By William A. Albrecht
When limestone is put on the soil, it accepts acidity from the clay, just as other minerals do in the rock weathering processes. As a carbonate, it changes the active acid, or hydrogen, into water, of which compound the hydrogen is not such a highly active acid element. Therefore, the limestone corrects or neutralizes the soil acidity.
It has, however, been shown that this neutralizing effect from the liming operation is not so much the particular benefit derived by the crop, because compounds of calcium that do not neutralize the acidity, like calcium chloride, calcium sulfate or gypsum, and even ordinary cement for example, can improve the legume crop as well as calcium carbonate. Liming the soil puts calcium (or both calcium and magnesium if dolomitic limestone is used) on the clay, and thereby makes up this shortage on the list of nourishment of the crop.
It feeds the plant this one nutrient that the better forage legumes need so badly for their good growth and which is so readily removed from soils under higher rainfalls. It is the calcium put in, more than the acidity put out, that comes as the beneficial effect from liming the soil.
Source: Albrecht on Calcium