Influence ofTithonia diversifoliaand triple superphosphate on dissolution and effectiveness of phosphate rock in acidic soil

An incubation and a pot experiment were conducted to evaluate the dissolution and agronomic effectiveness of a less reactive phosphate rock, Busumbu soft ore (BPR), in an Oxisol in Kenya. Resin (anion and anion + cation)‐extractable P and sequentially extracted P with 0.5MNaHCO3, 0.1MNaOH, and 1MHCl were analyzed. Dissolution was determined from the increase in anion resin (AER)–, NaHCO3‐, and NaOH‐extractable P in soil amended with PR compared with the control soil. Where P was applied, resin P significantly increased above the no‐P treatment. Busumbu‐PR solubility was low and did not increase significantly in 16 weeks. Anion + cation (ACER)‐extractable P was generally greater than AER‐P. The difference was greater for PR than for triple superphosphate (TSP). The ACER extraction may be a better estimate of plant P availability, particularly when poorly soluble P sources are used. Addition of P fertilizers alone or in combination withTithonia diversifolia(TSP, BPR, TSP +Tithonia, and BPR +Tithonia) increased the concentration of labile inorganic P pools (NaHCO3‐ and NaOH‐Pi). Cumulative evolved CO2was significantly correlated with cumulative N mineralized fromTithonia(r, 0.51,p< 0.05). Decrease in pH caused NH₄⁺ ‐N accumulation while NO₃⁻ ‐N remained low whereTithoniawas incorporated at all sampling times. However, when pH was increased, NH₄⁺ ‐N declined with a corresponding rise in NO₃⁻ ‐N.Tithoniasignificantly depressed soil exchangeable acidity relative to control with time. A significant increase (p< 0.05) was observed for P uptake but not dry‐mass production in maize where BPR was applied. The variations in yield and P uptake due to source and rates of application were statistically significant. At any given P rate, highest yields were obtained withTithoniaalone. Combination of Busumbu PR with TSP orTithoniadid not enhance the effectiveness of the PR. The poor dissolution and plant P uptake of BPR may be related to the high Fe content in the PR material.