Gypsum, chemically known as calcium sulfate dihydrate, is a naturally occurring mineral used as a soil amendment to improve soil quality. It is distinct from agricultural lime (calcium carbonate) because it is a neutral salt, meaning it enhances soil conditions without significantly altering the soil’s pH level. Gypsum’s primary function is to deliver soluble calcium and sulfur, which initiate beneficial physical and chemical changes within the soil structure. These actions lead to a more hospitable environment for root growth and nutrient availability for plants.
How Gypsum Improves Physical Soil Structure
The most immediate benefit of gypsum application is the improvement of soil’s physical structure, particularly in heavy clay soils. This structural enhancement is driven by the calcium ion released as the mineral dissolves in water. The highly charged calcium acts as a bridge between fine clay particles, causing them to clump together in a process called flocculation.
This flocculation creates larger, stable aggregates, which re-establishes pore spaces previously clogged by dispersed clay. The formation of these aggregates dramatically improves water infiltration and drainage, preventing water from pooling on the surface. Improved porosity also increases soil aeration, allowing oxygen to reach roots and beneficial microorganisms more effectively.
The structural improvement also mitigates surface crusting, a common issue where fine particles seal the surface upon drying. By binding these fine clay particles into aggregates, gypsum helps maintain a more porous surface layer, essential for uniform seed emergence and reducing water runoff. Gypsum provides a chemical solution that creates a more resilient and stable soil structure over time, unlike mechanical tillage, which only temporarily breaks up compaction.
Gypsum’s Role in Soil Chemistry and Nutrient Supply
Gypsum delivers two nutrients, calcium and sulfur, both necessary for healthy plant development. Calcium is integral to the formation of strong plant cell walls and is relatively immobile once inside the plant, requiring a constant external supply. The sulfate component provides sulfur, which plants need for the synthesis of proteins, enzymes, and vitamins.
A primary chemical application of gypsum is the remediation of sodic soils, characterized by an excess of sodium ions bound to the clay particles. Sodium acts as a dispersing agent, causing clay particles to break apart and clog soil pores. The soluble calcium in gypsum readily displaces the sodium ions from the clay exchange sites due to its stronger positive charge.
Once displaced, the sodium ions combine with the sulfate ions to form sodium sulfate, a highly soluble salt. This salt can then be leached out of the root zone with drainage water. This chemical exchange restores the beneficial aggregated structure of the soil. This is a specific chemical fix for sodic soils, a problem that cannot be addressed by simply adding lime.
Gypsum also mitigates aluminum toxicity, a problem prevalent in highly acidic subsoils. In these conditions, soluble aluminum restricts root growth and nutrient uptake. The high solubility of gypsum allows the calcium and sulfate to move quickly into the subsoil, a depth that lime cannot easily reach.
The sulfate ions form less toxic complexes with the aluminum, such as aluminum sulfate, effectively removing the aluminum from the soil solution. The calcium ions also displace aluminum from the cation exchange sites, allowing it to be leached below the root zone. This process, which does not require a change in soil pH, permits deeper, healthier root growth, allowing plants better access to water and nutrients.
Identifying When Gypsum Is Needed
Determining the need for gypsum involves both field observation and definitive soil testing. Visual indicators suggesting a need include standing water, poor water penetration, or a hard surface crust after the soil dries. Soils that are sticky after a light rain or have a cloudy appearance when water ponds are often experiencing clay dispersion and are likely to benefit from gypsum.
A simple, on-farm dispersion test can provide a quick assessment by placing a dry soil aggregate in a jar of water. If the water becomes cloudy or murky within a few hours due to dispersed clay, the soil is likely dispersive and responsive to gypsum application. This simple test is not a substitute for a full laboratory analysis.
The most reliable diagnostic tool is a comprehensive soil test, which confirms chemical imbalances. Specifically, a test should look for a high sodium adsorption ratio (SAR) or a high soil pH, typically above 8.4, which indicates a sodic issue. The test also confirms if the soil is deficient in calcium or sulfur, or if there is aluminum saturation in the subsoil.
Gypsum is not a universal fix. Highly sandy soils, which lack the clay particles gypsum acts upon, will not benefit structurally. Additionally, gypsum is ineffective when the goal is to raise the soil’s pH, as it is a neutral salt; lime is the appropriate amendment. Applying gypsum when the soil already has sufficient calcium or sulfur is unnecessary and could cause other nutrient imbalances.