution required to fertilize each greenhouse or zone, time required to deliver the required solution, quality of water to be used, portable versus stationary needs, maintenance requirements and cost of the injector. Always be sure to read the manufactures usage recommendations, specifications and maintenance recommendations for an injector.

Internal components of many injectors are made of stainless steel to resist corrosive fertilizers and other chemicals. If being used to inject acid, the injector selected must be equipped to handle acids. The type of acid to be injected and its concentration are important. Any internal parts of injectors to be used for sulfuric acid must be made of acid-resistant rubber, while for phosphoric acid injection these same parts may be made of stainless steel. If pesticides and insecticides are to be injected on a routine basis, the internal components of the injector should contain no plastic parts because many insecticides and fungicides contain a hydrocarbon base that may be harmful to PVC plastics.

It is often desirable to have the ability to inject several fertilizers (or other chemicals) at the same time that cannot be mixed together as concentrated solutions in the same stock tank. If certain components are mixed together in the same stock tank, they may form insoluble compounds that precipitate out of solution. In this situation, the incompatible fertilizers (or fertilizer salts) are kept in separate stock tanks and are injected directly into the irrigation stream with flowing water where their lower concentration and the water movement prevents precipitation. As a rule, sulfates should not be mixed with calcium in high concentrations (concentrated stock solutions) as the result is insoluble calcium sulfate (gypsum) and calcium phosphate. Phosphates should not be mixed with calcium or iron in high concentrations as insoluble calcium phosphate and iron phosphate result, respectively. Potassium bicarbonate should also not be mixed with other fertilizer salts as the resulting high pH may cause many fertilizer salts to precipitate out of solution. Because of these incompatibilities, multiple stock tanks and multiple injectors or injectors with multiple injection heads may be required.

The intake strainer and suction tube of the injector that leads into the fertilizer stock tanks should be positioned 2 to 3 inches above the bottom of the stock tank to avoid siphoning undissolved concentrated fertilizer salts. Be sure to periodically inspect the strainer on the suction tube to make sure it is not clogged. Finally, be sure to periodically test the injector to make sure that it is functioning correctly and injecting at the set ratio (see “Testing Fertilizer Injectors” in the “Mineral Nutrition” learning unit).

Stock tanks, holding tank and constructed lagoons
Stock tanks are used to prepare and store concentrated solutions (i.e. fertilizers) that can be blended with water using an injector to achieve a desired final concentration. The concentrated stock is usually injected directly into the water line where it is diluted and sent on to the crop. Other stock tanks may hold clear water used to irrigate or to dilute concentrated fertilizer solutions if required. Stock tank size depends on the injector ratio and daily water usage requirements. The stock tank should be large enough to allow the entire fertilization job to be completed with one batch of fertilizer concentrate. A larger stock tank is needed if a low injector ratio is used and if the injector is used frequently.

Stock tanks should be opaque since chelating agents in fertilizers used to keep micronutrients available to the plants break down if exposed to light. Stock tanks should be covered to prevent algae growth, to keep debris out of the stock tank and to minimize evaporation from the tank. Undissolved fertilizer salts can accumulate in the bottom of the stock tank, which results in large differences in fertilizer concentration. If a large stock tank is used, make sure that the fertilizer stock solution is well mixed using an agitator before use.

Holding tanks are generally used to store diluted fertilizer solutions that are immediately ready to be supplied to the crop (are at the desired concentration). The solution in the tank is usually pumped out to some type of closed recirculating irrigation system such as ebb-and-flow benches, Dutch trays or flood floors. These tanks need to have a capacity large enough to supply fertilizer to at least a complete greenhouse or irrigation zone in a greenhouse. After irrigation, the solution is typically filtered and returned to the holding tanks where it can be adjust if necessary and used to irrigate the next greenhouse or zone. As with stock tanks, holding tanks should be opaque to exclude light and covered.

In addition to tanks, water or fertilizer solutions may be stored in constructed storage lagoons lined with water impermeable liner and often covered with a light-excluding polypropylene cover. The cover blocks out light to prevent algal growth and keeps other types of debris from entering the irrigation system.

Filters
Irrigation water may contain contaminants (i.e. plant debris, algae, etc.) that can clog irrigation lines and emitters. To remove these materials and prevent clogging, the water is passed through a filter made of various types of wire mesh or paper, or through sand columns.

Solenoid valves
These devices are magnetic switches that control water flow. Solenoid valves can be connected to a timer or computer. When the timer or computer triggers an electric current and the electric current flows through the solenoid valve, it activates a magnet that in turn opens and allows water to flow.
  
Pressure regulators
Many emitters, drip tubes and drip tapes operate within certain water pressure ranges. The incoming water pressure is often higher than what the emitter, drip tube or drip tape is designed to handle. Excessive water pressures can break emitters or cause drip tapes to rupture. Therefore, pressure regulators are placed in the irrigation line to reduce water pressure to levels appropriate for the emitter, drip tubes or drip tapes being used.

Clocks/computer controls
Clocks or computerized controls are used to automatically control the frequency and timing of irrigation. These may be systems designed to control multiple greenhouses or zones in greenhouses or they may be mounted to specific irrigation devices such as is the case with individual controllers on irrigation booms.

Water meters
These devices measure the amount of water being used and are legally required in some states particularly where water use for agriculture is regulated.

Back-flow preventers
After liquid fertilization is terminated, suction in the water line may cause fertilizer solution to be pulled back into the water line resulting in fertilizer entering the potable water supply. To prevent this from occurring, back-flow preventers should be installed in the water line between the incoming water and the fertilizer injector.

Section 6: Special Considerations When Using Subirrigation and Recirulating Irrigation Systems

When using subirrigation and recirculating irrigation systems, it is important to remember that the method of irrigation impacts many other aspects of production.

When using subirrigation, leaching is either minimal or does not occur. Mineral elements that would normally leach out with overhead watering do not leach out to a significant extent with subirrigation. Because mineral elements are not lost through leaching, it is often recommended that fertilizer inputs be cut by 30% to 50% of the normally recommended rates used for overhead irrigation systems.

It is very important to monitor the E.C. of the substrate when using subirrigation systems since fertilizer salts can build up (since leaching does not occur). Because of evaporation from the substrate surface, the fertilizer salts will tend to move into the upper inch of the substrate. The E.C. in this portion of the substrate can be much higher than the E.C. of the lower portions of the substrate. Because of the elevated E.C. level, this region is often devoid of roots. A high E.C. in the upper portion of the substrate is not normally a problem. However, if subirrigation ceases and overhead irrigation is used, the salts in the upper substrate can leach down into the lower portions of the substrate and cause a rapid, and sometimes harmful, increase in the E.C. of the lower portions of the substrate. Also, when conducting substrate tests when subirrigation has been used, the upper ½ - 1 inch of substrate is usually discarded since this area will have an elevated E.C. level and not be representative of the substrate as a whole.

As discussed earlier in this unit, when using recirculating systems, the spread of plant pathogens (i.e. Pythium, Phytophthora, Erwinia, etc.) is a concern. If the irrigation system is used properly, there is minimal water movement from the substrate back into the irrigation solution and thus minimal opportunity for the spread of pathogens. However, if benches or floors are flooded for too long to too deep (which in reality often happens), the lower zones of the substrate become saturated and there can be movement of pathogenic organisms from the substrate back into the irrigation solution. Proper watering technique is important. Proper watering will also prevent the movement of fertilizer salts from the substrate into the irrigation solution. In most cases the E.C. of the irrigation solution is monitored to insure that fertilizer salts do not build up (or being depleted from it) in the irrigation solution, and the recirculated water of fertilizer solution is disinfested to prevent the spread of disease-causing organisms.

© M.R. Evans, 2008, 2009, 2011, 2014