Calcium sulfate

The use of gypsum (calcium sulfate), also called landplaster, to correct poor soil structure is a reasonably old practice. Gypsum can improve soil structure by increasing the aggregation of several small soil particles into larger particles. This can result in faster rates of water infiltration. Gypsum also is used as a source of calcium for soil applications on some crops. Peanuts in the southeastern U.S. are routinely sidedressed with about 1,000 pounds of gypsum annually. The use of gypsum to improve the calcium status of apple trees and fruits has been investigated in only two North American sites.

In Massachusetts, a series of studies has been conducted on Delicious and Cortland apple trees. In general, it appears that it takes 2 to 3 years for results to appear in the fruit. The treatments raised leaf and fruit calcium levels (20 percent and 10 percent, respectively), depressed leaf and fruit magnesium levels (20 percent and 5 percent, respectively), but had no effect on leaf and fruit potassium levels. Little or no effect was seen on fruit firmness at harvest or after storage, but bitter pit (50 percent reduction) and senescent breakdown after storage were reduced. Although high rates were used in early studies, in later studies it appeared that annual rates of as low as 3 to 4 tons per treated acre per year were effective. In a Nova Scotia study, annual applications of 5 tons per treated acre, for 5 years, raised leaf and fruit calcium levels in years 2 to 7 of the study but lowered magnesium levels.

For growers who have persistent calcium problems and who feel adventurous, the following treatment regime is suggested on an experimental basis. Apply 3 tons per treated acre, banded under the trees. This likely will be between 1.5 and 2.0 tons per acre of orchard. Follow a strict soil and leaf analysis program on an every-other-year basis to avoid nutritional problems. Special attention should be paid to magnesium, as gypsum may depress magnesium uptake.

Gypsum comes in various grades based largely on its color, with white grades being more expensive. For this use, the only relevant characteristic is the percentage of calcium in the product. The use of gypsum will not reduce the grower's need for lime.