How does lime actually raise soil pH?
Soil acidity isn’t just a number on a soil test. It’s driven by hydrogen (H⁺) occupying exchange sites on soil colloids.
When we apply traditional ag lime, whether high-calcium lime or dolomitic lime, the pH change does not come from calcium or magnesium alone. The real work is done by the carbonate portion of the material.
Carbonate reacts with acidity in the soil:
CO₃²⁻ + 2H⁺ → H₂O + CO₂
This reaction removes hydrogen from the system, reducing acidity and shifting pH toward neutral. Calcium and magnesium then occupy those exchange sites, increasing base saturation.
Calcium helps build aggregation and pore space. Magnesium also plays a structural role, but only when it remains in balance with calcium. Too much of either can create physical and nutritional challenges, which is why pH and base saturation should always be considered together.
Understanding this chemistry helps sort through product claims. If a product does not contain carbonate (or similar neutralizing chemistry), it cannot meaningfully raise soil pH. Gypsum is a great example. It supplies calcium and sulfur and can improve structure, but it does not neutralize acidity.
It’s also worth noting that some fast-reacting lime materials can create challenges if applied aggressively or without regard to soil buffering. Rapid pH shifts can promote precipitation reactions with micronutrients like iron and aluminum, temporarily reducing availability and, in some cases, contributing to surface sealing or structural issues. Rate, particle size, and overall soil balance matter.
In certain situations, a strategic blend of faster-reacting materials with slower, traditional lime sources can make sense. This can help address short-term pH limitations while supporting longer-term stability as less reactive materials continue to work. As with all soil decisions, this approach should be guided by testing, buffering capacity, and a clear plan.
Managing soil pH isn’t about quick fixes. It’s about understanding the chemistry, respecting the system, and choosing the right material, at the right rate, for the right soil.