The rates of nitrification and the extent to which a number of nitrogenous materials are nitrified in soil have been studied. Of the substances used ammonium sulphate was most rapidly and most completely nitrified under these conditions. A reputedly genuine sample of Peruvian guano was almost as rapid as ammonium sulphate. At the other end of the scale was horn meal, which was nitrified most slowly and in which the percentage of nitrogen converted was lowest. On a basis of nitrification alone there appears to be no justification for the traditional view that hoof and horn is slow- and that dried blood is quick-acting. Two samples of bone meal showed very different rates of decomposition. This, and the anomalous behaviour of hoof and horn separately as compared with mixed hoof and horn, suggest that differences occur in the behaviour of different samples of what are nominally the same materials. Some of the commercial implications of these findings are discussed.