It is now a well-established fact that the ammonia, which is formed in the cultivated soil by the breaking down of nitrogenous organic matter through the action of bacteria and moulds, is itself converted by oxidation into nitrates. The latter process, “nitrification“ proper, is effected in two stages by two distinct species of bacteria, the one carrying the oxidation to nitrite, and the other changing the latter compound into nitrate. It is also known that, in the absence of a base, nitrification does not take place. The most usual and most available soil base is calcium carbonate, which is, however, present in some arable soils to only a very small amount, less than ·05 per cent.; in such soils, and in others where its presence cannot be detected, nitrification is still active, a fact which seems to suggest that other substances may replace it and serve as bases for nitrification.