Which amendment improves sodic soils?

• A. Lime
• B. Compost
• C. Gypsum
• D. Organic mulch

Answer: (C)

In sodic soils the main problem is high exchangeable sodium that disperses clay and seals the soil, reducing infiltration and root growth. Gypsum is the best choice because it provides calcium ions (Ca2+) that replace sodium on the clay’s exchange sites. As calcium swaps onto the sites, the clay particles flocculate rather than disperse, improving soil structure, porosity, and infiltration. The sulfate component of gypsum is relatively neutral with respect to pH, so it won’t raise the pH in the way lime can; lime adds calcium but also tends to increase alkalinity, which can worsen nutrient availability in already alkaline sodic soils. Compost or organic mulch can help with organic matter and surface conditions, but they don’t directly fix the exchangeable sodium or the soil structure issues caused by sodicity. For gypsum to work effectively, you also need good drainage and leaching to remove the displaced sodium from the root zone after calcium has swapped onto the exchange sites.