1. Rees, W. J., and Sidrak, G. H., Plant and Soil, 8, 141 (1956).
2. Holliday, R., et al., Nature, 181, 1079 (1958).
3. Private communication from Central Electricity Generating Board Research Laboratories, Leatherhead.
4. Bear, F. E. (Ed.), “Chemistry of the Soil”, 329 (Reinhold Publishing Co., New York, 1955).