1. F.J. BERGERSEN and E. H. HIPSLEY,. Gen. Microbiol., 60, 61 (1970).
2. D. R. BIGGINS and J. R. POSTGATE, J. Gen. Microbiol., 56, 181 (1969).
3. H. BORTELS, Arch. Mikrobiol., 1, 333 (1930).
4. J. M. BREMNER, Methods of Soil Analysis. Agronomy Monograph 9 (C. A. BLACK, ed.) American Society of Agronomy, Madison, Wisconsin (1965).
5. D. BURK, J. Phys. Chem., 34, 1195 (1930).