1. Ademoroti, C.M.A. Environmental Chemistry and Toxicology. Foludex Press Ltd., Ibadan (1996).
2. Alloway, B.J. Heavy Metal in Soils. Halsted Press, John Wiley & Sons Inc., London (1996).
3. Chapel, A.: Foliar Fertilization, pp. 66-86, In: Matinus Nijhoff Dordrecht, A. Alexander (Ed.). Stuttgart (1986).
4. Juste, C. and Mench, M.: Long — term application of sewage sludge and its effects on metal uptake by crops. pp. 159-194. In: Biogeochemisby of TraceMetals. D. C. Adriano (Ed.). CRC Press, Boca Raton (1992).
5. Peech, M.: Hydrogen ion concentration. pp. 1167-1183. In: Methods of Soil Analysis Part I. C. A. Black (Ed.). Am. Soc. Agron. Inc. Agron., 9, Madison, Wisconsin, USA (1965)