1. Clark, J. S., and Peech, M., Soil Sci. Soc. Amer., Proc., 19, 171 (1955). Lindsay, W. L., and Moreno, E. C., ibid., 24, 177 (1960).
2. Aslyng, H. C., Veterinary and Agricultural College, Copenhagen, Yearbook (1954).
3. Wild, A., Trans. Fifth Int. Cong. Soil Sci., 2, 500 (1954).
4. Bjerrum, N., Damsgaard-Sørensen, P., and Tovborg Jensen, A., Beretn. 19, skand. naturforskarmötet i Helsingfors, 344 (1936).
5. Lucena-Conde, F., and Prat, L., Anal. Chim. Acta, 16, 473 (1957).