Soil pH and Gypsum Peanut Notes No. 212 2019

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Question about Soil pH and Gypsum:

A grower called me this week and said he heard at a peanut meeting last winter that liming peanut land to pH 6.5 was a good move to mitigate pH lowering effects from gypsum application. I never thought gypsum lowered pH. Just wanted to get your take on it in case it was a meeting you presented.

Jordan Answer:

There is some debate about gypsum lowering pH, but my impression is that there is a transient reduction for the first few weeks after it is applied. Then, pH increases back to the original level. With that said, having a pH of 6.5 would more than compensate for the lowering of the pH by calcium sulfate. I don’t think it is a major issue if you start with pH at or above 6.0. Although we don’t know exactly why (but I think it could be an additional lowering of pH), when gypsum is applied to a soil with pH 6.0 there is a positive impact on yield. When gypsum is applied to soil with pH 5.5, yields are lower than the no-gypsum control. At pH 5.0, it does not matter (see Table 3-3 on page 25 of 2019 Peanut Information.)  Based on those data, I don’t think the grower needs to lime to get a pH of 6.5. I think at pH 6.0, you would get the normal positive response to gypsum. At pH 5.5, something negative is going on, and I would worry that if I had a pH of 5.6 to 5.8 that I might negate the positives provided by gypsum.

Supplemental Information, August 1, 2019

Jordan Question to David Hardy:

Have I been erroneously stating that gypsum causes a transient decrease in pH in the pegging zone for the first few weeks after application? Can’t seem to find it in the literature. Have assumed a reduction of about 0.3 units for 2 weeks.

David Hardy Response:

Gypsum may cause a transient change in pH when measured in water.

We now use a weak salt solution (0.01 M CaCl2) in pH measurement. This should help negate any transient pH depression caused by salts. Typically, salt depression on pH measure in water is about 0.6 units but can range from 0 to 1.0 pH units. We now report an “adjusted” water pH on our report. We add 0.6 pH units to the salt pH measured in our lab and report it as pH on the report. The lime recommendation is based on this adjusted “water” pH.

2017 Soil pH Salt Info

As leaching occurs, the salt effect will dissipate.