Myths of slash and burn on physical degradation of savannah soils: impacts on Vertisols in north Cameroon

Abstract. A study was conducted to assess the impact of agricultural land use history on macro-aggregate (4.0–4.8 mm) stability in the 0–5 cm layers of Chromic and 'Hydromorphic' Vertisols in north Cameroon. Macroaggregate stability to water drop impact was determined and the ASI₅₀ index calculated. Macroaggregates from fallow and zero-tilled cropped soils disaggregated in a stepwise manner. Macroaggregates from ploughed cropped soils collapsed in one step into semi-liquefied microaggregates and primary particles. On both soil types, the ASI₅₀ index of samples from ploughed land was 10.0 mJ, compared to 16.4– 21.9 mJ from zero-tilled slash and burn land use. The stepwise disintegration of macroaggregates indicated the existence of a hierarchy of aggregation within the size range 2–5 mm. Slash and burn land use on zero-tilled Vertisols significantly increased sand-sized organic carbon content and the stability of macroaggregates to water impact.