Does adding acid to soil affect microbes?

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Photo: NRCS

Question: I farm out in central Colorado, and a fertilizer company suggested using sulfuric acid to lower the pH of my soil. What effects would this have both short- and long-term on soil microbes, soil health?

Also, is there any cost effective ways to reduce high levels of manganese and magnesium in your soil profile?

Answer: Sulfuric acid can be used to lower soil pH. The issue is can you afford to do it? The answer is almost always “no” because the amount of acid needed to lower the pH would be cost-prohibitive in terms of what is gained. At pH levels above 7.8, micronutrients like zinc, iron, manganese, etc., are less soluble and thus less available to plants. If you have a deficiency of one of them, it’s by far cheaper to purchase fertilizer than to try to decrease your soil pH. (By the way, the only micronutrients ever found to be deficient in Colorado soils are zinc and iron.)

The addition of the sulfuric acid would not damage the soil microbial population or “soil health” in the long-term. Additions of acid would likely decrease microbial population at the specific place where it was added, but the population would soon recover. Sulfuric acid and elemental sulfur are sometimes added to “sodic soils” to help in their reclamation. However, this practice is not intended to lower soil pH, but simply to dissolve some calcium from calcium carbonate.

The second part of the question regarding manganese and magnesium is puzzling to me. First of all, soils with pH levels above 7.8 do not have high levels of manganese. Manganese has limited solubility at these pH levels, and thus manganese cannot be at high levels in high pH soils.

Levels of available magnesium are generally high in soils with pH levels above 7.0, which is a positive characteristic. So, it’s extremely unlikely that you would want to reduce magnesium levels in your soil. There is an unfounded rumor created by pseudo-soil scientists that high levels of magnesium will limit calcium and potassium availability to plants in high pH soils. If this ever occurs it is extremely rare. Therefore I do not think you should be seeking ways to decrease either manganese or magnesium in your soils.

–Answered by Gary “Pete” Peterson, Colorado State University

Have a question for Soils Matter? Post it as a comment below, or email us at soils-matter@soils.org

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