Gypsum is one of the best sources of calcium, the most important of the secondary plant nutrients. But calcium is more than a nutrient. It is the major balancing element in plants and soils. It protects, within limits, from nutrient excess and deficiency, problems caused by both high and low pH and heavy metal contamination. Along with organic matter, calcium improves soil structure.
Pure gypsum is 23 percent calcium and 19 percent sulfate (CaSO4-2H2O). In the hierarchy of the 16 essential plant nutrients that begins with non-minerals hydrogen, oxygen and carbon, calcium is seventh and sulfur is ninth.
Calcium is the plant nutrient most likely to be unavailable to roots when needed. An essential nutrient itself, calcium also improves root uptake of other plant nutrients, especially ammonium nitrogen. Calcium does not move from old to new plant tissues, so a constant supply of soluble calcium is needed. The growing points of both roots and shoots are sensitive to insufficient calcium, but roots more so. The ratio of calcium to sodium is more critical than the actual concentration of calcium.
Symptoms of calcium deficiency include
Calcium is most often deficient in high-rainfall, acid-soil areas, such as the Pacific Northwest west of the Cascade Mountains, and east of the Mississippi River. These regions receive in excess of 40 inches of rainfall each year. Deficiency symptoms, such as tomato blossom end rot, often appear there after a period of heavy rainfall. Gardeners apply ground dolomite or limestone, primarily to raise soil pH, but also to replace lost calcium. But the calcium in limestone is not water soluble, however. It must be incorporated into the soil and brought into contact with soil acids before calcium ions (Ca++) are released.
Limestone does not migrate in the soil, and is effective only to the depth incorporated. In contrast, gypsum dissolves immediately in the moisture of the soil, allowing plant roots to absorb the calcium ions and the sulfate ions. Because it is dissolved in the soil solution, gypsum readily migrates into the lower depths in the soil.
Most western soils contain abundant calcium, but as very insoluble limestone in the form of calcium carbonate. Where pH is 7 or higher, the limestone is insoluble and the calcium it contains is not available to plant roots. Again, the calcium in applied gypsum is immediately available.
The other component of gypsum, sulfate, supplies plants with sulfur, which is essential to protein synthesis. Sulfur is a constituent of the amino acids cystine, methionine and cysteine. It is also essential for nodule formation on legume roots, and for the characteristic odors of plants such as garlic and onions. Sulfur deficiencies are less common than calcium deficiencies, but can occur throughout North America, particularly in high-rainfall areas.
If sulfur is applied as elemental sulfur, it is not available until the soil bacteria oxidize it to sulfuric acid. Surface-applied sulfur is oxidized faster than incorporated sulfur, but because the required bacteria are frequently not present in alkaline soils, the oxidation may be very slow. Gypsum supplies sulfur as sulfate, the form plant roots can absorb.
Symptoms of sulfur deficiency include
Gypsum can reclaim high-sodium or "sodic" soils. Soils with a high proportion of exchangeable sodium (Na+) lose structure. Once soil is tightly packed, water and air penetration is reduced and root growth suffers. Water often puddles on the surface of these soils.
To revitalize sodic soils, incorporate gypsum into the soil and then apply six or more inches of water. The calcium ions will replace sodium ions that are attached to the soil particles. The sodium (as sodium sulfate, NaSO4) washes away through the soil into the groundwater, and your topsoil is left more porous, so is more supportive of root growth.
Sodic soils occur almost exclusively in the arid and semiarid West. If you suspect your soil is sodic and would benefit from this gypsum-leaching treatment, order a laboratory soil test to confirm the presence of excess sodium. Applying gypsum to the surface of a compacted soil won't work.
Compacted soils are potential problems anywhere, and organic matter--such as compost--is generally the best remedy. Gypsum helps you get the most benefit from organic matter added to the soil. Soil crumbs and aggregates that give structure to soil are cemented together with glues that include salts such as calcium. Leaching from salt-free rainwater increases acidity and decreases the salinity of soils, and this can cause soil to lose its physical structure despite the presence of adequate organic matter. Correcting acidity with limestone helps, but limestone does not contribute calcium ions to the soil. Gypsum also protects the soil from compaction and other types of degradation where irrigation water is reclaimed or of low quality.
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.
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.