I have frequently cited the Maize Production Guide, published by the Zambian Ministry of Agriculture. It is the quintessential source of information on official recommendations, but to my knowledge is not available on-line. You can download my scanned copy here.
Zambia Agricultural Research Institute (ZARI). 2002. Maize production guide, in: Kaitisha, G.C., Gondwe, P., Mukwavi, M.V., Kaula, G.M. (Eds.), Permian exploration, boundaries, and stratigraphy. Soils and Crops Research, Ministry of Agriculture and Cooperatives, Mt. Makulu, Zambia. Available at www.afpconsulting-burke.com/maize-production-guide