Impacts of gypsum as a soil amendment on clayey soils

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Some producers have wondered about the potential benefits of gypsum applications to heavy-textured, clayey soils that have slow drainage. The potential benefits of using gypsum (calcium sulfate dihydrate) as a soil amendment on clayey soils include:

  • Enhanced soil aggregation
    • Improved infiltration
    • Reduced risk of runoff and erosion
  • Reduced aluminum toxicity in acidic soils
  • Source of soil calcium (Ca) and sulfur (S)

To test whether gypsum can provide any or all of these potential benefits, a one-year pilot study was recently conducted in Kansas. Similar experiments are underway in other states on clayey soils, and some unpublished data for one 2011 site location has been supplied by a Wisconsin researcher.  

Methods
A test was established on an Irwin silty clay loam soil in a producer’s field near Marion, Kansas in December 2010. The 2010 crop was conventionally tilled soybeans and the 2011 crop was no-till grain sorghum. The five treatments consisted of applying 0, 0.50, 2.15, 4.30, and 8.60 tons per acre of mined gypsum before planting.

Soil samples were collected prior to gypsum application and were analyzed for background cation concentrations (Table 1). Grain yield and grain nitrogen (N) and sulfur (SO4-S) content were determined. After harvest, soil samples were taken for wet-aggregate stability analysis and for soil fertility analyses, including: pH, potassium (K), Ca, Mg, sodium (Na) and cation exchange capacity (CEC). Additionally, soil hydraulic conductivity was measured.

Data was also obtained from a similar study conducted in Douglas Co., Wisconsin, near Lake Superior on a clay loam soil with 39% clay using flue gas desulfurization (FGD) gypsum applied at rates of 0, 0.5, 1.0, and 2.0 tons per acre in a tilled corn field. In that study, infiltration tests were conducted and samples were collected for wet aggregate stability and bulk density measurement approximately 12 weeks after gypsum application.

Results

Table 1. Soil test results prior to gypsum application (Kansas). 

pH

K

Ca

Mg

Na

CEC

Ca:Mg*

 

(ppm)

(ppm)

(ppm)

(ppm)

meq 100 g⁻¹

 

6.2

401

2526

1338

84

28

1.1

*The Ca:Mg ratio may look incorrect at first glance, but this ratio should be calculated after converting Ca and Mg concentrations to units of meq per 100 grams. To convert Ca ppm to Ca meq per 100 grams, divide by 200. To convert Mg ppm to Mg meq per 100 grams, divide by 120.

Sorghum yield and nutrient content. In the Kansas tests, no differences in sorghum yield and grain N and S content were observed among gypsum application rates (Table 2).

Table 2. Sorghum yield and nutrient content as impacted by gypsum application rate (Kansas). 

Gypsum Application

Sorghum Yield

Grain Nitrogen

Grain Sulfur

(tons per acre )

(bu per acre )

(% N)

(% SO4-S)

0.00

41.3 a

2.06 a

0.108 a

0.50

37.0 a

2.01 a

0.107 a

2.15

32.6 a

2.05 a

0.109 a

4.30

30.2 a

2.09 a

0.119 a

8.60

38.3 a

1.91 a

0.113 a

click image to zoomFigure 1. Effect of gypsum application on percentage of water-stable aggregates by size fraction (A) and MWD (B). In all of the figures below, treatments with different letters are significantly different at p ≤ 0.10. Wet-aggregate stability. Gypsum addition of 8.60 tons per acre  resulted in a greater percentage of large aggregates (>4.75 mm) relative to no gypsum application. Lesser percentages of small aggregates (<0.25 mm) were observed with gypsum application rates exceeding 0.50 tons per acre as compared to no gypsum addition (Fig. 1A). Likewise, gypsum application of 8.60 tons per acre increased mean weight diameter (MWD) of water-stable aggregates by 0.23 mm compared to no gypsum application (Fig. 1B).

Soil fertility. Unlike lime, gypsum does not impact soil pH. Any differences in soil pH observed in this study are most likely the result of within-field variation (Fig. 2A). Soil Ca increased by 883 ppm at the 4.30 tons per acre gypsum rate and by 2530 ppm at the 8.60 tons per acre application rate compared to no gypsum application (Fig. 2B). This result should be expected as gypsum can be composed of up to 23% Ca. On the other hand, soil Mg decreased by 143 ppm at the 4.30 tons per acre gypsum rate and by 389 ppm at the 8.60 tons per acre application rate compared to no gypsum addition (Fig. 2D). click image to zoomFigure 2. Impact of gypsum application rates on soil chemical properties at the Kansas site.

Infiltration. Gypsum application rate of 2.15 tons per acre resulted in lower hydraulic conductivity than the control (Fig. 3), however, this difference is most likely the result of inherent, within-field soil variability.

Figure 3. Soil hydraulic conductivity as impacted by gypsum application rate. Wisconsin Data. Addition of gypsum at this site appeared to improve soil physical properties by reducing bulk density and increasing infiltration and mean weight diameter of water stable aggregates; however, the effect was not found to be significant (data not shown). Additional tests were completed at other sites in watersheds draining to Lake Superior and Lake Michigan and those sites also exhibited no significant treatment effect on soil physical properties in the year of gypsum application. All sites had fine-textured soils that were relatively dense and had been in corn-wheat or corn-soybean rotations under tillage. These soils would be similar to those in Kansas but with a shorter growing season.

Conclusion
Although the highest rate of gypsum application in the Marion County, Kansas study (8.60 tons per acre) resulted in increased soil wet-aggregate stability, Ca concentration, and CEC the first year after application, no differences in sorghum grain yield or infiltration were observed. Additionally, gypsum application did not significantly alter soil physical properties in Wisconsin. However, there is the potential that some of the benefits might take longer than one growing season to be seen. To read more about gypsum application rates and gypsum as an amendment, you might be interested in a 2011 publication from Ohio State University by L. Chen and W.A. Dick: Gypsum as an Agricultural Amendment,  available at: http://ohioline.osu.edu/b945/b945.pdf 


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