2021 Coffey Road, Columbus, Ohio 43210

ANR-20-05

Katerina Dontsova

Post-doctoral Researcher

Yong Bok Lee

Post-doctoral Researcher

Brian K. Slater

State Extension Specialist

Jerry M. Bigham

Professor

Gypsum is hydrated calcium sulfate (CaSO4�2H2O), and is often

marketed as a soil �conditioner� for improving soil �tilth.�

Compared to most other calcium-rich soil amendments, such as

limestone, gypsum is relatively soluble in water, dissolving up to 2

g per liter. The solubility of gypsum, when either incorporated or

surface applied, permits a quick release of calcium (Ca2+) and

sulfate (SO4

2-) ions into the soil solution. The supply of dissolved

salt and Ca2+ ions, in particular, may reduce soil crusting (Figure

1) and otherwise benefit soil structure. The aggregation of clay

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Figure 1. Inhibition of soybean

seedling emergence by severe surface

crusting.

particles that help to form and stabilize soil structure is clearly

enhanced by the presence of calcium on clay exchange sites.

It is important to note that pure gypsum is not a liming agent, and

it cannot be used to raise soil pH. However, gypsum has the

potential to relieve aluminum (Al3+) toxicity in acid soils and to supply calcium and sulfur (S) for plant

nutrition. Some natural and synthetic sources of gypsum also contain other chemical compounds, such as

calcium carbonate (agricultural lime), calcium oxide (burned lime), or calcium hydroxide (hydrated lime).

These materials do have a liming effect when applied to soil, but are not discussed further in this publication.

The objectives of this fact sheet are to review possible sources of gypsum for agricultural use in Ohio, and to

report results from chemical and mineral analyses of representative samples.

There are several possible sources of gypsum currently available for agricultural use in Ohio. These include:

Natural gypsum mined from geologic deposits

Synthetic gypsum produced as a by-product of electricity generation

Recycled casting gypsum from various manufacturing processes

Recycled drywall gypsum

Natural Gypsum

Gypsum has been obtained by mining geologic deposits in northern Ohio, Michigan, and other locations for

many years. Mineral purity of natural samples varies with the local geology and the mining technology

employed at the site. Samples obtained from northern Ohio mines near Port Clinton were predominantly

gypsum, but also contained dolomite [CaMg(CO3)2] and quartz (SiO2) (Table 1). Small quantities of quartz

have no effect on soil properties, whereas dolomite is a liming agent and is a good source of magnesium (Mg).

Table 1. Mineralogical composition of gypsum samples.

Source Minerals*

 present

Synthetic gypsum1 gypsum, quartz

Natural gypsum2 gypsum, quartz, dolomite

Cast gypsum3 gypsum, quartz, anhydrite

Drywall gypsum4 gypsum, quartz, portlandite, calcite

1 Samples obtained from the W.H. Zimmer Station in Moscow, OH,

owned by Cinergy Corporation

2 Samples obtained from the Kwest Group at Port Clinton, OH

3 Samples obtained from Mansfield Plumbing Products, LLC of

Mansfield, OH

4 Samples obtained from Transfer Services, LLC of Columbus, OH

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* gypsum = CaSO4�2H2O, quartz = SiO2, dolomite = CaMg(CO3)2,

anhydrite = CaSO4, portlandite = Ca(OH)2, calcite = CaCO3

Synthetic Gypsum

Synthetic gypsum is produced at some coal-fired power plants as a by-product of pollution control measures.

The Clean Air Act Amendments of 1990 mandate that electrical utilities install systems for removal

(�scrubbing�) of sulfur dioxide (SO2) from flue gases that are generated during the burning of coal. The

resulting materials are termed flue gas desulfurization (FGD) by-products. Depending on the process, these

by-products can have a variety of mineral constituents. The forced oxidation procedure used at the W.H.

Zimmer Station in Moscow, Ohio, results in a high purity product (Table 1), and the material is marketed as

synthetic gypsum.

In the process used at Zimmer Station, the flue gases are first exposed to a slurry of hydrated lime, and

calcium sulfite (CaSO3�0.5H2O) is initially formed by capture of SO2 (Figure 2). The calcium sulfite is then

oxidized to form gypsum. During the oxidation process, washing of the by-product with water removes

undesirable chemical contaminants such as boron (B) and mercury (Hg). The final step of the process

involves partial removal of water by a combination of centrifugation and vacuum filtration.

Figure 2. The scrubbing process and gypsum production at Zimmer Station (Figure courtesy of CINERGY

Corp.).

The final product is available for drywall manufacturing or for agricultural applications. To be acceptable for

drywall manufacture, the material must have less than 600 parts per million (ppm) of total dissolved solids in

the pore water, and a water content of less than 15% by weight. Material that does not meet these criteria is

marketed as agricultural gypsum, and total dissolved solids is the major criterion for diverting material to

agricultural uses. Power plant gypsum in Ohio is permitted as a fertilizer material through the Ohio

Environmental Protection Agency, and is monitored by the Ohio Department of Agriculture for Ca and S

contents.

Cast gypsum

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Figure 3. Gypsum stockpiled in the

field for post-harvest application.

The manufacture of some products, such as plumbing fixtures, requires gypsum casts or molds. The used

molds can potentially be ground and recycled for other uses. The recycled material analyzed for this report

contained mostly gypsum with a small admixture of the mineral anhydrite (Table 1), probably due to the

dehydration of gypsum during the casting process. Anhydrite (CaSO4) is calcium sulfate without water of

hydration, and is usually similar in behavior to gypsum when applied to soils.

Drywall gypsum

Drywall consists of gypsum with a thin paper backing. About 30 billion square feet of gypsum wallboard are

manufactured each year in North America, and a considerable quantity is discarded during the construction of

homes, offices, and other structures. Up to 25% of the waste produced at new construction sites is drywall

material. The recycled drywall analyzed in this study was obtained entirely from new construction projects

and is regularly monitored by the Ohio Department of Agriculture as a fertilizer. The samples contained

quartz, calcium hydroxide [Ca(OH)2] or portlandite, and calcium carbonate (CaCO3) or calcite (Table 1).

Demolition drywall is another possible source of gypsum but should probably be avoided for land application

because of potential chemical contamination from paint or other wall coverings.

The cost and ease of land application are heavily dependent on

factors such as water content, particle size, and purity of the

gypsum product. The samples collected for this study included

products taken directly from the sources as well as materials

stockpiled in the field in preparation for land application (Figure

3). As a result, the water contents varied considerably (Table 2).

Water content was consistently small (< 1%) for the mined

gypsum and the recycled cast material, neither of which were

exposed to rainfall. Water contents of the synthetic FGD-gypsum

were below 10%, even though the manufacturer reports that the

product has an average water content of 12%. Drywall gypsum

contained 1% water at the recycling facility, whereas contents

were as high as 19% after storage in the field.

The waste drywall material was crushed and sorted through a

0.5-inch screen at the recycling center. For agricultural uses, this material is best spread with a wet lime

spreader. Fertilizer spreaders do not work well because the feeder holes are not sufficiently large to pass

larger particles. The synthetic FGD-gypsum, by contrast, generally has excellent spreading characteristics.

Most products sampled for this study were high purity and yielded < 3% water insoluble residues (Table 2).

The mined gypsum, on the other hand, contained up to 19% undissolved residue after 3 days equilibration in

acidified water. Much of the residue was dolomite, which did not dissolve due to large particle size and

relatively low solubility.

Table 2. Physical properties and price (as of 12/2004) of gypsum.

Material Water content1

 % Particle size Price $/ton Insoluble residue2 %

Synthetic gypsum 5.55 (3.04)3 120 μm 7.00 0.4 (0.2)

Natural gypsum 0.38 (0.48) NA 12.75 12.9 (8.1)

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Cast gypsum4 0.15 (0.21) NA NA 0.2

Drywall gypsum 10.1 (12.8) < 0.5 inch 11.00 2.2 (0.3)

1 Dried overnight at 60 degrees Celsius.

2 Following dissolution for three days at pH < 3.

3 Standard deviation included in parentheses.

4 Material is not yet available for sale for agricultural application.

NA = not available

All the materials tested would be excellent sources of Ca and S for plant nutrition (Table 3). Because of its

dolomite content, the mined gypsum is also a source of Mg.

Boron is a plant micronutrient and some crops have a relatively high demand for B; however, others can be

sensitive to elevated levels. Unwashed FGD by-products can have levels of B sufficiently high to result in

toxicity to corn. Washing of the by-product in the process of gypsum formation lowers B contents to safe

levels if recommended application rates are used.

Table 3. Selected macro- and micronutrient1 concentrations in the gypsum samples.

Measure Units

Museum

specimen2

Synthetic

gypsum

Natural

gypsum

Cast

gypsum

Drywall

gypsum

Ideal

analysis3

Calcium % 22.6 23.0 (0.0)4 19.1 (2.2) 22.4 (0.0) 21.9 (0.2) 23.3

Magnesium % 0.01 0.03 (0.01) 1.35 (0.30) 0.05

(0.00) 0.22 (0.01) 

Sulfur % 18.6 18.7 (0.1) 15.1 (1.2) 19.3 (0.2) 18.1 (0.3) 18.6

Boron ppm < 13.1 26.7 (8.7) 9.4 (0.9) 0.4 (0.4) 7.3 (4.5) 

Iron ppm < 1 264 (129) 1045 (148) 44 (7) 547 (92) 

Manganese ppm 0.1 5.5 (2.3) 14.6 (2.9) 9.1 (0.0) 9.4 (1.6) 

Phosphorus ppm 3.8 16.7 (9.4) 30.6 (7.6) 7.5 (0.3) 51.6 (3.5) 

1 Micronutrient data obtained by EPA method 3050 (USEPA, 1996).

2 The museum specimen is included as a pure sample of gypsum.

3 Calculated content in a 100% pure product.

4 Standard deviation included in parentheses.

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Chemical analyses of the gypsum materials collected in this study showed that trace metals were present at

low concentrations in all samples (Table 4). As a point of reference, the metal contents were much lower than

concentration limits identified by government regulations for land application of excellent quality biosolids

(USEPA, 1993), and calculated metal loadings with application rates of 2.23 ton ac-1yr-1 (5 Mg ha-1yr-1)

were 100 to 10,000x lower than annual loading rates permitted by these same regulations (see Part

503�Standards for the Use or Disposal of Sewage Sludge; 503.13, Tables 1�4, for details). Gypsum from any

of the sources examined could thus be applied without restriction for trace metal loading; however, samples

from a given source should always be tested prior to application. There is also no demonstrated benefit of

application rates greater than 2 ton ac-1yr-1 for agronomic or horticultural crop production in Ohio, and

biennial applications are probably adequate. Greater quantities could result in seedling damage to salt

intolerant species, especially if applied near the time of planting. Autumn applications are recommended.

Table 4. Trace metal content1 of gypsum from different sources compared with U.S. EPA Part 503 pollutant

concentration limits for excellent quality biosolids.

Pollutant (ppm = mg

kg-1)

Museum

specimen

Synthetic

gypsum

Natural

gypsum

Cast

gypsum

Drywall

gypsum

Part 503

Table 32

Arsenic < 0.52 0.56 (0.05)3 < 0.52 < 0.52 0.98 (0.11) 41

Cadmium < 0.48 < 0.48 < 0.48 < 0.48 < 0.48 39

Chromium 0.01 1.30 (0.85) 1.38 (0.32) 0.07

(0.00) 1.09 (0.09) 1200

Cobalt < 0.48 < 0.48 0.53 (0.04) < 0.48 < 0.48 NR4

Copper < 0.48 1.16 (0.66) 1.33 (0.30) 1.40

(0.21) 0.95 (0.14) 1500

Lead < 0.48 0.80 (.30) 2.92 (0.30) 0.57

(0.08) 0.70 (0.02) 300

Mercury < 0.26? < 0.26 < 0.26 < 0.26 < 0.26 17

Molybdenum < 0.24 0.51 (0.26) 1.28 (0.04) < 0.24 < 0.24 -----5

Nickel < 0.24 0.73 (0.18) 1.42 (0.23) < 0.24 0.83 (0.12) 420

Selenium < 1.45 5.51 (3.47) < 1.45 < 1.45 1.85 (0.04) 36

Zinc < 0.24 3.88 (2.78) 0.91 (0.49) < 0.24 3.08 (0.45) 2800

1 Data obtained by EPA method 3050 (USEPA, 1996).

2 Part 503�Standards for the Use or Disposal of Sewage Sludge; 503.13, Table 3. (USEPA, 1993).

3 Standard deviation included in parentheses.

4 NR = not regulated.

5 Ceiling concentration limit for molybdenum is 75 ppm; 503.13, Table 1. (USEPA, 1993).

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U.S. EPA. 1993. 40 CFR Part 503�Standards for the use and disposal of sewage sludge: Final rule. Federal

Register 58:9248�9415. Washington, DC.

U.S. EPA. 1996. Method 3050. Acid Digestion of Sediments, Sludges, Soils and Oils. SW-846. Washington,

DC.

This publication was produced through a cooperative effort between Ohio State University Extension and the

College of Food, Agricultural, and Environmental Sciences.

Click here for a PDF version of this Fact Sheet.

OSU Extension embraces human diversity and is committed to ensuring that all educational programs conducted by Ohio State University Extension are

available to clientele on a nondiscriminatory basis without regard to race, color, age, gender identity or expression, disability, religion, sexual orientation, national

origin, or veteran status.

Keith L. Smith, Associate Vice President for Agricultural Administration and Director, OSU Extension TDD No. 800-589-8292 (Ohio only) or 614-292-1868

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