1. Weston, R. L., Gadgil, P. D., Salter, B. R., and Goodman, G. T., from Brit. Ecol. Soc. Symp. No. 5 (edit. by Goodman, G. T., et al.) (Blackwell, Oxford, 1965).
2. Pitcairn, C. E. R., thesis, University of Wales (1969).
3. Gemmell, R. P., thesis, University of Wales (1971).
4. Himes, F. L., and Barber, S. A., Soil Sci. Soc. Amer. Proc., 21, 368 (1957).
5. Lucas, R. E., Soil Sci., 66, 119 (1948).