Lime and phosphate fertilizer are used to ameliorate P deficiency and Al toxicity in developed countries but their high cost precludes their use on many farms in Africa. Other cost-effective strategies therefore need to be explored. This book reports results of a study in Kenya where the effect of two organic materials (OMs); farmyard manure (FYM) and Tithonia diversifolia, when applied alone or in combination inorganic P sources, on soil chemical properties and maize yield was investigated. Overcoming P deficiency without reduction of exchangeable Al did not increase maize yield. Similarly, reduction of exchangeable Al without adequate application of P failed to increase the yield. Tithonia when applied alone, or in combination with inorganic P sources, was more effective in increasing maize yields than other treatments at similar P application rates because of its stronger ability to reduce exchangeable Al. Although FYM was more effective than tithonia in increasing the available soil P, it gave lower maize yields than tithonia, suggesting that the ability of an OM to lower the exchangeable Al more important in increasing maize yields than its ability to increase P availability.