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The Importance of High Quality Irrigation Water in Your Crop

The Tomato Magazine
August 2006

By Mary Peet, Brian Whipker, John Dole and Ted Bilderback– North Carolina State University

Plants need water, but high quality irrigation water is getting harder and harder to come by and keep. In some cases, existing wells or water supplies have changed in pH or alkalinity, resulting in problems with equipment or difficulty in controlling substrate pH and nutrient availability. With water resources shrinking and the demand for water increasing worldwide, neither water quality or water quantity can be guaranteed. The web-based and print resources listed at the end of this article will help you make decisions on whether and how to treat your water to grow successfully now and in the years to come!

The Basics:
All types of water supply (pond, cistern, municipal, well, lake, stream) can have water quality problems, and all require different types of treatment. At a minimum, irrigation water should be CLEAN! It should not contain plant or human pathogens, herbicides, heavy metals or industrial pollutants, such as gasoline, at levels that could damage plants or people. The less particulate matter (such as sand or organics) in the water, the less filtration that is needed to keep the drip irrigation system from clogging. Fig. 1 shows a media filter of a type frequently used in the field for drip and sprinkler irrigation. The media bed captures a wide variety of organic and inorganic materials and filtered water flows out.

With this type of system, material captured by the media filters builds up over time and is periodically removed from the system by reversing the flow of water (backflushing).
Even when no contaminants or particulate matter are present, certain water characteristics make it difficult to manage your fertility program and maintain emitters, filters, evaporative pads and fogging systems. This article discusses these characteristics: pH, alka linity, EC, salinity and macro and microelements. Everyone knows that before starting a greenhouse, the water should be tested. But how do you interpret those water tests? And what do you do if water doesn’t meet one or more quality specifications? There are a lot of water treatment options out there, but which is the best for your particular situation?

pH
The most important reason to treat water is to maintain rootzone pH in a range that optimizes availability of nutrients. As shown in Fig. 2, if the pH level is too low, plants take up more iron, manganese, zinc and copper, but less molybdenum, calcium and magnesium. When the pH is too high, plants may take up too much molybdenum and too little iron, manganese, zinc, copper and boron. A pH of 5.6-5.8 is optimal for tomatoes. Although people sometimes think of pH 7 water as being ‘neutral’, it is too high for good tomato production in soilless media. Researchers in the UK reported 25 percent declines in tomato yield at a pH level of 7.0 in rockwool systems, for example.

Factors in addition to the irrigation water pH that affect rootzone pH include fertilizers added to the irrigation water or present in the substrate, and plant uptake. The fertilizer bag should list the potential acidity on the label as pounds of calcium carbonate equivalent per ton. For example 20-8-20, like most blends, is an acidic fertilizer with a potential acidity of 400 pounds of calcium carbonate equivalent per ton. As a general rule, materials containing ammonium or phosphate, such as diammonium phosphate, are acidity forming in the substrate (i.e. they lower pH). Those containing nitrate or calcium are generally alkalinity forming (i.e. they raise pH). Alkalinity forming fertilizers include calcium nitrate, potassium nitrate and sodium nitrate. Ammonium nitrate, however, is acidity forming, rather than alkalinity forming, so it will reduce the pH. Potassium sulfate, superphosphate, calcium sulfate and potassium chloride are generally neutral and do not change pH very much. Thus, the pH of the irrigation water should also be checked after the fertilizer is injected.

Finally, the leachate from the containers should be checked to ensure the rootzone is not becoming too acid or alkaline from salt buildup, plant uptake or other causes. Use the standard recommendations as described in the references at the end of this article and the alkalinity calculator described below to establish baseline levels and adjust based on regular monitoring of the rootzone using the pour-through technique. Resources on using the pour-through technique are also listed at the end of this article. Often the amount of acid or base to inject has to be determined somewhat empirically. Generally in trying to correct and maintain rootzone pH, it is better to monitor closely and make small changes over time rather than suddenly making a drastic change.

Alkalinity
Water pH and alkalinity levels must be considered together. The various carbonate species, also called the hardness minerals, contributing to alkalinity of the water (carbonates, bicarbonates and carbonic acid) tend to buffer water from pH changes, which is good if they are present only in small amounts. If your water has excess alkalinity, however, the pH will tend to rise over time in the rootzone and more acid is necessary to bring the irrigation water pH down to the desired range. As discussed under pH changes, high pH reduces nutrient availability. At a high pH, minerals and iron also tend to come out of solution and are deposited as white scale or red stains on equipment or plants. They will also cause scale buildup inside of pipes, plug misters, foggers, emitters and irrigation lines. Publications listed at the end of this article provide guidelines on maximum levels of the various forms of carbonates and recommendations on the amount and types of acid needed to bring pH down at various levels of alkalinity. To make these calculations easier, you can download an Excel spreadsheet, alkalinity.xls. The spreadsheet allows you to enter your alkalinity levels and desired pH. It then calculates the amount of acid needed to bring the pH down. The buffering capacity can be hard to predict, so levels of acid addition to correct alkalinity may need to be determined somewhat empirically. It is better to start with low levels of acid addition and make adjustments as needed. This is a good job for the start of the season, before crops are present in the greenhouse! However, even after plants are growing well, weekly monitoring of rootzone conditions will allow the grower to correct problems with small changes before they damage the plant. Examples of monitoring charts are given in the Plant Root Zone Management guide listed at the end of the article.

Soluble Salts (EC)
Soluble salts are the total dissolved salts in the water as measured by their electrical conductivity. This water quality attribute is somewhat confusing to new growers because it describes both the nutrient level of the fertigation water and the sodium (salinity) level of the water. If the EC of the water supply is high, it may contain high concentrations of calcium, magnesium, sulfates, sodium or bicarbonate, not all of which contribute to plant growth. Monitoring EC in the rootzone is important because at high EC, both shoot and root growth are poor. A high EC makes it hard for plants to take up water, and in extreme cases, the plant will die. Fig. 3 shows a tomato leaf after the plant was ‘burned’ by the high salt level of added fertilizers. Seedlings are particularly susceptible to high EC. High rootzone EC can also result when there is not enough irrigation during leaching, when too much fertilizer is being applied or when the irrigation water itself is high in soluble salts. Generally, pour-though EC levels should be in the range of 2.0 to 3.5 mS/cm for actively growing crops, but only 0.5 mS/cm for seed germination and early seedling growth. Reverse osmosis systems are the most common option to reduce the EC of irrigation water, but these systems are somewhat complex and require pre-filtration and maintenance, as discussed. To convert EC units, you can use the conversion tools on the Greenhouse Vegetable Website: www.ces.ncsu.edu/greenhouse_veg/. An alkalinity and a fertilizer calculator are also available on this site.

Water Treatment Options
Should you treat your water? Most large greenhouses, especially those with recirculation systems, have elaborate water treatment systems, as shown in Fig. 4, Even small, non-recirculating systems require at least acid injection for pH control and filtration to remove particulates. Depending on the specific water quality problem and the overall operation, a number of water treatment technologies may be required to produce consistently high quality water. Manufacturers of water treatment equipment will analyze your water, and then often recommend that you need to purchase their systems. Should you believe them and invest money in a water treatment system? Before investing, have your water tested by an independent commercial laboratory or your state water testing facility and consult the water quality guidelines in HIL 557 (see below). If your water is above the suggested limits, here are some questions you can ask yourself before making an expensive investment:

Can I use acid injection to remove carbonates and bicarbonates?
As described above, and in HILS 557 and 558, injecting acids, most commonly sulfuric, is the most common way to eliminate moderate levels of carbonates and bicarbonates and can be the most cost-effective. Unless the alkalinity levels are extreme or iron, manganese, hydrogen sulfide or other contaminants are present in the water, acid addition may be all that is necessary.

Do I have an alternative water source available?
Both high alkalinity and high EC can potentially be reduced by substitution of another water source or dilution with higher quality water. Well water is the most likely water source to contain iron, manganese, calcium, magnesium and organics. Municipal water, surface water or a different well may be cost-effective alternatives to expensive water treatment systems for alkalinity in some cases. Municipal water prices and chlorine levels vary widely, however, so using municipal water may not be practical in some areas. Tomatoes are fairly tolerant of the chlorine injected into municipal water but levels over 70 ppm Cl- are not recommended for any crops. With municipal water, it is still necessary to control algae because polyphosphates are added to prevent lime deposits by keeping the lime in solution. Over time phosphates break down sufficiently to support algae growth. If municipal water is used, other means, such as copper ionization, ozone or various algaecides must be used to control algae growth. Blending municipal water or rainwater with well water may be practical in some cases to reduce alkalinity or EC. It may also be possible to blend in pond water which usually does not contain excessive levels of alkalinity or nutrients. However, herbicide runoff is a potential problem, and, in any case, pond water will require filtration before use, as shown in Fig. 1, and algae control.

What is reverse osmosis (RO) and how will it improve my water?
Reverse osmosis is the most common method for reducing high EC in irrigation water. With this system, dissolved and suspended substances are separated from water and discarded. Reverse osmosis can remove up to 99 percent of salts by forcing water through a semi-permeable membrane. It also removes organic material, particulates and bacteria. The water is completely pure at this point and will have EC and total dissolved solids readings of zero. It is easy to make fertilizer calculations using RO water, because the amount injected is exactly what the plant experiences. Dr. Moyhuddin Mirza in Alberta recommends correcting high EC water with reverse osmosis when: sodium is over 200 ppm, SAR (sodium adsorption ratio) is over 5 or the water is not usable for some other reason. It is generally not a good idea to use only reverse osmosis water for irrigation. Dr. Mirza recommends adding only enough reverse osmosis water to keep the sodium concentration below 50 ppm. One problem with using only reverse osmosis water is that it has no buffering capacity. For this reason, Dr. Mirza recommends adding enough potassium bicarbonate to raise the bicarbonate level to 50 ppm if only reverse osmosis water is used. Reverse osmosis systems also require proper pre-treatment filtration, regular maintenance and safe disposal of the briney wastewater. Large industrial water treatment systems can be quite complex, especially when utilized as part of a recirculation system. There are many decisions to be made in selecting a reverse osmosis system, however, including the type of membrane and the system capacity. While small systems for homeowners are available for under $1,000, they can only produce up to 150 gallons per day of RO water. The system also requires large amounts of water: up to 4 gallons of water to produce 1 gallon of RO water. Large storage areas are also required for the processed water.

Can I solve my water quality problems by changing cultural and water management practices?
Increasing the amount of extra water applied to the containers (leaching fraction) will reduce buildup in the media, although it wastes more water. When salts start to build up in the substrate, applying plain water to the containers will help remove accumulated salt, if the irrigation water itself is not too salty. In some cases, running the irrigation system once weekly without injecting any fertilizer will also reduce salt buildup. Seedlings are more sensitive to high EC than established plants, so purchasing transplants or using municipal water or RO water when establishing plants may also be a useful practice. Reducing the amount of added fertilizers, especially calcium and magnesium, if these are already high in the irrigation water, will also reduce alkalinity and EC problems. If micronutrients are high in the irrigation water, be especially careful about adding any additional micronutrients to avoid toxicities. If pH is too high, acidity-forming fertilizers, such as ammonium nitrogen forms, can be chosen. However with tomatoes, ammonium should not represent more than 25 percent of the total nitrogen.

If the main water quality problem is buildup of mineral deposits on the evaporative cooler pads from high carbonates, increasing the bleed-off rate using the bleed-off valve, will reduce buildup, though again at the cost of extra water usage. Information on managing evaporative pad systems is also included at the end of the article.

What if I just need to remove iron, manganese and hydrogen sulfide?
Subsurface water high in certain forms of iron and manganese will oxidize on contact with air to form insoluble, rust-colored deposits which stain and clog equipment, including drip emitters. There are several ways to treat this type of water. Commercially available oxidizing filters precipitate out iron, manganese and hydrogen sulfide, then remove them from the water by mechanical filtration. Some of these systems also reduce pathogens and bacteria through ozone and filtration. Using a similar approach, iron and manganese can be removed from well water prior to adding fertilizers by spraying into a holding tank or pond. Aeration causes the iron and manganese to oxidize and form an insoluble precipitate. The precipitate is then allowed to settle to the bottom before water is withdrawn for irrigation.

Can I get by just removing calcium and magnesium with water softeners?
‘ Hard’ water contains high levels of calcium and magnesium left as white deposits after water evaporates. In home water softening systems, the calcium and magnesium are replaced with sodium to avoid clogging of pipes and water heaters, unsightly deposits of ‘lime scale’ and to help soap and detergents dissolve. Iron may also be removed with the proper softeners. Water softening is basically an ion exchange process in which the calcium and magnesium ions are exchanged for either sodium or potassium ions (NaCl or KCl). Most home units use sodium and are not appropriate for greenhouse use. Softening systems using potassium can be used, however. Water softeners will not reduce the EC, but the potassium added by the softener can be subtracted from the amount of fertilizer potassium added. Removing the calcium and magnesium also avoids competition with potassium. However the plant will still need recommended levels of calcium and magnesium for good growth.

What about the other types of water treatment systems I see advertised?
There are a lot of different water treatment systems out there in addition to those listed here. Some only remove particular types of water impurities, such as iron, manganese and hydrogen sulfide. Others claim to increase yields dramatically, but may only be effective if there is a problem with high calcium in the system or some other imbalance. As with any large purchase, the best policy is to research the product, avoid expensive systems if there are cheaper alternatives, and check on where the products were tested and are currently being used. Check also on the return policies if the unit does not function as promised.

Resources
Water Quality Information leaflets
Two excellent bulletins on water quality and a very useful alkalinity calculator are available online in either an .html or .pdf format at: http://www.ces.ncsu.edu/depts/hort/hil/flowers-index.html. The site has lots of useful information, but for water quality information, consult HIL 558 Alkalinity Control for Irrigation Water used in Nurseries and Greenhouses and HIL 557 Water Considerations for Container Production of Plants.

Water quality publications
Two excellent print publications: Plant Root Zone Management and Pour-Through Management. These can be ordered online at: http://www.nccfga.org/publications.html for $35 and $15, respectively. These water quality topics are discussed in much more detail in the Root Zone Management Guide, and monitoring is illustrated in the Pour-through guide. The publications also provide guidance on long-term strategies for monitoring and correcting pH and EC problems in the rootzone.

Maintaining Kool-Cel pads: http://www.cooling-pads.usgr.com/cooling-pads.html

Greenhouse Product News
Water Quality Zone

Go to www.gpmmag.com and select water quality on the list of zones. You can view articles in the archives, look at water treatment systems and supplies, view frequently asked questions and email your own questions.

Editor’s note: Mary Peet, Brian Whipker, John Dole and Ted Bilderback are all with the Department of Horticulture, North Carolina State University, Raleigh, N.C. Peet, who submitted this material, is a professor and can be contacted by phone at (919) 515-5362 or by e-mail at mary_peet@ncsu.edu.

© 2006 Columbia Publishing

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