On occasion I receive questions on the addition of Gypsum (calcium sulfate) to soils. Recommendations made to add gypsum should only be done after conducting the proper tests and determining the addition of gypsum would be beneficial.

The amount of gypsum to add to a soil should be based on the level of sodium, calcium and magnesium in the soil. Soils with an excess of sodium ions, compared to calcium and magnesium ions, remain in a dispersed condition, almost impermeable to rain or irrigation water. A 'dispersed' soil is extremely sticky when wet, tends to crust, and becomes very hard and cloddy when dry. Water infiltration is usually severely restricted. Dispersion caused by sodium may result in poor physical soil conditions and water and air do not readily move through the soil. High levels of sodium may also be toxic to plant cells.

Elevated concentrations of sodium ions create a plant growth hazard which is measured by one of two methods. The more common method, the Sodium Adsorption Ratio (SAR), is the proportion of sodium (Na) ions compared to the concentration of calcium (Ca) plus magnesium (Mg). The second method of estimating the sodium hazard is called the exchangeable sodium percentage (ESP).

ESP refers to the concentration of sodium ions on cation exchange (CEC) sites. This analysis depends on an accurate measurement of the CEC, a pH dependent measurement. If the CEC value is determined by the testing laboratory using a different pH than that existing in the field from which the soil was collected, CEC could be incorrect. In addition, in salt-affected soils, cations not associated with the CEC may be dissolved, thus resulting in a higher CEC than actually exists. (Carrow & Duncan, page 20).

When the SAR rises above a certain level, serious soil problems occur and plants have difficulty absorbing water. An SAR "value of 15 or greater indicates an excess of sodium will be adsorbed by the soil clay particles. Excess sodium can cause soil to be hard and cloddy when dry, to crust badly, and to take water very slowly." Extracted from: Soil Test Explanation. December, 1999. Clangor, P.M., and Fillet, R.C. Colorado State Cooperative Extension Publication No. 0.502. Carrow and Duncan report Na⁺ can produce adverse effects on soil structure even when the SAR is near 5 due in part to type and content of clay. Montmorillonite, vermiculite, illite and mica-derived clays are more sensitive to Na⁺ than other clays.

When the SAR test indicates an excess of sodium, the addition of gypsum to free the sodium and allow it to be leached from the soil, may be necessary. Gypsum should only be applied if a soil test indicates a deficiency. Many Western Colorado soils have adequate gypsum to correct a sodium problem; the soil needs only to be tilled and the salts leached with a good quality water to remove sodium from the soil.

A number of computations are necessary to determine the SAR of a soil and whether the addition of gypsum is appropriate. The following example is based on a soil test report provided by an out-of-state laboratory. In this instance, the lab recommended the addition of gypsum to correct a high sodium ("alkali") problem.

The lab reported the following:

• 100 pounds/A of sodium (Na)
• 5000 pounds/A of calcium (Ca)
• 1500 pounds/A of magnesium (Mg)

These reading are best converted to parts per million (ppm). An acre of soil (43,560 square feet/4047 square meters), 6 inches deep (15 cm), weighs 2,000,000 pounds (907200 kg). An acre of soil 6 inches deep contains 21,780 cubic feet of soil (616.74 cubic meters). The pounds of nutrient reported per acre therefore must be divided by 2 to produce the ppm of that nutrients.

• 100 pounds/Acre of Na divided by 2 = 50 ppm of Sodium
• 5000 pounds/A of Ca divided by 2 = 2500 ppm of Calcium
• 1500 pounds/A of Mg divided by 2= 750 ppm of Magnesium

The next step converts ppm to meq. Milliequivalent (meq) is determined by dividing the ppm by the equivalent weight of the element. Equivalent weight is determined by dividing the molecular weight by the valence.

• The molecular weight of sodium = 23; the valence of this element = 1; thus the equivalent weight of sodium = 23/1 = 23
• The molecular weight of calcium = 40; the valence of this element = 2; thus the equivalent weight of calcium = 40/2 = 20
• The molecular weight of magnesium = 24; the valence of this element = 2; thus the equivalent weight of magnesium = 24/2 = 12

The milliequivalent weight of Na, Ca, and Mg in this example are:

• 50 ppm of Sodium divided by 23 = 2.17
• 2500 ppm of Calcium divided by 20 = 125
• 750 ppm of Magnesium divided by 12 = 62.5

The next step in this process involves the determination of the SAR of the soil. The SAR of a soil is defined as the milliequivalent weight of Sodium divided by the square root of the milliequivalent weight of Calcium + the milliequivalent weight of Magnesium divided by 2. Thus the sodium absorption ratio (SAR) is calculated as follows:

To determine the SAR of the example in this paper, the following steps are necessary:

1. 
   
2. 125 + 62.5 (Ca + Mg)/2 = 93.75
3. The square root of 93.75 = 9.68 & 2.17 divided by 9.68 = 0.22

Thus the SAR = 0.22

Based on the SAR, the addition of gypsum for this soil is not necessary. Not all soils laboratories provide quality results. In this case, the laboratory was in error.

If the SAR was determined to be such that plant damage would result, the soils laboratory must determine the amount of gypsum present in the soil prior to recommending the addition of gypsum. In this instance, no test for gypsum was conducted.

The water analysis you receive from an analytical laboratory should provide the Adjusted SAR in addition to the SAR calculated as discussed above. The Adjusted SAR takes into account the carbonate and bicarbonate ions in the water. These ions cause the Ca and Mg ions to precipitate out of water resulting in a higher sodicity. The Adjusted SAR should be used when determining if the water is appropriate for irrigation purposes.

The Adjusted SAR of irrigation water should be less than 10, especially if young plants are to be grown (Davidson, et al.). Table 14 on page 140 of the Ball Red Book, indicates 50 ppm (mg/l) of sodium in irrigation water is too high for most crops.

The following conversion table does not permit the determination of the Adjusted SAR. The Adjusted SAR needs to be provided by the analytical laboratory.

The following is a quick and easy JavaScript conversion table to determine the SAR of a soil.

Insert the pounds per acre or ppm of sodium, calcium and magnesium as reported on the soil test, and click submit. The computations given above automatically will be done for you, and the SAR value will be given.

If the SAR indicates excessive sodium, a follow-up test by a soils laboratory to determine the quantity of gypsum already present in the soil as well as the amount of gypsum [calcium chloride (CaCl₂), magnesium chloride (MgCl₂), or phosphogypsum] necessary to alleviate the sodium problem must be conducted. Phosphogypsum is a by-product of the reaction between sulfuric acid and phosphate rock in the manufacture of phosphoric acid. This compound consists mostly of calcium sulfate.

Thanks to Dan Champion, Former Extension Irrigation Agent, Colorado River Salinity Control Project, Grand Junction, for his advise and review of this paper.

References used:

Balba, A.M. 1995. Management of Problem Soils in Arid Ecosystems. Lewis Publishers, Boca Raton, FL.
Ball, V. 1998. Ball RedBook, 16th Edition. Ball Publishing Batavia, Il.
Carrow, R.N., and Duncan, R.R. 1998. Salt-affected Turfgrass Sites: Assessment and Management. Ann Arbor Press, Chelsea, MI.
Davidson, H., Mecklenburg, R. & Peterson, C. 2000. Nursery Management Administration and Culture. Prentice hall, Upper Saddle River, NJ. page 68.
Donahue, R.L., R.C., R.R.C.and Tulloch, R.W. 1990. Our Soils and Their Management. Interstate Publishers, Inc. Danville, Ill.
Munshower, F.F. 1994. Practical Handbook of Disturbed Land Revegetation. Lewis Publishers, Boca Raton, FL.
Richards, L.A. 1954. Diagnosis and Improvement of Saline and Alkali Soils. Agriculture Handbook No. 60. USDA, Washington, DC.

Soil Test Interpretations - New Mexico State University
Salt-affected Soils - Colorado State University Cooperative Extension

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