pH as a measure for the requirement of Calcium (Ca) in Agriculture
Calcium is an essential plant macro nutrient. It is also the dominant cation in most agricultural soils representing about 80% of the basic (alkaline) component (Percentage Base Saturation).
Consequently the measure of pH can be used to approximate the requirement for Calcium. It is important to remember that the pH is the potential for hydrogen and not Ca; it measures only the ration between the acidic and basic ions that are in solution. It does not determine the cause or quantity of a given ion responsible for the pH. It is just that in SOILS the dominant cation is Ca, which conveniently happens to be a macro nutrient. It does not mean that pH and Ca are interchangeable!!
The pH measure is used to approximate the requirement of Ca based on the general rule that Ca is the dominant cation and therefore changes in pH are likely to be indicative of Ca losses.
Thus pH is a measure that in Agriculture is used to determine the ration between acidic and basic components in solution. As Ca is usually (fortunately) the basic cation pH serves as a useful tool in determining lime requirements. It’s not a great tool but an easy and very much misunderstood one in agriculture. A good article on such can be found in the growersminerals.com Autumn 2003 Newsletter (V16 I4): Rely on pH for Calcium Needs? (James Halbeisen 2003)
Free Cultural Works (CC-BY-NC-SA) Malcolm McEwen 2011