1. Allison, L. E. 1965 Organic carbonIn Methods of soil analysis. Ed. C. A. Black. Am. Soc. Agron. Madison, Wisconsin. p. 1374–1375.
2. Castro, R. U. 1977 Zinc deficiency in rice: a review of research at the international rice research institute. IRRI Res. Pap. Ser.9, 18 p.
3. Chapman, H. D. 1965 Cation exchange capacity.In Methods of soil analysis. Ed. C. A. Black. Am. Soc. Agron. Madison, Wisconsin. p. 294–985.
4. Day, P. R. 1965. Particle fractionation and particle-size analysis.In Methods of soil analysis. Ed. C. A. Black. Am. Soc. Agron. Madison, Wisconsin. p. 552–555.
5. IRRI Saturday Seminar;O. Giron,1976