Allanblackia stuhlmannii – a tree under current domestication: what are the soil requirements?

Sammanfattning: Allanblackia is a genus of trees that grows in the rainforests of West, Central and Eastern Africa. Its big fruits contain seeds very rich in oil which has been used by local communities for cooking and making soaps. The native stands are threatened by overexploitation and the demand for the oil is much greater than the supply. Domestication programs, aiming at introducing the trees to small holder agroforestry systems, have been started for some of the species. The main aim of this study was to investigate the soil requirements of Allanblackia stuhlmannii in terms of chemical and physical parameters, as revealed by the soil conditions in native stands. Another aim was to see if these varied along an altitudinal transect. The aim was also to conduct soil profile descriptions and collect climatic data. The hypotheses were that the soils would be acidic and highly weathered with low concentrations of plant available nutrients. Soils on lower altitudes were hypothesized to be less acidic and have higher concentration of nutrients. The soil description was expected to result in a Ferralsol or Acrisol. The sampling and collection of data were conducted at five sites within the Amani Nature Reserve in the East Usambara, Tanzania. The soils were analyzed for bicarbonate extractable P, organic C, total N, electric conductivity, pH, C:N ratio and bulk density. Water infiltration rate was also measured. Soil samples from the soil profiles were also analyzed for texture, effective cation exchange capacity, exchangeable base cations, Al, Fe and Mn, effective base saturation, exchangeable acidity and oxalate and dithionite extractable Al, Fe and Mn. The results showed that the soils had very low pH, generally below 4.5. Overall the concentration of nutrients was very low but there were higher concentrations of P, organic C and total N in the upper 10 cm compared to lower depths. There were differences between sites for all parameters but not a clear altitudinal effect. A slight altitudinal effect was observed for P in the upper 5 cm, with higher concentrations at lower altitudes, and for pH with lower pH at lower altitudes. The two soil profiles were classified as Ferralsol and Acrisol, which are typical red, highly weathered and leached tropical soils with poor chemical status. The climate was found to be of humid tropical character with rain during all months of the year. This was a pilot study linking soil samples to native stands of Allanblackia stuhlmannii. Hopefully these results will be useful in the future domestication process by providing knowledge about the tree’s requirement on soil and climate. This information can help farmers successfully integrate the tree in their farming system and thereby improve their farm income as well as preserve the native forests.