By Joe Traynor
It is well known that excessive soil lacidity, resulting in a low soil pH, impairs root growth and reduces crop yields. Ideally, soil pH should be in the 6.5 to 7.5 range, with a pH of 7.0 being neutral. When pH drops below 5.5, lime, or calcium carbonate, is applied to raise soil pH.
Lime application and incorporation is easily accomplished in most situations, but for drip-irrigated crops raising soil pH can be a vexing problem. Lime must be incorporated into the soil to be effective and because lime is relatively insoluble, it cannot be applied through drippers.
Many California soils have a good supply of lime in its virgin state – enough to maintain soil pH in a favorable range for many years. On the many soils that don’t have natural lime, growers planning a drip-irrigated permanent crop should incorporate lime into the projected drip line at the rate of 10 tons per acre. On established vineyards, continued use of ammonium-based nitrogen fertilizers over the years coupled with low-salt irrigation water can drop soil pH below 5.5. On vineyards, sulfur application for mildew control will also acidify low-lime soils over a period of years.
When soil pH on established drip-irrigated vineyards drops below 5.5, incorporating lime into the wetted zone is logistically difficult. A possible solution to this dilemma is water-run potassium carbonate. Potassium carbonate (K2C03) is very soluble and applying it through drippers should be feasible. Potassium carbonate can’t compete with lime on a cost basis, nor can it compete as a potassium fertilizer, however the combination fertilizer/amendment properties of potassium carbonate make it an intriguing material for raising soil pH on drip-irrigated crops with a high potassium requirement, such as bearing vineyards.
There are some caveats to running potassium carbonate through drippers, however. If the irrigation water has a moderate calcium level, lime can precipitate out and clog drippers, albeit where high calcium waters are used, low pH problems are not encountered. Once potassium carbonate is added to water, much of the carbonate will convert to bicarbonate and bicarbonate can have a deleterious effect on soils that are not well supplied with calcium. Low pH soils are usually low-calcium soils and calcium levels should be increased on such soils by adding gypsum, or calcium sulfate, and/or by using calcium nitrate as the nitrogen fertilizer. By adding calcium to the soil – followed by or preceded by potassium carbonate – you are essentially making your own lime, or calcium carbonate, in the soil.
Ammonium-based nitrogen fertilizers should never be applied with potassium carbonate because NH3 volatilization could occur. Potassium carbonate is hygroscopic and will clump as it picks up moisture from the air, however the 50-pound bags in which it is sold minimizes this problem. Also, because of its high alkalinity, potassium carbonate is more irritating to skin, eyes and lungs than other potassium materials.
Potassium carbonate has fungicidal properties and it is possible that water-run potassium carbonate could suppress phytophthora or even nematodes.
Fungicidal Properties of K2CO3
Potassium carbonate foliar sprays could also have a place in vineyard management as nutritional sprays and/or fungicides. Once potassium carbonate is added to water some of it is converted to potassium bicarbonate and that compound has been shown to have fungicidal properties. The amount of bicarbonate in a solution is dependent on the pH of the solution. Simply measuring the pH of a spray solution will determine whether potassium carbonate or potassium bicarbonate is dominant. In a lab test, 5 pounds of potassium carbonate in 100 gallons of water gave a pH of 10.9 with tap water and 11.2 with distilled water. One pound in 100 gallons gave a pH of 10.1 and 10.5, respectively. The pH of a beaker of potassium carbonate solution was lowered from 10.5 to 9.0, thereby converting carbonate to bicarbonate, by bubbling in CO2 blown in through a straw for about a minute. Adding dry ice would have the same effect.
Potassium carbonate is relatively unstable in solution since it picks up CO2 from the air. After standing in the lab for a week, the pH of a beaker of potassium carbonate solution dropped from 10.9 to 9.1. It is noted that all irrigation water, whether well water or river water, contains some bicarbonate and the amount found in the water will affect the final pH of any spray solution.
The fungicidal properties of bicarbonates have been known for years. At least two potassium bicarbonate products are currently registered as fungicides: Kaligreen (from Monterey Chemical; cost around $5.50/lb) and Armicarb (from Armand Products; cost around $7/lb). Kaligreen contains 18 percent “inert ingredients” and Armicarb contains 15 percent, with these inert ingredients including spreaders, stickers and other adjuvants that enhance the products’ effectiveness. Recommended application rates are 2 to 5 pounds of material per acre and the manufacturers recommend keeping the pH of the spray solution in the 7.5 to 8.5 range to maintain bicarbonates in solution. Because of its higher analysis, about one-third less potassium carbonate would be needed than potassium bicarbonate. Both Kaligreen and Armicarb are used to eradicate powdery mildew on grapes and on other crops, but their fungicidal activity has not been widely tested against orchard diseases.
If the fungicidal properties of potassium bicarbonate are in part a pH effect, then it is quite possible that potassium carbonate would be even more effective as a fungicide due to its higher pH in a spray solution. It is also possible, however, that the higher pH of potassium carbonate could have a phytotoxic effect on plant tissue. Certainly, the optimum pH for both fungicidal activity and for eliminating phytotoxicity should be studied and determined.
The relatively low rates of potassium carbonate at 2 to 5 pounds per acre recommended for Kaligreen or Armicarb for mildew control would provide only a small nutritional boost from potassium. 20 pounds per acre of material, whether potassium bicarbonate or potassium carbonate, would be best as a nutritional spray and could provide more fungicidal activity, but growers should wait for test results before trying such high rates to be sure there are no phytotoxicity problems.
It should be stressed that although potassium bicarbonate is registered as a fungicide, potassium carbonate is not. Potassium bicarbonate is used as a food additive and therefore could easily jump through the registration hoops. Potassium bicarbonate is also approved for use in organic production while potassium carbonate is not registered for use as a fungicide or foliar spray. It shouldn’t be difficult to convince authorities that when potassium carbonate is added to water and the pH adjusted to 7.5 to 8.5, the resulting solution is essentially potassium bicarbonate.
Because of its high solubility and relatively low cost when compared to potassium bicarbonate, potassium carbonate is an intriguing material in vineyard management. Bearing vineyards have a high potassium requirement with an estimated 100 pounds or more of potassium removed each year by the crop, and in situations where both a pH boost and a nutritional boost were needed, water-run potassium carbonate could be the ideal answer, especially for drip-irrigated plantings.
The proven fungicidal properties of potassium bicarbonate also make potassium carbonate worth looking at as a combination nutritional/fungicide spray. Certainly more work needs to be done to evaluate potential phytotoxicity problems with potassium carbonate and to determine the optimum pH of the spray solution when potassium carbonate is used.
Joe Traynor is a certified professional soil scientist, crop scientist and agronomist listed with the American Registry of Certified Professionals in Agronomy, Crops and Soils, Ltd. He holds multiple degrees from the University of California, Davis, is a member of the American Society for Horticultural Science, and is the author of Ideas in Soil and Plant Nutrition, published by Kovak Books.
The author would like to thank Growers Testing Service, Visalia, Calif. for running the lab tests described herein.