High Soil pH – Can we fix this problem?


loweringphgypsumWhile some areas apply lime (Calcium carbonate) to increase the soil pH and improve crop yields, other areas of the northern plains wonder about soils with a high pH. Soils with a pH higher than 7.3 can cause some issues with phosphorus availability in the soil. When phosphorus fertilizer is applied to a high pH soil, a chemical reaction occurs between the phosphorus and the calcium carbonate in the soil. The result is some of the phosphorus forms compounds with calcium that are not very soluble. This reduces the amount of phosphorus the crop can get from the fertilizer applied. If we were able to lower the soil pH to less than 7.3 and neutralize all of the calcium carbonate, we would reduce the problem of phosphorus fertilizer being tied up in low solubility calcium compounds.

There are several products that are promoted as being able to lower the soil pH. Research has been done at several universities through the years to figure out if any of these products actually lower the soil pH permanently. In April of 2005, AGVISE Laboratories established a long-term project to determine if applying elemental sulfur and gypsum can temporarily or permanently lower soil pH. This project was established on a soil with a soil pH of 8.0 and a calcium carbonate content of 1.5%. The elemental sulfur and the gypsum were tilled into the soil to a depth of 6″ on April 4, 2005. The treatment area was seeded into alfalfa so that there will be no soil disturbance.

In the figures, you can see that two years after application, only the elemental sulfur at a rate of 10,000 pounds per acre has decreased the soil pH and kept it there. We know from the science, that elemental sulfur will react in the soil to form sulfuric acid (very strong acid). Once the sulfuric acid is formed, it will react with the calcium carbonate in the soil. If there is enough elemental sulfur to form sulfuric acid and react with all of the carbonate in the soil, the soil pH will continue to decrease over time and stay at the lower pH value in the future. In the two years since the elemental sulfur was applied, the soil pH has decreased about 0.4 units. This decrease in soil pH has not affected the growth of the alfalfa in the treated area. There were no differences in the nutrient levels in the alfalfa between the check and the areas treated with elemental sulfur.

We also included two rates of gypsum in this project due to companies that keep telling people that gypsum will lower the soil pH. As you can see in the figure, even the 5000 lb/a rate of gypsum has had no affect on the soil pH over two years. Good science tells us that when gypsum is applied to a soil it slowly dissolves into calcium and sulfate sulfur. There is no chemical reaction that happens when gypsum salt dissolves in the soil solution. With no chemical reaction, there is nothing that will make the soil pH decrease. In the first two years of this demonstration project, no rate of gypsum has had any affect on the soil pH. We intend to keep this project going for twenty years or longer. This depends on how long people keep promoting gypsum as a product which will lower the soil pH.