Gypsum is a naturally occurring mineral widely used in agriculture as a soil amendment to enhance soil properties and plant nutrition. It is a soluble source of calcium and sulfur for the soil. Its primary function is to improve the physical and chemical characteristics of the soil environment. Gypsum is a neutral salt, meaning its application does not significantly change the soil’s acidity or alkalinity, unlike agricultural lime. This makes it suitable for improving soil health where a \(\text{pH}\) adjustment is not desired.
Improving Soil Structure and Counteracting Sodium
One distinct use of gypsum is the reclamation of sodic soils, which have high concentrations of exchangeable sodium ions bound to clay particles. When sodium dominates the exchange sites, the soil clay disperses when wet. This dispersion destroys the pore structure, leading to poor water infiltration, surface crusting, and reduced aeration. As gypsum dissolves, it releases positively charged calcium ions (\(\text{Ca}^{2+}\)) into the soil solution. The divalent calcium ion displaces the sodium from the negatively charged surfaces of the clay because it has a stronger positive charge.
This chemical exchange converts sodic clay into calcic clay, rapidly improving the soil’s physical structure. The displaced sodium ions, dissolved in the soil water, are then flushed out of the root zone through leaching with irrigation or rainfall. The calcium ions promote the flocculation of clay particles, causing microscopic particles to clump together into larger, stable aggregates. This aggregation creates more stable soil peds and increases the volume of macropores.
The resulting improvement in soil structure enhances the movement of water and air through the soil profile. Better water infiltration reduces surface runoff and erosion, while improved aeration supports healthier root growth and microbial activity. This effect is beneficial in heavy clay soils that tend to compact easily or exhibit poor drainage. The structural benefits of this calcium-sodium exchange are often seen quickly, leading to softer, more workable soil.
Supplying Calcium and Sulfur Nutrients
Beyond its role in soil structure remediation, gypsum is a direct source of two secondary macronutrients: calcium and sulfur. Calcium is integral to the formation and stability of plant cell walls, providing structural integrity. Adequate calcium supply is important for the growth of new roots and shoots, as it is relatively immobile within the plant once deposited. A deficiency can manifest as disorders like blossom-end rot in tomatoes or tip burn in lettuce.
Sulfur plays a central role in several metabolic processes within the plant. It is a constituent of amino acids, such as cysteine and methionine, which are the building blocks of proteins. Sulfur is also necessary for the formation of chlorophyll, the pigment responsible for photosynthesis. Gypsum provides sulfur in the readily available sulfate (\(\text{SO}_4^{2-}\)) form, which plants absorb immediately. This makes it an effective supplement where organic matter mineralization or atmospheric deposition does not provide sufficient sulfur.
Mitigating Aluminum Toxicity in Acidic Conditions
Gypsum offers a unique benefit in managing acidic soils, particularly concerning aluminum toxicity in the subsoil. In soils with a \(\text{pH}\) below 5.5, aluminum becomes soluble and highly toxic to plant roots, causing stunting and inhibiting water and nutrient uptake. While traditional liming materials neutralize acidity by raising the \(\text{pH}\) of the topsoil, they are often slow to move deeper into the subsoil. Gypsum, being more soluble than lime, can move rapidly down through the soil with water.
The sulfate ion (\(\text{SO}_4^{2-}\)) released by the dissolved gypsum is responsible for this deep-reaching effect. It binds with the toxic, soluble aluminum ions (\(\text{Al}^{3+}\)) to form non-toxic complexes, such as aluminum sulfate. These complexes are then leached below the effective rooting depth, detoxifying the subsoil. This process allows plant roots to grow deeper, accessing more water and nutrients. Gypsum’s ability to address this subsoil constraint is an advantage over surface-applied lime in many weathered, acid soil regions.
How to Apply Gypsum Effectively
Effective use of gypsum begins with a comprehensive soil test, measuring soil \(\text{pH}\), exchangeable sodium percentage (\(\text{ESP}\)), and existing calcium and sulfur levels. The application rate should be tailored to the specific soil problem. Sodic soil reclamation often requires higher rates, sometimes up to 3 to 5 tons per acre spread over multiple applications, while routine nutrient supplementation may require 0.5 to 1 ton per acre.
The most common application method is surface broadcasting using a standard fertilizer spreader, followed by incorporation into the topsoil for immediate sodic soil remediation. In no-till systems, surface application relies on rainfall or irrigation to dissolve the gypsum and carry the calcium ions into the soil profile. The frequency of application depends on the severity of the issue and the soil type, with some soils requiring annual low-rate applications. Gypsum offers little benefit in sandy soils that already have excellent drainage and structure. It should not be relied upon to correct low soil \(\text{pH}\) across the entire profile, as its primary chemical action is not to neutralize acidity like lime does.