Dr. Claudio C. Pasian
The relationship between water quality and fertility is the Rube Goldberg machine of floriculture. The quality of the water that floriculture growers use to irrigate their crops significantly effects fertility, seed germination, plant growth, and plant susceptibility to pests and diseases. In many cases, water quality is the limiting factor in floral crop production.
Water quality is a common problem. The information in this fact sheet deals strictly with pH and alkalinity-two important factors still not well understood by many growers.
The pH is a measure of the concentration of the hydrogen protons (H+) and hydroxide (OH-) in a solution. If hydrogen protons predominate, the solution is acidic. If hydroxides predominate, the solution is basic. pH is measured on a 0 to 14 scale. A pH of 7 indicates neutrality. pH values below 7 indicate acidic conditions; pH values above 7 indicate basic conditions.
Water alkalinity is a measure of the capacity of the water to neutralize acids. If an acid is added to water, the pH of the water will decrease because the acid adds H+. However, the decrease in pH will be more pronounced and faster for waters with low alkalinity (low capacity to neutralize an acid) than for waters with high alkalinity (high capacity to neutralize an acid). A larger amount of acid will be required to reduce one pH unit of a given volume of water with high alkalinity than the same volume of water with low alkalinity.
Water pH, water alkalinity, and the pH of the growing mix are closely connected. Over time, all three factors affect fertility. For example, high or low soil pH values usually produce minor element deficiencies or toxicities. Of the two, water alkalinity is more important than water pH in influencing media pH. Water with high pH but low alkalinity will have little effect on media pH over time. However, highly alkaline irrigation water, regardless of its pH, may have a profound effect on media pH. Water pH is a poor indicator of the capacity of water to modify the pH of the growing mix; alkalinity, however, is an excellent indicator. Growers who intend to manage media pH must know the alkalinity of their water. Water pH can also affect the stability of some pesticides and growth regulators. Acidifying the spray water is advisable when water pH is 7 or higher and alkalinity is 150 ppm or more.
Two common units are used to express water alkalinity: 1) milligrams per liter of calcium carbonate equivalents (mg/l CaCO3); or 2) milliequivalents per liter of calcium carbonate equivalents (meq/l CaCO3). Some laboratories and authors measure water alkalinity as mg/l of calcium carbonate and state the measure as mg/l of calcium bicarbonate. Growers who make their own acidity calculations should be aware of these different ways of expressing water alkalinity.
There is no "best" alkalinity level. To determine optimum alkalinity, growers must consider the crop, the size of the container, the leaching fraction, the growing media, and the type of fertilizer used. As a rule of thumb (and depending on whom you talk with), alkalinity above 120 ppm (1 ppm = 1 mg/l) can cause problems. Below that value, alkalinity should not be a problem unless the water is used to irrigate plugs. Water with alkalinity levels of 80 ppm or less should be used with plugs because of the reduced buffer capacity of the small volumes of growing medium found in each plug cell. If a short-term crop is grown in a mix that has a low initial pH using acidic fertilizers, alkalinity levels of 140 - 160 ppm may be acceptable.
The more that high alkalinity water is applied to a crop, the faster that growing medium pH rises. Consequently, the effect of water alkalinity on media pH will be different on crops grown in warm weather (with high transpiration) than on crops grown in cooler environments (with less transpiration).
Growers using water with high alkalinity levels can compensate for the effect of alkalinity by using fertilizers of acidic reaction. However, fertilizers of acidic reaction have high levels of ammonia-nitrogen, which is poorly tolerated by some crops, especially in cool environments.
Usually growers use water from one of three sources: 1) a well, 2) a municipal facility, and 3) a pond. All three sources produce water whose characteristics may change over time. The quality of well water may vary with the season and/or climate. For example, solutes tend to dilute in rainy years, and vice versa in dry years.
The environment also impacts the quality of pond water. During dry periods, the concentration of the solutes increases as water evaporates. This is more important if most of the water comes from a spring, because spring water contains solutes.
Some municipalities may have more than one source of water. Some may even blend water from different sources. Growers in these areas should know if and when a change of source is effected, particularly if one of the sources has inadequate water characteristics, such as high alkalinity. As a rule of thumb, growers should test their irrigation water at least twice a year, keep the results, and find trends over time or correlations with climatic events.
Growers can buy easy-to-use pH meters and alkalinity titration test kits to check water and growing mix characteristics routinely. At least once a year, growers should also send samples to a reputable laboratory to have complete and accurate water tests designated for growing characteristics (not human drinking water characteristics).
A common way of neutralizing pH and alkalinity is by adding acids to the irrigation water. Several acids can be used: 85 percent or 75 percent phosphoric acid, 35 percent sulfuric acid, 67 percent nitric acid, or a 50/50 mix of phosphoric and sulfuric acid. The grower should choose an acid based on availability, price, and safety. Both sulfuric and nitric acid are extremely caustic and have to be handled with caution. Phosphoric and sulfuric acids add phosphorus and sulfur to the growing medium.
Corrections on the fertilizer regime have to be made to compensate for these extra nutrients. Phosphorus and sulfur are not phytotoxic, even at high levels. However, they increase the electrical conductivity of the mix and, if leached, become environmental pollutants.
These acids and the water carrying them are corrosive. Plastic pipes are recommended for injecting acids. If in doubt, check with the supplier before using an injector with acids. Always wear protective eyeware and protective clothes while handling acids. Always add acid to water. Never add water to acid-- the acid can boil out of control and splash back on you.
Usually, laboratories that conduct water, soil, and plant tissue analyses recommend how much acid to apply based on the alkalinity of the water. Extension agents or floriculture specialists are also available to advise growers. Contact Claudio Pasian, floriculture state specialist, for specific questions on how much acid to add in certain conditions (phone: 614-292-9941; e-mail: firstname.lastname@example.org).
Alternatives to adding acid to water include: 1) reverse osmosis, which is expensive and can be used only for small volumes of water and with high-value crops; and 2) using rain water, which is almost pure water. The solute content of rain water is negligible, making rain water ideal for direct irrigation or to dilute poor-quality water.
In the Netherlands, the majority of growers use rain water-- a factor that contributes to the high quality of Dutch crops. Ohio usually has high levels of rainfall, and several growers are collecting and using rain water for irrigation. Some have cisterns; others have ponds. Some use rain water directly for irrigation; others use it to dilute their well water. Growers interested in using rain water for irrigation can contact growers who are already using it for that purpose.
Water softeners replace calcium and magnesium with sodium. They do not reduce alkalinity or soluble salts. Softening is not recommended to treat irrigation waters.
In summary, growers must know the pH and alkalinity of their irrigation water because of its capacity to affect media pH and fertility. Growers should test media and water pH regularly with simple, inexpensive pH meters and alkalinity kits. At least once a year, growers should send samples to be tested by a reputable laboratory. Based on test results, growers should act in one of the following ways: change water source, if possible; collect and use rain water; or acidify the water used for irrigation. If a grower plans to expand facilities or move, a water test should be among the top items to consider. If a property has a poor water source, little can be done to remedy the situation.
All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.
Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.
TDD No. 800-589-8292 (Ohio only) or 614-292-6181