We tested the effects of two organic materials (OMs) of varying chemical characteristics that is, farmyard manure (FYM) and Tithonia diversifolia (tithonia), when applied alone or in combination with three inorganic P sources, that is, triple superphosphate (TSP), Minjingu phosphate rock (MPR), and Busumbu phosphate rock (BPR) on soil pH, exchangeable acidity, exchangeable Al, and P availability in an incubation study. FYM and tithonia increased the soil pH and reduced the exchangeable acidity and Al in the short term, but the inorganic P sources did not significantly affect these parameters. The effectiveness of the inorganic P sources in increasing P availability followed the order, TSP？>？MPR？>？BPR, while among the OMs, FYM was more effective than tithonia. There was no evidence of synergism in terms of increased available P when organic and inorganic P sources were combined. The combination of OMs with inorganic P fertilizers may, however, have other benefits associated with integrated soil fertility management. 1. Introduction Soil acidity and phosphorus deficiencies limit crop production in many tropical soils [1]. Lime and inorganic phosphate fertilizers are used in developed countries to remedy these problems. However, due to increasing costs and unavailability when needed, their use among smallholder farmers in developing countries is not widespread. This coupled with concerns for environmental protection and sustainability has renewed interest in the use of alternative cheaper locally available materials. The use of phosphate rocks (PR) and organic materials has in particular received increased attention in recent years in eastern Africa [2–4]. In addition to provision of P, PRs have Ca and Mg which makes them assume a significant role as a potential tool for sustaining soil productivity by reducing soil acidity through its liming effect [5]. Although most OMs are low in P, they can influence soil parameters such as soil pH, exchangeable Al, and Ca, which greatly influence crop growth [3]. There are a number of PR deposits of variable reactivity in eastern Africa which, however, differ greatly in their suitability as sources of P in P-deficient soils [6]. The most promising of these PRs are Minjingu in northern Tanzania and Busumbu in eastern Uganda [7], but their low solubility makes them unsuitable for direct application [1]. Techniques aiming to increase the solubility of BPR through blending with soluble phosphate fertilizers such as TSP or partial acidulation are likely not to have positive effects in terms of increasing P availability and
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