A 2-year field experiment was conducted to evaluate the effects of Tithonia diversifolia green manure combined with either Minjingu phosphate rock (MPR) or triple super phosphate (TSP) on soil chemical properties that influence P availability, P pools and maize grain yields, on a Chromic Acrisol in Morogoro, Tanzania. Leafy biomass of tithonia was applied before maize planting for two consecutive growing seasons. Treatments compared were the control, MPR and TSP each at 80 kg P ha )1 ; tithonia alone at 2.5, 5.0, and 7.5 Mg ha )1 dry matter and tithonia combined with MPR or TSP at 40 kg P ha )1 . Tithonia led to significant increases in soil pH, exchangeable Ca, labile (resin and NaHCO 3 -Pi), and moderately labile inorganic P (NaOH-Pi). It reduced exchangeable Al and P sorption. Application of MPR alone had liming effects and resulted in increase in labile P. Combining tithonia with MPR had similar but more intense effects. Triple superphosphate alone led to acidification and this was reversed when TSP was co-applied with tithonia. Increasing the application rates of tithonia either alone or in combination with TSP or MPR led to more pronounced liming effects but the differences between 2.5 and 5.0 Mg tithonia ha )1 were not significant due to moisture stress that was experienced during the season. The P and Ca concentrations of the maize plants at tasselling increased with the application of tithonia alone or combined with MPR or TSP, and were significantly correlated with maize grain yields (r = 0.75 and 0.64 for MPR and TSP, respectively). Tithonia added consecutively for 2 years increased total maize grain yields by 70% compared to that in the control. The relative agronomic effectiveness (RAE) of MPR increased from 46% in the first year of application to >142% in the second year, indicating that the initially slow dissolution of MPR improved by combined application of tithonia and MPR, attributed to reduction of P sorption. It is concluded that tithonia can enhance P availability from the Chromic Acrisol through modification of soil properties associated with P transformation and availability. In cases where tithonia is found within the farmers' fields its combined application with MPR can increase maize yields at a much-reduced cost associated with tithonia procurement.

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This work was carried out in collaboration between all authors. Author EMM designed the study, wrote the protocol and wrote the first draft of the manuscript. Authors JPM, ES and CKG guided on data collection, evaluated and interpretations while author MKN was involved in data collection, data analysis, interpretation and review. All authors read and approved the final manuscript.

A b s t r a c t A r t i c l e I n f oThis study was conducted to characterize dominant soil type at Dakawa Irrigation Scheme (DIS), Tanzania. Typical soil at DIS was identified, excavated and described using FAO (2006) Guidelines. Six disturbed samples were taken from soil profile horizons together with three undisturbed core samples for laboratory analysis. The pedon developed from Neogene alluvial parent materials under ustic moisture and isohyperthermic temperature regimes. The pedon was very deep (> 150 cm), moderately well drained, with gray to very dark gray soil colours. CaCO 3 concretions characteristic of calcic horizon were observed in subsoil. Soil texture was sandy clay loam throughout the pedon. pH was rated as medium (5.5 -7.0) to very high (7.0 -8.5) respectively for topsoil and subsoil. OC and N levels were very low; CEC and exchangeable bases were low to medium. Bulk densities were high (1.8 gcm -3 ) to very high (>1.9 gcm -3 ) for topsoil and subsoil, respectively. Study on moisture retention properties showed that subsurface (95 -100 cm) and intermediate (45 -50 cm) horizons retained more water than surface (0 -5 cm) soil, congruent with increasing clay content with depth. According to Soil Taxonomy and World Reference Base for Soil Resources, pedon at DIS classified as Vertic Calciustepts and Haplic Vertic Cambisols, respectively. These taxa reflect properties that may guide use and management of soils at DIS including selection of crops and fertilizers and rates to use. Organic fertilizers were recommended to increase organic matter content in soil. Use of acidifying inorganic fertilizers and gypsum as soil amendments should also be taken onboard to correct emerging sodicity.

PWM and CKG guided on data collection, evaluation and interpretations while author MKN was involved in data collection, data analysis, interpretation and review. All authors read and approved the final manuscript.