Should there be an efficiency factor for granulized gypsum when compared to PRO CAL 40? Over the years we have frequently gotten this question. One reason is when comparing granulized lime and agricultural lime it has been common to use an efficiency factor. All liming materials are based on an ECCE (Effective Calcium Carbonate Equivalent). This value is expressed in either % of a ton or pounds and is based on particle size and purity. The more pure and the smaller the particle size the higher the ECCE value and the faster response you will see. The efficiency factor for granulized lime is assuming that it has a higher percent in the smallest particle size. Although most agricultural lime contains some in this smallest particle size also, these small particles could become air borne during application and not land on your field.
What about gypsum? There is no such value as an ECCE, but in effect, the same holds true for gypsum. The more pure the source and the smaller particle size gives you the most value and the fastest reactivity in the soil. Therefore, a laboratory analysis of gypsum evaluates these two factors also for effectiveness. There is some mined gypsum on the market that is quite course.
So getting back to our original question: Should there be any efficiency factor for granulized gypsum products when compared to PRO CAL 40? The short answer is “no”. PRO CAL 40 is one of the most pure forms of gypsum you can buy and the particle size is very small (<160 microns) so granulized gypsum has no advantage over PRO CAL 40 other than it can be spread at lower rates easier and can be mixed with other granulized fertilizer materials. Since PRO CAL 40 is less than $20 per ton and granulized costs more than $150 per ton, PRO CAL 40 is by far, more cost effective as a soil amendment, hands down!! To improve soil conditions, higher rates of gypsum (>1000 pounds/acre) need to be applied, making PRO CAL 40 the product of choice.