1. Morrison, T. M., Nature, 205, 1027 (1965).
2. Bollard, E. G., J. Exp. Bot., 4, 363 (1953).
3. Jones, O. P., and Lacey, H. J., J. Exp. Bot. (in the press).
4. Varley, J. A., Analyst, 91, 119 (1966).
5. Jackson, M. L., Soil Chemical Analysis, 151 (Prentice Hall, Inc., Englewood Cliffs, 1958).