Composts with wood ash addition: A risk or a chance for ameliorating acid tropical soils?

Within the framework aiming at assessing the potential risk of the use of wood ash amended composts to tropical acid soils, a greenhouse experiment was conducted with two acid soils (Oxisol and Ultisol) collected from Cameroon and amended with three types of compost 3:1 (w/w) prepared with 0, 8 and 16% wood ash prior to composting, respectively. They were planted with soybean (Glycine max L.) for 100 d. Addition of organic waste composts increased soil pH, SOM, WHC, EC, total C and N. Concentrations of exchangeable Al and NH4+–N decreased while those of NO3−–N and phosphorous increased. The exchangeable cation (Ca, Mg, Na and K) and micronutrient (Zn and Cu) concentrations increased while concentrations of Fe and Mn decreased. The C/N ratio of treated soils ranged from 12 to 14. Soil pH, concentrations of Fe and Mn were higher; EC and nitrate concentration were lower under ash compost amendment. Eight percent ash amendment to compost yielded best results in many aspects. Concentration of exchangeable Al was most decreased in soils treated with compost without ash. Plant biomass production was not significantly enhanced by compost amendment. This lack of an effect may be attributed to a soil pH (> 7.7) beyond the optimum for soybean (6.2). Summarising the results, composts with ash addition could serve the purpose of ameliorating acid tropical soils, albeit the optimal amendment rates still need to be addressed in future research.