Lime is used to increase soil pH. The amount of lime to apply depends upon the buffer pH value. Buffer pH is an indicator of the soil's ability to resist pH changes. Generally, soils with a higher buffer pH require more lime. Universities and certified soil testing laboratories have databases that relate buffer pH to lime requirements. Cation exchange capacity .
Cations have a positive charge.
Plant nutrients that are cations include ammonium, potassium, calcium, magnesium, copper, iron, manganese, and zinc. Soils generally have a negative charge, so they are able to hold cations and keep them from being leached away with water. Both units are a measure of the positive charge that can be held by the soil. Soils with a higher CEC are generally more fertile, since they are capable of providing more nutrients to crops. CEC is also needed for following label restrictions on pesticides.