The important part played by calcium carbonate in maintaining the fertility of the soil makes it of interest to investigate all the causes which determine both its formation and decomposition. A number of processes which are continually going forward in the soil tend to diminish the stock of calcium carbonate present; yet, in spite of this, and without frequent additions of chalk or lime from other sources, most soils, under normal conditions, remain more or less in a state of fertility. An equilibrium is set up by which the withdrawal of calcium carbonate is balanced by the results of another set of actions which restore base to the soil. This enables many soils which contain only very small quantities of lime to retain their neutral reaction and so to produce fair crops.