Gardening Articles: Care :: Soil, Water, & Fertilizer

Improving Soil With Gypsum (page 3 of 3)

by Garn Wallace with Shelly Stiles

Gypsum and pH

In most situations gypsum has little effect on soil pH. Use limestone to raise pH of acid soils, and elemental sulfur to lower pH of alkaline soils. However, if soil is alkaline because of high bicarbonate ions, or if irrigation water contains bicarbonate, the soluble calcium in gypsum reacts with the bicarbonate to form insoluble calcium carbonate. The pH of the soil is decreased to the range of 7.5 to 7.8 when bicarbonate is reduced. Also, limestone does not move through the soil to lower depths. Gypsum is the best way to supply calcium to soil and roots below cultivation depth.

Gypsum as Metals Protector

Heavy metals, such as cadmium, copper, lead and nickel, are toxic to plants in excess amounts, as well as to the people who eat the plants. Soils in or near some industrialized areas and soils very heavily amended with municipal sewage sludge might contain toxic amounts of heavy metals. Plants generally absorb less of the heavy metals from soil when gypsum is applied. Metals also become more toxic in highly acid soils. In those cases, use limestone to reduce acidity and metal toxicity. Some of the heavy metals plants absorb never move farther than the roots, the reason you should not grow root crops, such as carrots, in contaminated soil.

Using Gypsum

A calcium analysis of your garden's soil or an assay of calcium uptake in your plants is the best way to gauge your garden's gypsum needs. Most Cooperative Extension Service soil tests include calcium if you request it. Either very low (below about 5.5) or very high (above 8.2) pH values are also indicative of a need for calcium. If your soil pH is alkaline (above 7.0), make sure your lab doesn't rely on an acidic testing agent, as many do. The results won't accurately portray what goes on in your garden.

Natural-mined and finely milled gypsum costs $3 to $4 for a 10-pound bag, about the price of limestone. It's also available in 50- and 80-pound bags. Apply 10 pounds per 1,000 square feet of garden area up to three times each growing season to correct calcium deficiencies. Over-application of gypsum is generally not a problem because it has low solubility and the increase in salinity is low. Look for high-purity, natural-mined gypsum. Although natural-mined gypsum is considered "organic" by the country's organic certification programs, gypsum-based by-products and recycled materials are not.

Garn Wallace, Ph.D., operates Wallace Laboratories in El Segundo, California. Shelly Stiles is a writer living in Buskirk, New York.

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