Soil desilication affects the nutrient and silicon status of cassava in South Kivu, DRC.

Cassava (Manihot esculenta) is widely cropped in many tropical countries. It can be planted and harvested throughout the year making it a major crop for food production and safety. Surplus production can generate income, helping to improve livelihoods. Yet, in DRC, the cassava value chain is poorly developed and its production faces many threats as it lacks support services at almost all levels, although R&D organizations are involved at farm level in production, training, soil improvement and disease control. Though nutrient supply increases productivity, cassava is said to thrive on poor soils. Soil infertility is therefore a major constraint in most cassava growing areas.

Here we highlight the relationship between soil weathering stage and nutrient status of cassava plants in three agroecological zones in South Kivu, DRC. Zones (Z) 1 and 2 encompass ferrallitic soils derived from, respectively, old basalt and gneiss in highlands around Bukavu. Zone 3 includes a variety of soils derived from lacustrine deposits in the Uvira plain. The soils key out as Ferralsol (Z1), Acrisol (Z2), Cambisol and Fluvisol (Z3). Through a survey of 720 households, we identified farms with similar management and selected 120 plots (40/zone) for topsoil-foliar sampling.

In Z1 and Z2, the soils are poor in silt as their texture ranges from sandy clay to fine clay. The soils in Z3 are lighter: loamy sand to sandy clay loam. Our data confirm that the Z1-2 soils have reached an advanced degree of weathering, with weathering indices (TRB, Si/(Al+Fe), CIA, BDI, Parker Index) typical for the ferrallitic domain. In contrast, the soils in Z3 are moderately weathered with mineral reserves 10 times higher. Desilication is strong in Z1-2, but particularly in Z1 where gibbsite occurs with kaolinite. In contrast, Z3 soils contain weatherable minerals (mica, feldspar, plagioclase). The contents of leaf Ca, Mg and K are higher in Z3 than in Z1-2 while Ca depletion correlates with a relative excess of K, suggesting a Ca-K antagonism in cassava. Strong desilication occurs in Z1 soils where bioavailable silicon is extremely low. Yet, we could extract plant phytoliths in all sites, with varying coatings of aluminum, which thus seems to be taken up by cassava.