Gypsum: an old product with a new use

Gypsum, scientifically known as calcium sulfate (CaSO4), has been a staple in various industries for centuries. The anhydrite form of refined gypsum contains 29.4% calcium (Ca) and 23.5% sulfur (S), while the more commonly used gypsum with water in its molecular structure (CaSO4·2H2O) consists of approximately 23.3% calcium and 18.5% sulfur, often referred to as plaster of Paris. In agricultural contexts, gypsum fertilizers may contain additional impurities, leading to grades that typically offer around 22% calcium and 17% sulfur. Its low solubility is vital, as it prevents immediate dissolution upon contact with water, allowing the material to soften without losing integrity. Gypsum is a neutral salt resulting from a strong acid and a strong base, thus maintaining soil pH levels and not affecting its acidity. The dissolution in water or soil is represented by the reaction: CaSO4·2H2O = Ca2+ + SO42- + 2H2O, where calcium ions (Ca2+) and sulfate ions (SO42-) are released without altering hydrogen ions (H+), which means gypsum does not perform as a liming agent or acidifier.

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Gypsum Utilization as a Fertilizer

As a fertilizer, gypsum plays a significant role in supplying crops with accessible forms of calcium and sulfur. When soil deficiencies of these elements are evident, the application of gypsum can lead to enhanced crop yields. However, in regions like Iowa, the situation is more complicated, as research indicates that calcium deficiencies are uncommon. Traditional methods, such as the application of limestone (CaCO3), usually address low calcium levels effectively. Issues regarding soil acidity often arise before any calcium deficiencies, which illustrates that liming concurrently manages calcium needs as well. Data indicated in Table 1 demonstrates that exchangeable calcium levels in a variety of Iowa soils are generally high.

When looking at sulfur levels, studies conducted over 35 years across numerous Iowa field trials have shown minimal and isolated positive responses in corn or soybean yields with sulfur fertilization (e.g., gypsum). Table 2 reflects recent findings on sulfur trials involving corn and soybeans, confirming no significant yield increases from gypsum or calcium applications in Iowa. Hence, if the levels of calcium and sulfur in the soil are satisfactory, additional gypsum application for fertilization is unnecessary.

Gypsum's Role as a Soil Amendment

The integrity of soil structure is influenced significantly by exchangeable cations, which are positively charged ions. Multivalent cations such as Ca2+, Mg2+, Zn2+, and Al3+ effectively bond soil particles due to electrostatic interactions with negatively charged sites found in soil clay and organic matter. This interaction contrasts with monovalent cations (like sodium Na+), which, due to their single positive charge, can disrupt soil structure when excess amounts occupy exchangeable sites. In many areas of Iowa, sodium excess is not an issue; in saline-sodic regions, however, gypsum can play a pivotal role in reclamation. By introducing substantial quantities of calcium ions, gypsum displaces sodium ions from exchange sites. This process is beneficial, particularly when followed by thorough water flushing to wash away both salts and sodium, highlighting why gypsum is favored over limestone due to its increased solubility. However, care must be taken in soils with low permeability when attempting this method, and in soils with excessive soluble salt, gypsum can exacerbate the problem.

Moreover, soil health benefits significantly from organic material, which aids in compacting soil particles. High organic matter content in Iowa soils contributes to the overall structure just as much as multivalent cations do. Surface soil structure can be negatively impacted by the physical force of raindrops, with surface residue serving as a natural barrier and economically beneficial means of preserving water infiltration. Enhancing crop residue and limiting tillage proves more advantageous than gypsum application for soil improvement.

To summarize, gypsum is an effective source of calcium and sulfur, but its necessity in Iowa soils is mitigated due to the naturally adequate supply of these nutrients, where the application of excess fertilizers is not warranted.

Table 1. Exchangeable calcium and magnesium of several Iowa soils.

Soil

Table 2. Corn and soybean yield response to gypsum and elemental S application, average across six sites in Iowa.

This article originally appeared on pages 46-47 of the IC-490 (5) -- April 21, issue.

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The Advantages of Gypsum in Agriculture

According to Dr. Warren Dick, a retired soil scientist from The Ohio State University, gypsum can contribute significantly to water quality improvements when properly utilized in agricultural soils. Beyond its primary function as a source of calcium and sulfur, gypsum aids in controlling phosphorus runoff, which is critical for protecting water bodies from nutrient loading. Gypsum, a mineral composed of calcium sulfate dihydrate, has vast applications in various sectors. It can be sourced from geological deposits, phosphogypsum via phosphoric acid production, as well as through recycling processes from industrial applications such as wallboard or flue gas desulfurization.

The benefits of using gypsum cannot be understated. Over the years, agricultural practices relying on rainfall for sulfur inputs have diminished significantly, necessitating supplementation to prevent deficiencies. Monitoring studies in Ohio showed a decline in sulfur deposition from approximately 31 lbs per acre to around 16 lbs per acre in recent years. This trend necessitates a renewed focus on including gypsum to meet crop demands. Calcium and sulfur are crucial for crop health, impacting everything from plant strength to yield. Sufficient sulfur levels in soil are vital to prevent deficiencies leading to subpar growth.

Gypsum's ability to promote soil aggregate formation enhances soil structure, which is beneficial for root development, water retention, and overall crop health. The introduction of calcium assists in displacing sodium ions that might otherwise hinder soil stability while restoring its salinity balance. The application of gypsum can also assist in greater water infiltration, reduce erosion risks by binding soil particles, and significantly improve nutrient-holding capacity, directly affecting crop productivity.

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