Effet in vitro de la roche verte de Gangila sur les propriétés physico-chimiques des sols sableux de l'hinterland de Kinshasa (RD Congo)

Kasongo, R. K., Kanyankogote, P., Verdoodt, A., Baert. G., Dumon, M. and Van Ranst, E. 2012. In vitro effect of Gangila green rocks on the physico-chemical properties of sandy soils in the hinterland of Kinshasa (DR Congo). Can. J. Soil Sci. 92: 787–797. The green rocks of Gangila (Lower Congo) are mainly made up of amphibolites and green schists and have a high nutrient capacity. A laboratory experiment was conducted to study the effects of these rock fertilizers on various physico-chemical properties of the sandy soils of Kinshasa's hinterland, DR Congo. These soils are classified as Orthic Regosols in the Canadian system of soil classification (Arenosols in WRB) and are characterized by a low chemical fertility, a very limited water-holding capacity, and an acid reaction resulting in Al and Mn toxicities. Four rates (0.0, 12.5, 25.0, and 50.0 Mg ha⁻¹) of finely ground green rocks were mixed with topsoil (0–25 cm), and incubated and leached for 3, 6, 9, 12, 15, and 18 mo, respectively. The use of these ground rocks as amendment resulted in a significant increase in pH (>5.5), available phosphorus, sum of basic cations (SBC) and CEC, and significantly increased the water retention by about 3 % (volume). Increases of available P by up to 50 %, SBC by 2,2 cmolc kg⁻¹ and CEC by 75% were obtained in soils amended with 50.0 Mg ha⁻¹ of green rock after 6 mo of incubation. Furthermore, the increased soil pH and decreased pH₀ (point of zero charge) due to silicate sorption, brought about a significant increase of negative charges, CECB (basic cation exchange capacity) and CECT (total cation exchange capacity) with complete neutralization of phytotoxic exchangeable Al (pH−H₂O>5,5). The leaching study revealed that the application of ground rocks of Gangila significantly affected the cation dynamics and immobilized certain micronutrients (Fe, Mn, Cu, Zn). The relative loss of monovalent cations (K⁺, Na⁺) was significantly higher compared with that of the bivalent ones (Ca⁺⁺, Mg⁺⁺). Green rocks of Gangila can be used as a natural amendment to increase soil fertility of sandy tropical soils.