1. Alexander, M. (1977) Introduction to Soil Microbiology, 2nd edn, Wiley, New York.
2. Alsaadawi, I. S., Al-Uqaili, J. K., AlRubeaa, A. J. et al. (1986) Allelopathic suppression of weed and nitrification by selected cultivars of Sorghum bicolor (L.) Moench. J. Chem. Ecol., 12, 209–19.
3. Bate, G. C. and Heelas, B. V. (1975) Studies on the nitrate nutrition of two indigenous Rhodesian grasses. J. Appl. Ecol., 12, 941–52.
4. Beggs, J. P. (1964) Spectacular clover establishment with formalin treatment suggests growth inhibitor in soil. New Zealand J. Agric., 108, 529–35.
5. Blum, U. And Rice, E. L. (1969) Inhibition of symbiotic nitrogen-fixation by gallic and tannic acid, and possible roles in old-field succession. Bull. Torrey Bot. Club, 96, 531–44.