Gypsum Magic – Part 2

We continue to receive questions on gypsum and what affect, positive or negative, gypsum will have on soil properties, crop yields etc. To answer some of the questions we are receiving on the affect gypsum will have on soil pH, soluble salt level etc., we recently conducted a laboratory experiment. We used two very productive soils from the Northwood, ND area. We added various amounts of gypsum to the soil. The soil was kept moist and at room temperature for 45 days so that chemical reactions would continue at a fast rate. All treatments were done in triplicate. The data from this project is shown in the table below.

Many growers have been told that applying gypsum will lower soil pH. It is very obvious from the tables, that adding gypsum has little or no affect on soil pH (even at 36000 lb/a). We did not expect gypsum to change the soil pH because it is a neutral salt. The only situation where you would expect an application of gypsum to lower the soil pH would be in a sodium affected soil where the pH is 8.6 or higher before the gypsum is added.

In many situations, growers have been told that applying gypsum at a rate of 300 lb/a for several years in a row is a good practice. In the table below, you can see what happened to the salt level in the soil as the gypsum rate increased. Even the 300 lb/a rate of gypsum increased the salt level slightly. In a poorly drained soil, where very little water is moved through the soil profile, no salts are moved out of the soil profile. If you apply gypsum on an annual basis, the salt level will continue to increase and yields will decrease due to a higher salt level in the soil (caused by the gypsum).

In our next newsletter, we will explain how applying gypsum can change the ratio and percentages of nutrients on your soil test report, but does not increase crop yield. If you can’t wait until then, please give one of our technical staff a call!

SOIL 1 SOIL 2
Gypsum Rate
CaSO4
pH Salt Level Gypsum Rate
CaSO4
pH Salt Level
Check 7.50 .24 Check 7.77 .81
300 lb/a 7.53 .32 300 lb/a 7.80 .83
6000 lb/a 7.30 .92 6000 lb/a 7.70 1.09
18000 lb/a 7.30 1.17 18000 lb/a 7.60 1.39
36000 lb/a 7.37 1.31 36000 lb/a 7.73 1.48