Une comparaison des modes de gestion des terres au Sud Cameroun montre que les agroforêts-cacao sont moins dommageables à l'environnement que les autres formes de gestion des sols. A cet effet, une étude en vue de valider les perceptions paysannes des essences ligneuses fertilitaires associées dans les agroforêts à base de cacaoyers a été effectuée dans deux sites écologiquement différents au sud Cameroun entre mars et novembre 2008. Dans chaque site, 20 systèmes cacaoyers étaient prospectés et dans chacun de ces systèmes, toutes les essences ligneuses étaient identifiées. Chaque paysan classait parmi les essences identifiées les dix premières ayant le potentiel d'amélioration de la fertilité des sols le plus élevé, et indiquait les attributs fonctionnels qui y concourent. Leur fréquence et leur distribution ont été déterminées. 52 essences différentes ont été identifiées dans les systèmes cacaoyers de la zone de transition forêtsavane (Bokito) contre 195 à Ngomedjap situé dans la zone de forêt dense humide. Les essences fruitières telles que Dacryodes edulis, Elaesis guineensis, Citrus sinensis, Citrus reticulata, Mangifera indica et Persea americana sont plus fréquentes dans les deux sites. La classification des paysans des essences fertilitaires est basée sur certains attributs fonctionnels tels que le système racinaire et la surface foliaire. La comparaison entre la classification générale des paysans par site et la mycotrophie de ces essences est presque identique. Des études complémentaires incluant les aspects physico-chimiques et même certains autres facteurs biologiques de la fertilité des sols des systèmes cacaoyers s'avèrent nécessaires pour affiner la validité des perceptions paysannes des essences associées fertilitaires.

This study was designed to compare the phosphorous fixation capacity of three soils series named Tyele, Minkonmingon and Mekoto in the south region of Cameroon and to determine the soil properties that are the main predictors of the P activity of those soils. Five adsorption equations viz. Linear, Langmuir, Van Huay, Freundlich and Temkin were used to describe P adsorption processes. The results of the study showed that maximum adsorbed P of Minkonmingon, Tyele and Mekoto was 936.09, 311.15 and 823.37 mg kg-1 of soils respectively with the mean of 690.20 mg kg-1 of soils. By applying various models, P adsorption data revealed that for low concentration range, the Freundlich equation show a better fit followed by the Langmuir, Van Huay, Temkin and Linear equations. It can be thus concluded from the adsorption and Spearman correlation analysis that the soil of Minkonmingon has a greater capacity to fix P followed by those of Mekoto and Tyele respectively with silt, exchangeable acidity, free aluminum, potassium and soil organic carbon and pH KCl being the main predictors of P activity in these soils.

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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.