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The addition of limestone is the correct way to improve low pH soils. Limestone, primarily composed of calcium carbonate, is effective in raising soil pH levels, making them less acidic. This adjustment is crucial because many plants thrive in neutral to slightly alkaline conditions, and low pH can hinder nutrient availability and negatively affect plant growth.

When limestone is applied to acidic soils, it reacts with hydrogen ions in the soil solution, effectively neutralizing the acidity and increasing the pH. This process not only improves the overall soil structure but also facilitates better nutrient uptake by plants, as certain nutrients become more available when the pH is adjusted.

Other options, such as compost and gypsum, do not significantly change pH levels. While compost can improve soil structure and provide nutrients, it does not have a major impact on acidity. Gypsum is primarily used to improve soil structure and drainage in clay soils but does not affect pH levels. Sulfur, on the other hand, actually lowers soil pH, making it less suitable for addressing low pH issues.