1. Harmsen, G. W., and van Schreven, D. A., Adv. Agronomy, 7, 299 (1955).
2. Bremner, J. M., Methods of Soil Analysis, edit. by Black, C. A. (Amer. Soc. Agron., Madison; in the press).
3. Fitts, J. W., Iowa State Coll. J. Sci., 27, 172 (1953).
4. Hagin, J., and Halevy, J., Israel J. Agric. Res. (Ktavim), 11, 81 (1961).
5. Waring, S. A., and Bremner, J. M., Nature, 201, 951 (1964).