Physico-chemical and biological degradation of the soils of the Lower Sambirano plain, Ambanja District, North-West of Madagascar

Geological and Environment

The soils of the Lower Sambirano plain are degraded following the pressures of human activities and frequent flooding. This study aims to provide diagnostic elements on the physico-chemical properties of the soils of the study area in order to assess the state of their degradation. Soil samples on 17 profiles distributed in 7 different sites were taken and analyzed at the FOFIFA Madagascar Laboratory. The analysis results obtained were compared with the reference values. These results show that dark colored soils ranging from dark brown (10YR, 6/3) to black (10YR, 2/1), sandy loam to sandy clay loam soils (balanced textures) and sandy loam soils (balanced textures) and clayey to clayey (clayey texture), acid to slightly basic pH (4.5 to 7.4). These soils are very rich in assimilable phosphorus, manganese, and rich in total calcium, potassium, CEC and V. This indicates that the nutrient retention capacity is sufficient. Then, they are moderately rich in organic carbon, organic matter, magnesium and a sum of exchangeable cations (S). Indeed, the contents of total nitrogen and particularly of sodium proved to be deficient but despite this, the C/N ratio is sufficient. All of these results lead to the conclusion that the soils of the study area have average but very fragile fertility. However, additional morphoscopic analysis on other samples and that of the Al and Si elements are recommended to check whether these soils are toxic. Accompanying measures to restore these soils are therefore necessary in view of the risks they may incur.

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