Structural stability of Andisols and Andic Ferralsols from Mount Bambouto, West Cameroon highlands

Andisols and andic ferralsols occur in Mount Bambouto (2740 m asl), developed in upper and mid mountain respectively. They are very much in demand for agricultural activities. The objective of this paper is to investigate the structural stability of these soils, in order to access the sensitivity of their surface to degradation. Soils develop under fallow were selected on the three rock types (basalt, trachyte and rhyolitic deposits) for aggregates stability analysed by wet-sieve method. The results show that the soils aggregates in the study sites vary from stable (Mean Weight Diameter, MWD, between 1.3 and 2 mm) to very stable (MWD > 2mm). Aggregate sizes are correlated with soils and rock types, and soil structural stability is highly tied with rock types. Andisols are characterized by high proportion of macroaggregates (~50wt-% for particle-size 5-2 mm, at 30-50 cm depth). Soils developed on basalt are enriched in finest aggregates; soils developed on trachyte have aggregates of different sizes uniformly distributed. Soil upper rhyolitic deposits presents the coarse clay texture and hence low aggregates stability due to the quartz mineral. The soil aggregates from the rhyolitic deposits could be highest vulnerable during heavy rain storms. In the other hand, transformation of Andisols to Andic ferralsols toward the downslope is known and this transformation occurred with SOC mineralization and more clay-fraction formation; at the same time andic ferralsols became richer in microaggregates, and less stable.Key words: soil aggregate, structural stability, volcanic soil, Andisol, Andic ferralsol