A field study was carried out in 1999 and 2000 in the village of Akok 180 km south of Yaoundé in the humid forest zone of southern Cameroon to assess changes in particle size distribution and soil chemical properties related to five different land-use systems namely: a secondary forest, a young forest, a cocoa farm, a Chromolaena fallow and a mixed food crop field. The results showed that land-use systems significantly affect the soil particle distribution and the highest proportions of sand and silt fractions were found in soils of young forests, the highest clay content was found under cocoa farms (P<0.0001). Soil pH H2O , exchangeable Ca and Mg, Al saturation and ECEC in the first, second and third 10 cm layers varied significantly with land-use systems (P<0.0001); organic carbon in the first and the second 10 cm layers significantly differ with the landuse systems (P=0.0115); and available P in the second and third 10 cm layers varied significantly with land-use systems (P<0.0007). Moreover cocoa farms had the highest pH, the lowest Al saturation, the highest soil Ca, Mg, ECEC and P contents compared to the other land-use systems. Based on their pH level and their high base saturation levels, soils of cocoa farms seem to present a higher fertility status than those of the rest of land-use systems. It is assumed that permanent vegetation coverage and/or exposure to heavy rains of the soils, disturbances due to land preparation, organic matter supplied by the vegetation, nutrient cycling, nutrient restitution and nutrient uptake could be differentiation factors.

Soil structure is the key for controlling soil quality. To assess changes in soil structure and its related indices under different land-use systems, an on-farm investigation was carried out on an Oxisol of Southern Province of Cameroon. Six land-use systems (LUS) consisting of a primary forest, a 30-year old secondary forest, a 25-year old cocoa field, a 15-year old natural fallow, a 3-year old Chromolaena odorata fallow and a 2-month old groundnut (Arachis hypogea) field were tested. Soil samples were collected at 0-5 and 5-10 cm depths and were used to determine particle size distribution (hydrometer method), bulk density and aggregate stability. Soil resistance to penetrometer was measured at the same depths in the field using a hand penetrometer. It was found that the primary forest and the cocoa field were associated with highest clay contents (74.6 and 52.0%, respectively) compared to other LUS. However, bulk density was significantly higher under cocoa field (1.09-1.26 g.cm-3) as compared to primary forest (0.72-0.89 g.cm-3). Soil resistance to penetrometer was the lowest under forests (1.2-5.2 bars) as compared to cropped fields (9-12.5 bars) at 0-5 cm depth. Similar trend was observed at 5-10 cm depth. The proportion of aggregates less than 2 mm in diameter was the least under primary forest (27%) as compared to groundnut field (52%). In contrast, soil aggregates under primary forest and cocoa field were more stable, with the highest mean weight diameter (MWD) of 3.37 and 3.00 mm, respectively.