Water quality is one of the most critical factors affecting the production of nursery and greenhouse crops. This is particularly true in Texas, where growers must combat a variety of water quality problems. In the past, producers were forced to use the water they had available, regardless of quality. Today, however, the use of treated irrigation water is increasing.
Generally speaking, there are three major areas that Texas growers may address. These include: pH, alkalinity, and soluble salts.
Alkalinity and pH largely determine the efficacy of pesticides and plant growth regulators. However, the most common effect of these properties is on the solubility of fertilizer in the growing medium. This problem occurs most frequently with the micronutrients as well as magnesium.
What is pH?
pH is a measurement of the concentration of hydrogen ions (H+) in solution. Since this represents a logarithmic expression, H+ concentration at pH 6.0 is 10 times greater than at pH 7.0 and 100 times greater than at pH 8.0. In this relationship, pH has no direct effect on plant growth. However, pH does affect the form/availability of nutrient elements in irrigation water, fertilizer solutions and the growing medium.
The pH of irrigation water should usually be within the range of 5.5 to 6.5. These levels enhance the solubility of most micronutrients and avoids a steady increase in the pH of the growing medium. This pH range also optimizes the solubility of nutrients in concentrated fertilizer stock solutions.
pH and Alkalinity
pH is often described in terms of acidity. This is based on the ability of certain acids to dissociate or ionize into H+ ions and associated anions.
Alkalinity is a measure of a water’s capacity to neutralize these acids. Chemically, this is expressed in parts per million (ppm) of calcium carbonate equivalents (CaCO3). Bicarbonates, carbonates and hydroxides are the primary chemicals that contribute to the alkalinity of water.
Sound confusing? Well, simply stated, alkalinity affects the ability to reduce pH by neutralizing added acids. A more graphic example of this relationship is presented in Table 1.
| Sample | Existing pH | Alkalinity ppm CaCO3 | Acid Required |
|---|---|---|---|
| * Number of ml. of 0.1 N H2SO4/100 ml. water. | |||
| A | 9.3 | 71 | 1.2* |
| B | 8.3 | 310 | 6.0 |
As you will note, sample B is a full pH unit lower than sample A, but because of the neutralizing effect of CaCO3 , it requires five times more acid to lower the pH to 5.0. High alkalinity can cause the precipitation of nutrients in concentrated fertilizer solutions, increased pH of the growing medium (which in turn reduces the availability of micronutrients), reduced efficacy of pesticides and growth regulators, and, in some severe cases, foliar residue.
Adjusting the pH of Irrigation Water
To optimize fertility and combat the other adverse effects of high pH/ alkalinity, it is possible to treat irrigation water by injecting acid. Although phosphoric and nitric acids have some application, sulfuric acid is the most commonly used. At present, there are several “acid compatible” injectors on the market. Their prices range from $200 to $300 into the thousands, depending upon your needs. 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 good quality pH meter is essential. To calculate the amount of acid required to achieve the desired pH, first fill a 5-gallon bucket with irrigation water, then slowly add the type of acid you wish to inject and stir the water to insure complete mixing. Measure the pH of the water and continue until the desired pH is obtained. The quantity of acid required may be quite small. Using sulfuric acid, as little as 0.5 ounces may be required to reduce the pH from 7.0 to 4.0.
When the quantity of acid required to correct the pH of the sample has been measured, it is a simple operation to calculate the amount of acid to inject into the system, assuming the amount of water passing into the system is known. (Texas AgriLife Extension Soil and Water Testing Lab has recently established a titration process to assist in determining the amount of acid required. Contact your county Extension agent for more information.)
When using acid injection, be sure to acidify the water up-line from the nutrient injection point. This will optimize the solubility of fertilizer running through the system. Additionally, use acidified water to mix all fertilizer and pesticide solutions.
Soluble Salts
The presence of high soluble salts in irrigation water is one of the most limiting factors in the production of nursery and greenhouse crops. Although management techniques may be used to deal with some of these problems, certain situations require more drastic action.
Many producers are now using water treated through 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. This source of essential elements is extremely important in supplementing a basic fertility program, as well as those nutrients in the growing medium.
When micronurients are eliminated from irrigation water through the RO process, plants may be subject to a wide range of nutrient deficiencies. These may occur as the result of a low supply of a particular element or because of an imbalance between nutrients. Identifying and correcting deficiencies can be tricky in these “super clean” systems.
As a potential solution to this situation, many growers now blend their RO water with other sources (i.e. well, city, river, etc.). By mixing their treated water with the normal source, growers can supply many of the needed nutrients and still reduce soluble salts to an acceptable level. At present a 50- 50 mix seems desirable but further reductions in the amount of RO water used may be feasible. An additional benefit to this approach is that the cost per gallon is considerably reduced.
The use of RO water has some significant limitations. However, for situations where high soluble salts are a problem, this method of water treatment has tremendous potential. Growers are cautioned that reverse osmosis is not the solution to everything, but used wisely, can be a valuable tool in producing quality crops.
Although the costs associated with 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.
