1. CHISHOLM, D., MacPHEE, A. and MacEACHERN, C., Can. J. Agr. Sci. 35, 433 (1955).
2. EDWARDS, C., Soils and Fert. 6, 451 (1964).
3. STEWART, D., CHISHOLM, D. and RAGAB, M., Nature 229, 47 (1971).
4. WOLFE, H. and DURHAM, W., Wash. State Hort. Assoc. Proc. 62, 91 (1966).
5. AVERELL, P. and NORRIS, M., Anal. Chem. 20, 753 (1948).