Ologbo [Nigeria] extension, reconnaissance soil survey

Bourgeon Gérard. 2005. Ologbo [Nigeria] extension, reconnaissance soil survey. Montpellier : CIRAD, 24 p. N° de rapport : CP_SIC 1864

Résumé : Presco has concluded the acquisition of about 6,000 ha at Ologbo to develop a new oil palm plantation. The first field visits by the CIRAD engineer (Miss A. Verwilghen) revealed the existence of large zones affected by waterlogging. On consulting the available satellite images, it was found that, in addition to floodplains associated with the main rivers around the site, which were ruled out, the zone earmarked for the new plantation effectively had a dense network of bottomlands, a network not found in the zone of the existing estate at Obaretin. These findings led to a soil study being recommended, to avoid planting on soils marked by water saturation over long periods. Soil investigations were carried out for the south-west section (around 6,000 ha) of the newly acquired land, in an area mostly covered with forest of different types (and by their degraded stages), in order to select areas suitable for the oil palm plantation. Due to this dense vegetation cover, it was decided to open ground traverses especially for this survey, in order to achieve good overall distribution of soil pits. Satellite images were used to prepare this work: the main ground traverses were established in the median section of the interfluves and other traverses, perpendicular to them, completed the observation network (see map). The opening of ground traverses and excavation of soil pits were almost finished prior to the arrival of the soil scientist on site. The soil pits were described, 4 of them sampled ( one more profile was sampled later at the Obaretin plantation) and samples were analysed at the CIRAD soil laboratories in Montpellier (Physicochemistry analysis laboratory). Complete soil profile descriptions and analytical results are given in the Appendix. The following main conclusions can be drawn, based on these field observations, completed by the analysis of a few profiles. 1) Most of the soils in the study zone correspond to red plateau soils (profiles Ologbo O and Ologbo 28), known as 'Terres de Barre' in Togo and Benin, where they occupy large areas. These soils are strictly similar to those on which the Obaretin estate has been planted (Obaretin profile). They amount to approximately 3,370 ha of the study area and extend largely to the northnorth- east, beyond the survey zone. 2) Floodplains alongside the main rivers (mostly located outside the study zone, though around 580 ha are within it), along with bottomlands which sink deeply into the 'Terres de Barre' plateau, correspond to swamps which are categorically not to be recommended for planting (no profiles analysed). 3) Between these two major situations, there are two others which we shall call intermediate, be it as much in terms of geography as in land use: a - a sandy connecting slope between the 'Terres de Barre' plateau and the floodplain of the Ossiomo river (approximately 1,230 ha). In addition to the very sandy texture of its soils, the slope displays numerous hydromorphic traits, varying in degree, ranging from deep waterlogging, revealed by mottling, to virtually continuous waterlogging in mini-swamps which cannot be precisely delimited on the map either from satellite images or from the soil observations conducted during the survey. Two profiles of this slope have been analysed: the first, Ologbo 56, corresponds to a non-hydromorphic sandy soil, the second, Ologbo 62, to a soil with a slightly more balanced texture, but with deep waterlogging. When developing the land to set up the new plantation, these soils can be included if the policy adopted is to maximize the area planted to oil palm, but only once surveys have been conducted to locate the mini-swamps, and avoid them. Any soils such as those of the sandy slope that might serve as a reference do not appear to exist in the Obaretin estate, but it is reasonable to assume that yields will be lower there than those obtained on the plateau 'Terres de Barre'. If, however, the chosen policy is to leave buffer zones between natural biodiversity reserves, wetlands to be protected, and the plantation, the sandy slope could ideally be used for that purpose. b - steep slopes linking the 'Terres de Barre' plateau and the bottomlands sinking into it, which should be avoided because of their topography. The slopes are of very limited width (i.e. from the plateau to the bottornlands. They have been systematically shown on the map using dotted lines, based on rare observations made in the ground transverses. Their representation is therefore subject to later adjustments if other information becomes available. Once again, leaving these slopes as they are, to be used as filter zones between planted areas and the humid ecosystem, namely the bottornland, is the most appropriate course of action. The following report presents the different arguments that helped to lead to the conclusions summarized above. It also contains some information on how the study was conducted, along with considerations regarding the scientific classification of the soils studied, insofar as this is an essential step making it possible to (i) establish the diagnosis (ii) make subsequent comparisons or undertake bibliographical research for soils of the same type.

Mots-clés Agrovoc : Enquête pédologique

Mots-clés géographiques Agrovoc : Nigéria

Classification Agris : P31 - Levés et cartographie des sols

Auteurs et affiliations

• Bourgeon Gérard, CIRAD-CA-UPR Recyclage et risque (FRA)

Source : Cirad - Agritrop (https://agritrop.cirad.fr/528832/)

Métriques

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