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Technical Library: Irrigation #2

Troubleshooting Your Water Supply

from "Water Management Guidelines for the Greenhouse Industry",
by the Texas State Agricultural Extension Service.

Sizing up soluble salts and pH problems in your
irrigation water is half the battle. You also have to
figure out how to fix them.

WATER quality is one of the most critical factors affecting greenhouse crops. In the past, producers were forced to use the water they had available to their houses, regardless of the quality. Today, however, more growers are using treated water - that is, they're taking painstaking care to track and adjust several water properties. Here are a few tips for making your water the best it can be for your crops.

Fixing pH Of Irrigation Water

THE PROBLEM : PH has no direct effect on plant growth, but it does affect the form and availability of nutrient elements in irrigation water, fertilizer solutions, and the growing medium. Alkalinity and PH most often affect the solubility of fertilizer in the growing medium, particularly the micronutrients and magnesium. These factors also largely determine how effective your pesticides and plant growth regulators will be.

The PH of irrigation water usually should be within the range of 5.5 to 6.5. This level enhances the solubility of most micronutrients and prevents a steady increase in the PH of the growing medium. This PH range also optimizes the solubility of nutrients in concentrated fertilizer stock solutions.

THE SOLUTION: Inject acid in your irrigation water to optimize fertility and combat other adverse effects of high pH/alkalinity. Phosphoric and nitric acids are sometimes helpful, but sulfuric acid is the most commonly used. There are several "acid compatible" injectors on the market. Some of these systems consist of a flow meter, injector, and PH meter to automatically adjust the amount of acid used.

The first step in evaluating acid injection is to have your water tested (for information contact your county extension agent). In addition, a quality PH meter is essential. To calculate the amount of acid required to achieve a desired PH:

  1. Fill a 5-gallon bucket with irrigation water.
  2. Slowly add the type of acid you wish to inject and stir the water to ensure complete mixing.
  3. Measure the PH with each acid addition - until you reach the desired PH. The quantity of acid required may be quite small; as little as 0.5 ounces of sulfuric acid may be required to reduce the PH from 7.0 to 4.0.
  4. Going by the quantity of acid required to correct the PH of the sample, calculate the amount of acid to inject into the system - this assumes you know the amount of water passing into your irrigation system.
Acid should be injected into the irrigation system up-line from the nutrient injection point. This will increase the solubility of fertilizer running through the system. (Also use acidified water to mix all fertilizer and pesticide solutions.)

Soluble Salts

THE PROBLEM: A high soluble salt level in irrigation water is one of the most limiting factors in greenhouse production. The symptoms of too many soluble salts begin as a mild chlorosis of the foliage, which later progresses to a necrosis of leaf tips and margins. Once the salts reach a toxic level, they cause a characteristic "bum." Roots may also be injured. This often predisposes the plant to a wide range of root diseases. Extreme injury may also interfere with water uptake and cause excessive wilting Inspect root systems regularly to monitor any possible effects of soluble salts.

What causes the salts to accumulate?

Irrigation water is a major contributor of salts to the growing medium. Water primarily contains salts of Na, Ca, and Mo. although others might be present.

Fertilizers are forms of salts and therefore contribute to the total soluble salt content. Depending on the salt content of irrigation water used, growers must adjust fertility levels to avoid salt accumulations. Fertilizers are often classified by the amount of total salts they contain. This "salt index" can be used to determine the amount of salts contributed to the growing medium.

Growing media, formulated from a variety of materials (peat, perlite, vermiculite, pine bark), do not usually contain excessive quantities of soluble salts. Make sure, however, to monitor the quality of media components carefully. In some cases, you'll need to leach a medium thoroughly before using it.

For a quantitative evaluation of this process, determine the electrical conductivity (EQ of the leachate. When the EC is less than 2.0 millimhos, the medium is free of excessive salts. THE SOLUTION:

  • Use growing media with as many large pores as possible to facilitate good drainage. Such media are easily leached.
  • When irrigating, apply enough water so that large quantities drain out of the container. Apply approximately 15%-20% more water than the container can hold at each irrigation if the salt hazard is high.
  • Monitor the water content of the ,growing medium. Remember, the concentration of soluble salts in plant tissue increases as moisture levels decrease. In the presence of excessive soluble salts, the growing medium should not be allowed to dry out.
  • Do not overfertilize. Base your nutrient applications on the quality of the irrigation water as well as the fertilizer's salt index. Most fertility regimes used for potted greenhouse crops apply between 150 and 300 ppm N.
  • Be particularly careful when producing salt-sensitive plants. Each species has a distinct response to salt accumulation. A few plants that cannot tolerate soluble salts are: chlorophytum, African violets, calceolaria, geraniums, and petunias.
  • Try water treated though a process known as reverse osmosis (RO) to remove potentially harmful salts. RO water is cheaper than distilled or deionized water, and the overall quality is the same. While it is possible to purchase an RO system, most units now in operation are under lease.
Unfortunately, the use of RO water does not solve all the problems associated with soluble salts. In fact, it can create some very unique situations that are, in many respects, more difficult to correct.

Growers generally take for granted the micronutrients present in irrigation water. When micronutrients are eliminated from irrigation water through the RO process, plants may be subject to a wide range of nutrient deficiencies.

As a possible solution to this problem, many growers now blend their RO water with other sources such as well, city, river, etc. At present, a 5050 mix seems to work, but further reductions in the percentage of RO water used may be feasible.

While the use of RO water has some significant limitations, it also has tremendous potential where high soluble salts are a problem.

Although the costs of treating irrigation water are substantial, increased quality and reduced losses often offset the required investment. If growers are to maintain profitability, they must continue to evaluate improved cultural techniques for production.

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