The other significant difference between anhydrite gypsum and dihydrate gypsum is, in some cases, cost. In California, that difference may depend largely on location. The real cost of applying calcium sulfate is a combination of the material price, the shipping cost, and the farmer’s own spreading costs.

Material Cost
Of these three, material cost is usually only about one third. To calculate the true material cost of a calcium sulfate product, you must first determine how much of the specific product you need to satisfy the gypsum equivalence required by the crop and soil conditions. It is important to check the purity, or gypsum equivalence, of the specific product you are purchasing. Although products of high purity such as 95% are available, there are others, also sold as “gypsum,” that may actually be as low as 50% gypsum.

A product that contains anhydrite gypsum will probably have a higher gypsum equivalence than a dihydrate gypsum product, requiring less of the anhydrite gypsum product to meet the soil’s needs.

Shipping Cost
Since material is only about one third of the total application cost, shipping and spreading have a very big impact. Again, the proportion of calcium sulfate in the product is important. With dihydrate gypsum, 21% of the shipping cost pays to transport water that has no nutrient value. 100 lbs. of anhydrite gypsum delivers the same amount of calcium sulfate as 126 lbs. of dihydrate gypsum, so dihydrate costs 26% more to ship than anhydrite gypsum.

Spreading Cost
Comparing the two at a spread cost of $70/t, dihydrate has an effective cost of $88.53 per unit of calcium sulfate. Moreover, anhydrite gypsum’s slower dissolution rate and longer persistence in the soil may allow fewer applications, further lowering annual spreading costs. Anhydrite gypsum is the more cost-effective way to apply calcium sulfate, both in terms of immediate application cost and long-term application frequency.

The Location Factor
In California, location has a huge effect on shipping cost. The geography of the state means that calcium sulfate must cross the mountains from one of the two main mining sources: anhydrite gypsum from northern Nevada, or dihydrate gypsum from southern Utah, southern Nevada, and southern California. A grower’s specific location in California often determines which source is more affordable in terms of shipping, and therefore, whether anhydrite gypsum or dihydrate gypsum is more cost effective.

If, hypothetically, you were located an equal distance from the two main mining sources, and the cost of shipping were the same from either source, then anhydrite gypsum would be a better deal because, ton for ton, it has more ‘active ingredient’ (calcium sulfate) than dihydrate gypsum, and it lasts longer, so you may reduce long-term spreading costs.

However, in Southern California, dihydrate gypsum mined in southern Utah-Nevada-California may cost less for combined materials and shipping. Further north, anhydrite gypsum mined in Carson City, NV is a much better deal. The specific economics have to be worked out on a case-by-case basis, keeping in mind the calcium sulfate requirement (gypsum equivalence), shipping costs, and spreading costs that apply locally.