Recently, there have been numerous claims about the benefits of gypsum applied to the soils of Minnesota.  According to the promotional material, gypsum will: 1) improve production on high pH soils by lowering soil pH, 2) increase the ratio of calcium to magnesium, 3) reduce the severity of iron chlorosis, and 4) improve soil tilth and permeability.  There is an assumption that yields will increase if all of these changes occur. 

Are these claims accurate?  Are they supported by field data? Have crop producers been missing something that's really important if they don't use gypsum?

It's relatively easy to predict the impact of gypsum on crop production in Minnesota if we have an understanding of some fundamentals of chemistry in soil.  Some of those fundamental facts are outlined in the paragraphs that follow.

Gypsum is calcium sulfate.  In the natural state without refinement, there is some water attached to it.  The chemical shorthand version is written as Ca SO ₄ ^. 2H₂O.  When added to soil, it dissolves and breaks apart (disassociates) into the calcium (Ca⁺⁺) and sulfate (SO₄ ⁼ ) ions. There is no generation of excess hydrogen (H ⁺) ions.  At the same time, there is no neutralization of H ⁺ ions when gypsum is dissolved in soils.  Soil pH is a measure of hydrogen ions in soil.  If hydrogen is neither added to nor subtracted from the soil system, there should be no change in soil pH.  Gypsum is a neutral salt and a change in soil pH should not be expected when this product is added to most Minnesota soils.

When dissolved in soils, the calcium is attracted to the negative electrical changes (see MNCN07 for a diagrammatic representation of these changes). The sulfate, like nitrate-nitrogen, is free to move with the soil water and can be leached if soils are sandy.

It is possible that some of the calcium from gypsum will replace some magnesium (Mg ⁺⁺) from the exchange sites.  Is this exchange helpful or harmful to crop growth?   If you believe that it is helpful, you have to believe that magnesium is harmful or that the calcium/magnesium ratio is important.  Research in Minnesota has shown that the opposite is true.  In Minnesota the amount of magnesium in soils does not have any impact on crop production and the calcium/magnesium ratio is not important.

The application of gypsum has been evaluated recently in South Dakota.  Two rates of gypsum (300, 1500 lb./acre) were broadcast and incorporated before planting.  The measured yields are summarized in the following table.

Influence of added gypsum on crop yields.  South Dakota, 2002.

In evaluating this yield information, it's important to pay attention to the statistical analysis.  The NS means that the treatment used (rate of gypsum applied)  had no effect on yield.  In other words, the differences in yields shown in the table are the result of random variability of yield at the experimental site rather than the amount of gypsum applied.  The conclusion from the research conducted in South Dakota is quite clear.  Application of gypsum, regardless of rate, had no effect on crop yield.

Gypsum has some value ---- in limited situations.  It is a good source of sulfur if soils are sandy and there is a need for sulfur in a fertilizer program.

Gypsum has been shown to be of value in reclaiming sodic soils (soils that have high amounts of sodium) attracted to the exchange sites.  In this reclaimation process, the calcium from gypsum replaces the sodium which can be flushed from the soil system with large amounts of water.  The amount of gypsum needed to accomplish this reclaimation ranges from 1 to 3 tons per acre.  This is 6 to 20 times the rate recommended for use of gypsum in Minnesota.

There are very few acres of sodic soils in Minnesota.  So, this limited acreage does not justify the use of gypsum in this state.

So, rather than be awed by the slick sales claims, it's important to step back and ask, "what does our understanding of the fundamentals of chemistry tell us about the use of this product?"  Gypsum is not a knight in shining armor.  Except for use as a source of sulfur for crop production on sandy soils, this product has no real benefit in Minnesota.