Many cotton growers are concerned that repeated use of dolomite lime leads to excessively high soil magnesium levels that could depress yields.

In a three-year study by University of Missouri scientists, however, “no significant difference was found between calcite and dolomite lime,” and the researchers concluded: “Cotton farmers should not be concerned about potential negative effects of magnesium in dolomite lime material.”

Gene Stevens, agronomist at the MU Delta Research Center, Portageville, Mo., will publish the study results along with co-researchers David Dunn of the Delta Center Soil Testing Laboratory and MU soil scientist Peter Motavalli.

Some crop managers have long believed that cotton fiber quality and lint yield are adversely affected by the higher magnesium content of dolomite lime (calcium magnesium carbonate) compared to that of calcite lime (calcium carbonate).

Stevens noted that dolomite is sometimes more readily available than calcite. “Transportation expenses to haul ground limestone from quarries to production fields are a major part of the liming costs for cotton farmers,” he wrote. “The least expensive liming material — calcite versus dolomite — often depends on the closest quarry producing each type of lime.”

The MU scientists conducted two three-year experiments. The first studied the ratios of magnesium to calcium in the soil and how they affect potassium uptake by cotton roots and subsequent cotton yield. Gypsum (calcium sulphate) and Epson Salt (magnesium sulfate) were used to produce variations in the ratio of soil calcium to magnesium.

The second experiment compared the effectiveness of calcite and dolomite lime applications to increase cotton yield on acid soils. Both experiments included untreated fields for a basis of comparison.

The researchers discovered that dolomite lime applications could be helpful to correct soil acidity and increase magnesium in magnesium deficient soils. “Otherwise, magnesium content in lime is not a factor in lime recommendations,” Stevens wrote.

In the lime-source experiment, they took weekly pH measurements and found that “dolomite lime reacted slower than calcite lime but produced the same cotton yield response as calcite lime,” he wrote. “By the end of the first growing season, all limed plots had pH values above 6.0.”

“Applying lime as needed to correct soil acidity in cotton fields is more important than whether calcite or dolomite lime is used,” the MU team concluded. “Farmers should apply lime as soon as possible when soil test results show soil pH is below critical levels.”

The experiments also shattered another myth about lime applications in cotton, Stevens observed. “Sometimes farmers postpone liming before planting until after harvest because of a general belief in the agricultural community that lime reacts too slowly to provide first-year yield benefits.” The MU team found that calcite and dolomite lime applied and incorporated shortly before planting increased lint yields approximately 128 to 162 pounds-per-acre in the first growing season.