1. Greene, G., and Steelink, C., J. Org. Chem., 27, 170 (1962).
2. Morrison, R. I., J. Soil Sci., 14, 201 (1963).
3. Burges, A., et al., Nature, 199, 696 (1963).
4. Flaig, W., Maataloustieteellinen Aikakauskirja, 33, 1 (1961).
5. Waksman, S. A., Humus (Baillière, Tindall and Cox, London, 1936).