Understanding the difference between water pH and alkalinity

Photo © thinkstock.comUnderstanding irrigation water pH and alkalinity as it affects greenhouse crop fertility has become one of the most important and confusing topics in crop production today. pH and alkalinity are important factors in determining the suitability of water for irrigating plants.
 

Water pH
pH is a measure of the concentration of hydrogen ions (H+) in water and other liquids. In general, water for irrigation should have a pH between 5.0 and 7.0. Water with a pH below 7.0 is termed “acidic” and water with a pH above 7.0 is termed “basic”. Water with a pH of 7.0 is neutral. 
 

Water alkalinity
Alkalinity is a measure of the water’s ability to neutralize acidity. An alkalinity test measures the level of bicarbonates, carbonates and hydroxides in water. Test results are generally expressed as parts per million of calcium carbonate (CaCO3). The desirable range for irrigation water is 0 to 100 ppm CaCO3. Levels between 30 and 60 ppm CaCO3 are considered optimum for most plants.

Water pH tests
Some growers perform their own water pH tests using various types of meters available from greenhouse supply companies. A pH test by itself is useful but it is not an indication of alkalinity. Irrigation water tests should always include both pH and alkalinity tests.

Water with high alkalinity (i.e., high levels of bicarbonates or carbonates) always has a pH value of 7 or above. But water with a high pH doesn’t necessarily have high alkalinity.
It’s quite common for irrigation water to have a high pH (7-8) and low alkalinity (less than 100 ppm CaCO3). In some cases both pH and alkalinity may be high.

What management steps a grower should take depends on the type of water used to irrigate the plants and the crops being grown.
 

High pH, low alkalinity water
Water with a high pH (7-8) and low alkalinity (less than 100 ppm CaCO3) has three potential effects and required actions.

One potential effect is no effect at all. Most greenhouse plants can be irrigated with this water with no adverse effects on growth or quality. The only required action would be to test the water several times a year to make sure there are no large or sudden changes in the pH or alkalinity.

A second effect might be the appearance of chlorosis on plug seedlings or liners of some species like annual vinca, petunia and calibrachoa, which prefer a low growing medium pH. The growing medium in very small cell volumes is poorly buffered to pH changes so that the medium pH may increase with high pH water. Normally this problem can be solved by using a fertilizer like 20-10-20 or other acidic fertilizer that is about 50 percent nitrate-nitrogen. If a large part of a grower’s business is producing plugs and liners for sale to other growers, water acidification may be a consideration if chlorosis occurs regularly on some plant species. Only species known to develop chlorosis due to high water pH should be irrigated with acidified water.

A third effect might be deficiencies of calcium (Ca) and/or magnesium (Mg). Water with high alkalinity can be an important source of these two elements, so if water tests show “low” for alkalinity, the Ca and Mg levels should be checked too. If Ca and Mg are low, the corrective action would be to fertilize with a Cal-Mag fertilizer. The best example of a sensitive Ca and Mg deficient crop would be poinsettia. Plants develop edge burn on the bracts or interveinal chlorosis on the leaves if Ca or Mg, respectively, are deficient.

High pH, high alkalinity water
High pH and high alkalinity water (150+ ppm CaCO3) has the greatest effect on species that require low growing medium pH and are prone to iron chlorosis. Some important greenhouse crops sharing these two characteristics include petunia, calibrachoa, scaveola, bacopa and snapdragon. Irrigating with high alkalinity water tends to increase the growing medium pH because of the liming effect caused by the carbonates and bicarbonates (sources of alkalinity) in the water.

Corrective actions are meant to lower the growing medium pH by using acidic fertilizers, avoiding overliming and in some cases by water acidification. Also, application of an iron chelate fertilizer solution to prevent or correct iron chlorosis is a very effective action.

These corrective actions are meant for the very small group of species listed above. No action would be needed for most greenhouse crops because they are not susceptible to iron chlorosis. In fact, irrigating with this water might help prevent iron and manganese toxicity on marigolds and geraniums and provide supplemental Ca and Mg to crops with a special need for these elements.

Left: Water acidification is a very effective means of preventing iron chlorosis on plug seedlings and liners and crops like snapdragon. Right: Acid water should not be applied to crops like geranium that rarely have iron chlorosis since they may be injured by too much iron and manganese when the growing medium pH is low.

Water acidification
Water acidification is a very effective means of preventing iron chlorosis on plug seedlings and liners and petunia, calibrachoa, scaveola, bacopa and snapdragon. It is not a good a good idea to irrigate all crops with acidified water.

Acid water should not be applied to crops that rarely have iron chlorosis. This is especially true of marigold and geranium, which are injured by too much iron and manganese when the growing medium pH is low. If a large number of plants are being produced that are prone to iron chlorosis, water acidification is worth considering especially if the water consistently tests “high” for both pH and alkalinity.

Impact of water pH on chemicals
Water pH is a critical factor in the effectiveness of a number of common greenhouse chemicals including Azatin, Avid, Cleary’s 3336, A-Rest, Florel/Pistill and others. Poor performance of greenhouse chemicals might also be explained by pest resistance, application method, rate or development stage of the pest (pesticides) or the plant (plant growth regulators).
 

Douglas Cox is associate professor, University of Massachusetts, Department of Plant, Soil, and Insect Sciences, (413) 545-5214; dcox@pssci.umass.edu.

Adapted from the article “Q&A: Water pH and Alkalinity” in Floral Notes Newsletter, Nov.-Dec. 2010, Vol. 23, No. 3.

References
UMass Extension Floriculture. Adjusting alkalinity with acids. www.umass.edu/umext/floriculture/fact_sheets/greenhouse_management/adjalkal.html

UMass Extension Floriculture. Effects of pH on pesticides and growth regulators. www.umass.edu/umext/floriculture/fact_sheets/greenhouse_management/ph_pesticides.htm

North Carolina State University. Alkalinity control for irrigation water used in nurseries and greenhouses.
www.ces.ncsu.edu/depts/hort/hil/hil-558.html