Effects of natural fallow on topsoil properties and subsequent crop yields in a forest Oxisol of Southern Cameroon

Studies were conducted on farmers' agricultural practices in the Tropenbos Cameroon Programme situated in the humid forest zone of Southern Cameroon, in order to provide information to be used in the management planning of the area. This paper describes changes occurring in soil characteristics and soil productivity of Oxisols during the natural fallow period within the shifting cultivation system. A synchronised approach analyzing at the same time fallows of different duration was used. Soil characteristics from fallow of various ages were statistically compared. Stepwise analyses of multiple linear regressions were used to evaluate relationships between soil properties’ variation and crop (maize, groundnut, cocoyam and cassava) yields from mixed crop fields following each fallow type. The results showed that (i) ashes from burned vegetation biomass at the beginning of the cropping period act like lime fertilizer and significantly (p<0.05) increase soil properties such as pH, exchangeable bases (especially calcium), and decrease exchangeable acidity; (ii) there is a slow but almost irreversible decrease in topsoil clay content during the cropping period; and (iii) at the age 7 to 9 year bush fallow, there is a morphological recovery of the topsoil thin (3-5 cm) A1 organic horizon, which was destroyed by tillage during the cropping period. For subsequent mixed cropping, crop yields were generally very low due to the low quality of breeding varieties in use. Fallows of more than 15 years appeared to be not suitable for groundnut cultivation. On the whole, the productivity index (LER) showed higher crop productivity following bush fallow (7-9 years). The multiple linear regression analyses showed that the dry biomass yields of maize and groundnut were related to soil chemical properties while grain yields correlated mostly with soil physical properties. Cocoyam yield correlated positively with soil CEC and organic matter. For a sustainable management of the area, there are possibilities to intensify crop production and productivity within a rotational bush fallow (BF) system using for example, an integrated management approach combining application of mineral and organic fertilizer.