Gypsum and how it corrects aluminum toxicity in the subsoil

How does calcium remediate pH problems associated with an acid subsoil?

Soils can be acid or alkaline. Acid soils occur where there has been a lot of rainfall over the centuries. Acid soils are highly weathered, and many minerals and bases have been leached. Alkaline soils develop in arid areas that lack rainfall. Alkaline soils can be managed but not corrected. Acid soils can be neutralized with lime.

Plant growth comparison: Aluminum vs. Calcium

Growers apply lime to neutralize or raise pH. They generally apply enough lime to raise pH to 6.2 to 6.5 for a period of 4 years and then reapply. Neutralizing pH only targets the top 6 to 8 inches of soil and does nothing for the subsoil that can still be acid. Acidity, particularly when below 5.5, reduces nutrient solubility while increasing solubility of aluminum and iron, which can be damaging for growing root tips. While gypsum can’t increase the pH of acid subsoils, in can reduce aluminum toxicity in these same soils.

If acid neutralization is the goal, then lime is the material of choice. However lime is not very soluble or mobile in the soil. Therefore, neutralizing acid subsoils requires incorporation and deep mixing in the subsoil. This is not something that most farmers can easily or cost-effectively accomplish. Gypsum is more soluble and mobile in a shorter time frame and will leach into the subsoil by irrigation or rainfall.

In acid subsoils, soluble aluminum is toxic to plant roots and causes root pruning. Calcium will displace aluminum, allowing it to be leached below the rooting depths as long as there is enough moisture to draw it downwards. The more calcium that leaches through the subsoil, the more aluminum is leached. Lime can do the same in the topsoil but not the subsoil.

Screen shot 2010-11-24 at 7.57.20 AM
Global Aluminum Toxicity in Soil

Gypsum is calcium sulfate, and lime is calcium carbonate. Both are soil amendments, and both provide calcium that can be used to displace excess aluminum in acid subsoil. However, only gypsum is soluble enough to move quickly down into the subsoil, and you can see the benefits in a few months. Lime is less soluble and can take 18 months to move down and give the same benefit as gypsum.


Dr. Davidson posts articles on soil management and subjects to gypsum. If you have suggestions for topics or questions, feel free to contact him at [email protected] or call 402-649-5919.