When most of us hear the word ‘bicarbonate’ we think of baking soda. In this case, we’d only be half right.
Baking soda (NaHCO3) is sodium bicarbonate. Sodium is salt. Salt is bad for your garden. Period.
Potassium bicarbonate is something else entirely.
What is potassium bicarbonate?
Also known as potassium hydrogen carbonate or potassium acid carbonate, potassium bicarbonate (KHCO3) is a white, alkaline solid used in fire extinguishers, wine making, to make club soda, and to neutralize acids. As a base, potassium bicarbonate is at the high end of the pH scale. If you have alkaline soil, studies have shown that using potassium bicarbonate does not alter sodium levels in your soil, plants, or runoff water. If your garden plants commonly suffer from fungal diseases, potassium bicarbonate may be exactly what they need.
Potassium bicarbonate is an organic fungicide used mostly to prevent fungal diseases, such as Alternaria blight, apple scab, black spot, blights, botrytis, downy mildews, molds, phytophthora, powdery mildew, and Septoria leaf spot. Once these diseases are in place, they are very difficult to get rid of, through potassium bicarbonate can certainly improve the situation.
Research has shown that bicarbonates do, when they cover a leaf completely, slow the growth of and occasionally kill mold spores outright. For potassium bicarbonate to work effectively, it must be in solution. While some people promote the use of homemade mixes that use horticultural oils for this purpose, those oils can lead to phytotoxicity (plant poisoning), oily buildup on the leaves and in the soil, and the mix requires constant shaking to keep the oil and water mixed while applying. The ideal mixture of solution and treatment is sold under the name GreenCure®.
Unlike sodium bicarbonate, which leaves behind a salt residue, ammonium and potassium bicarbonates contain nitrogen, an important plant nutrient.
Can you make your own potassium bicarbonate spray?
You can, but you shouldn't. As a big proponent of DIY just about everything, I must say that this case is an exception to that rule. While you can certainly find recipes for your own potassium bicarbonate mixture online, making it properly is not as simple as advertised. The truth is, it took years of research to determine the proper balance of ingredients that allow the antifungal action to occur, while sticking to plants, but not killing them in the process.
The fine folks at The Garden Professors Blog Facebook Page directed me towards some good information along the same lines. Bottom line: potassium bicarbonate sprays are an effective prevention and treatment of many fungal diseases. But these mixtures are not something you should be trying to concoct at home. Instead, read labels and buy a product that will protect and not harm your plants.
If you decide to use potassium bicarbonate in your garden, UC Davis recommends no more than 8 treatments a year.
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