COMPARING AGRICULTURAL GYPSUM WITH LIMESTONE AND DOLOMITE (part 1)

 By Brent Rouppet, Ph.D.

(Part 1 of a 3 Part Series)

There are two key soil amendments in agriculture: (1) gypsum (calcium sulfate dihydrate [CaSO4 H2O]), and (2) liming materials such as limestone (CaCO3) and, dolomite ((CaMg(CO3)2). Liming materials are applied to neutralize soil acidity, but in situations where Ca is required without the need for correcting soil acidity, then gypsum is necessary.

These soil amendments should never be confused. Routine and frequent application of gypsum is required for the sustainability of all irrigated soils1; and while limestone, dolomite and other liming materials are necessary to neutralize soil acidity problems, improper usage of these liming amendments will actually result in harm to crops and plants.

The differences between gypsum and liming materials, and their uses in agriculture: 1. Limestone, dolomite, and other liming materials. Liming products are used and necessary in agriculture when the pH of the soil becomes too acidic for optimal plant growth and production. For most soils worldwide optimum pH is 6.2. At this pH the essential plant nutrients are most available; so in most cases in agriculture we strive to maintain a pH as close to 6.2 as practical.

If the soils become too acidic (have pH values lower than 6.2) then limestone, dolomite, or other liming products are applied to bring the pH back to the optimal range for crop/plant growth and production.

An example of the chemical reaction:

CaCO3 (limestone) + CO2 + H2O + 2H+ (acid) Ca2+ + CO2 + H2O

Acidic Soils:

• Soils become acidic when they are leached, especially in areas of higher rainfall. The more leached the soils are, the more strongly acidic they are.

• Strongly acidic soils have (1) few basic cations (calcium, potassium, magnesium and sodium) available in the root zone; (2) higher amounts of aluminum, hydrogen, and manganese; and (3) have lower amounts of more easily leached nutrients: sulfur, boron, zinc, molybdenum, and chlorine.

• Levels of toxic aluminum and manganese iron increase as pH levels do down.

• Most microbial processes, including nitrogen fixation, are slowed down by strong acidity.

Limestone, dolomite and other liming materials should never be applied to soils when the pH levels are above 7.0. At higher soil pH values the carbonate in limestone and dolomite will actually burn the crops, and even cause plant death in more severe cases.

The grower must be aware of which soil amendment products are needed (and how much) for application for specific uses to maximize crop production, and not harm plant growth and production.

Note: Dolomite is also used worldwide as a fertilizer source for magnesium (and calcium), but should only be used when soil pH values are less than 7.0.

Again, at pH values higher than 7.0 the carbonate in dolomite will actually burn the crops. Since limestone and dolomite are 150 times less soluble than gypsum, they generally are not a preferred source of calcium as a fertilizer.