Bicarbonate: Is it a problem in your soil?

Bicarbonate is not a plant nutrient. It actually tends to be a problem in soils. If you live on the West or the East Coast then you probably don't have to worry about bicarbonates.

But if you live in a place where the precipitation is less than evapotranspiration (in other words, a place that's relatively arid with high pH soils), then take note. Bicarbonate is often the largest issue in the Midwest and the West.

Bicarbonate primarily comes from irrigation water. The molecule is HCO3- and it has a pH increasing effect.

There are a few main reasons bicarbonate is problematic.

Bicarbonate originates from calcite parent material and is a form of carbonic acid. Bicarbonate tends to increase pH over time. It tends to precipitate out positively charged ions, specifically calcium, iron, magnesium.

Bicarbonate actually pulls calcium off the clay colloid, and then that calcium is often replaced with something like sodium, which is not good. It reduces the plant availability of calcium and magnesium and creates calcium bicarbonate and magnesium bicarbonate. With iron, the pH increasing effect reduces the availability of iron and it also interferes with iron metabolism. Iron chlorosis is most often seen when bicarbonates are high.

On the Logan Labs paste test, I like to see bicarbonate less than 120ppm. But I usually want to see it above about 50ppm. Why? It turns out it can be a source of calcium and magnesium since the dissociation of calcium and magnesium bicarbonate produces ions in solution. When you see bicarbonate over 150ppm, I would consider ways to reduce it.

How do you reduce Bicarbonate? Irrigation water.

Bicarbonates are the primary source of alkalinity in water. Note: alkalinity is different than alkaline, which just means the pH is greater than 7. Alkalinity is the water’s resistance to pH change. It's a summation of bicarbonates and carbonates. You won't see any carbonates in your irrigation water unless the pH is over 8.3. To reduce bicarbonates in irrigation water, there are a few techniques. Consider acidifying your water. I like organic acids. You can use phosphoric, nitric, or sulfuric acids. However, sulfuric and nitric are dangerous to work with. Phosphoric acid is a major source of groundwater pollution. I can write a recommendation of how much acid will reduce your pH based on how much bicarbonate is in your water. Citric acid is the go-to for small organic growers, and sulfur burners are the best options for larger growers of any kind.

Foliar Applications

If you are doing foliar applications, you don't want to use high bicarbonate water. More specifically, high alkalinity water (with high bicarbonates) is going to precipitate out a large portion of those nutrients and you'll be wasting your money. Use clean water for irrigation with low bicarbonate levels.

Sodium Adsorption Ratio

Another thing to consider is the sodium absorption ratio (SAR). If you have high sodium in your irrigation water, bicarbonates are actually going to increase the SAR, which reduces the permeability of your soil. It affects soil structure and permeability, especially when combined with sodium. There is a metric called the adjusted SAR, which accounts for bicarbonates. If you have high bicarbonate, you should be using the adjusted SAR. I have three different calculations for adjusted SAR.

The point is this: the combo of sodium and bicarbonate is even more problematic on soil structure and permeability than sodium alone.

If you are in the arid West or the Midwest, get your irrigation water tested. You can do that on my website and figure out your bicarbonate level, and if it's going to be an issue for you.