Calcium sulfate is a multi-tasking mineral, both a fertilizer and a soil amendment that treats numerous soil problems.

Fertilizer
As a fertilizer, it provides two nutrients that plants require for growth, calcium and sulfur. Although most commercial fertilizer products focus on nitrogen, phosphorous and potassium to feed plants, there are in fact
16 elements necessary for plant growth, sometimes referred to as essential nutrients. The ones used in large quantities by plants are called macro nutrients, and they include calcium and sulfur.

Calcium and sulfur are naturally present in soils in many locations, to varying degrees. However, crops use the
minerals—in essence, mining the soil. If nutrients are not either replenished naturally or applied by growers, the soil gradually gets depleted.

Calcium Nutrition
Of the two, plant demand for calcium is greater. According to Brent Rouppet, PhD, a soil fertility scientist and agriculture consultant located in the Sacramento area, “calcium is used in a fairly large amount by crops. Some crops will remove about two hundred pounds per acre annually.” (This translates to about 680 lbs. of anhydrite gypsum, or 860 lbs. of dihydrate gypsum.) He points out that California has only been farmed intensively since World War II, about 75 years, but that is long enough to deplete the soil. “The soil probably had several thousand pounds or more per acre when it was virgin soil, in most cases. But if you keep farming that year after year and keep mining it off year after year, you get into a deficit situation.”

Calcium moves slowly through plants, requiring that it is constantly available to the roots in order to reach the tips of the plant.1 For this reason, it is important to have a high enough base calcium level in the soil, and to keep replenishing it as the crops are using it.

Surface Crusting
Calcium decreases surface crusting2 and the surface sealing that comes with it.3 This improves air and water penetration. It reduces runoff and erosion.4 Decreasing surface crusting improves seedling emergence and establishment.5 Studies have found that decreased crusting from calcium sulfate treatment may speed up harvest by several days.

Sulfur Nutrition
The sulfur requirement of most plants is lower, and in some places such as Southern California, sulfur is replenished by the irrigation water. In Northern California, however, water coming from mountain streams is very pure and supplies little mineral content.

According to Garn Wallace, PhD, biochemist and owner of Wallace Labs in El Segundo, CA, a laboratory specializing in soil analysis and consulting, “If you’re being fed zero sulfur in your water supply, you have to introduce the sulfur for the crop.” Crop use may be in the range of 50-60 pounds per acre per cropping cycle (roughly 240 lbs. of anhydrite gypsum or 290 lbs. of dihydrate gypsum). Of course, for any specific crop and acreage, a grower should do appropriate testing to determine the correct application rate.

Soil Treatment
Calcium sulfate’s other contribution is as a soil amendment, useful both for modifying soil chemistry and improving soil structure. These actions partially overlap.

Sodium Problems
Soil problems and deficiencies vary by region. In Southern California, one of the most important uses of calcium sulfate is to treat sodic soils. Soil scientist Brent Rouppet considers sodium “the number one problem in California, especially in the Southern San Joaquin Valley. Not only is it toxic to plants in most cases, it’s terribly destructive for soil structure. If there’s a lot of sodium, and not too much calcium, the structure can get very massive, roots don’t want to grow into that massive soil. Water certainly doesn’t want to penetrate into that soil. Oxygen can’t get into that soil.”

Sodium also increases surface crusting, causing water to run off rather than penetrate the soil. This lowers wateruse efficiency and increases erosion. Crusting can also impede seedling emergence. The sodium is attached to clay particles in the soil. Calcium sulfate breaks sodium loose from the clay by replacing sodium with calcium (and forming sodium sulfate). The sodium can then leach out of the soil, effectively driven down through the soil profile below the root zone.

Magnesium Problems
In Northern California, sodium is not as severe a problem, but there is too much magnesium. According to soil scientist Brent Rouppet, “Magnesium is not quite as bad as sodium, but if there’s too much magnesium and not enough calcium, magnesium will also destroy the soil structure, and we end up with massive soil.” Biochemist Garn Wallace also suggests that sodium and magnesium can decrease uptake of calcium. “By adding calcium, we can overcome the inhibitions caused by sodium or magnesium, so it has effect on nutrition as well as structure.”

Cation Exchange Capacity
Calcium sulfate also improves the cation exchange capacity of soil, making more “landing sites” in the soil that let it bind and retain certain cations that are nutrients. Important cations include calcium (Ca++), hydrogen (H+), magnesium (Mg++), potassium (K+), sodium (Na+) & ammonium (NH4+).

Soil Structure
In terms of soil structure, calcium helps with soil flocculation,6 encouraging clay particles to form into tiny clumps (called micro-aggregates) instead of forming large, densely-packed regions. The micro-aggregates have spaces between them where both water and air can penetrate. This more structured soil allows better water penetration, which increases the effectiveness of irrigation, and improves water-use efficiency. It reduces runoff and erosion.7 It allows better soil aeration, and deeper root formation. Calcium improves compacted soils. It increases the range of soil water content that will allow tilling.8

Aluminum Toxicity
The addition of calcium sulfate helps reduce aluminum toxicity by reacting with soluble aluminum in the soil and removing it from the soil solution.9 Aluminum toxicity attacks small plant roots, making them less able to take up the nutrients they need. Brent Rouppet cites the example of vineyard soils suffering aluminum toxicity “that showed up as a phosphorous deficiency, because the plants could not take up phosphorous.” Eliminating aluminum toxicity make plants healthier and crops more productive.