Ammonium Bicarbonate

What does ammonium bicarbonate have to offer your garden?

Ammonium bicarbonate (Wikipedia) CC BY-SA 4.0

In the garden, bicarbonates are touted as cure-all treatments of powdery mildew, gray mold, septoria leaf spot, and other fungal diseases, particularly sodium bicarbonate (baking soda) and potassium bicarbonate. The truth is, baking soda is a type of salt and always a bad idea in the garden. Potassium bicarbonate, on the other hand, is an effective organic fungicide. 

But what about ammonium bicarbonate? What are bicarbonates, anyway?

Bicarbonates

In chemistry, the word ‘bicarbonate’ is inaccurate and outdated. It was first coined in 1814 by a chemist who observed that there is twice as much carbon as sodium in sodium bicarbonate. After different types of bicarbonates were identified, with different ratios, the observation became irrelevant, but the habit lives on.

Bicarbonates are the main form of dissolved inorganic carbon in the ocean. In freshwater plants, bicarbonates are released into the water as part of photosynthesis. This can shift the water’s pH to toxic alkaline levels. This continues until nightfall, when photosynthesis stops and respiration releases carbon dioxide, causing pH to return to normal. Bicarbonates commonly act as pH buffers in the human body [plop, plop, fizz, fizz] and in soil. 

Antifungal bicarbs

Bicarbonates are lumped together, incorrectly, as antifungal treatments because they are alkaline. Most fungi prefer a slightly acidic environment. Baking soda and other bicarbonates will raise pH temporarily, but the effect is fleeting and fungi take up where they left off as soon as the baking soda is washed or blown off. Applying enough baking soda to kill powdery mildew would end up adding toxic levels of salt to your soil. Baking soda should be kept out of the garden. Potassium bicarbonate, on the other hand, is an effective organic fungicide. ​

Powdery mildew on butternut squash leaf

Salt of Hartshorn

Ammonium bicarbonate used to be the leavening agent of choice, before baking powder hit store shelves. Still used today in flatbreads, German Lebkuchen, Danish Christmas cookies, and Swedish "drömmar" biscuits, ammonium bicarbonate is often referred to in older cookbooks as salt of hartshorn or hornsalt. This form of ammonium bicarbonate used to be made by dry-distilling horns, hooves, leather, and hair.

Like baking soda, ammonium bicarbonate is a salt. It is the bicarbonate salt of an ammonium ion. Ammonium bicarbonate breaks down easily into carbon dioxide, water, and ammonia. It breaks down so quickly that it must be stored cold. In flatbread baking, the ammonia smell dissipates during cooking, but that smell is why ammonium bicarbonate is not used in making bulkier, yeast breads. [Ammonia toast, anyone?]

Ammonium bicarbonate fertilizer

In the plant world, ammonia means nitrogen. This makes ammonia bicarbonate sound like a good idea as a fertilizer, right? In China, ammonium bicarbonate is used as an inexpensive fertilizer. But, because of its instability, it is being phased out in favor of urea. Also, ammonium bicarbonate is an eye, skin, and lung irritant. If you were to use it (against my advice) be sure to wear protective clothing and a respirator. 

High soil bicarbonate levels commonly occur when soil or irrigation water have a pH of 7.5 or higher. Alkaline soil and irrigation water tend to have lots of bicarbonate and carbonate ions floating around. These ions tend to attach themselves to and transform calcium and magnesium into less soluble forms that are difficult for plants to use. Also, as these minerals are altered, they leave salt behind in your soil. Not good. When these conditions occur, chelated fertilizers should be avoided. The level of bicarbonates in your soil also determines how much acid is needed to acidify the soil. 

If powdery mildew or other fungal diseases are causing problems in your garden, forget the baking soda and ammonium bicarbonate, Instead, space and prune plants for better air flow and apply potassium bicarbonate, Bordeaux mixture, sulfur, fixed copper, or milk. You may also want to apply insecticidal soap (not dish soap) to reduce the spread of disease by ants.