Paddy soils from basaltic colluvium in the Highland of Madagascar

Three soils under lowland rice cultivation, found on the Highland of Madagascar, in the Vakinankaratra region, were investigated, in order to help understanding their ecology and to assess their fertility. The parent material of the three soils consists of basaltic colluvium. According to the FAO-UNESCO (1988), these soils have been classified as Ferralic Cambisol (D2) and Dystric Gleysol (D3 er D4). They are very pool in alkali cations, and rich in aluminium, iron and clay. The clay is mainly composed of gibbsite, kaolinite as well as of little amounts of goethite and quartz. This composition underlines the high degree of intensive soil weathering. The low fertility of the investigated soils is due to very low contents of available P, Ca, Mg, K and N, as well as to the high content of iron and manganese, which are toxic for rice plants under the anaerobic conditions of the soils during flooding. We therefore recommend to add to the soil beside NPK fertilizers, limestone powder in order to obtain a pH-value between 6 and 7. This will lead to an increase of Ca, Mg, and K as well as to a better plant availability of phosphorus and a decrease of the toxic effects of iron and manganese.