Wednesday, November 15, 2006 - 10:45 AM

Chemistry and Mineralogy of Selected Kenyan Acid Soils.

Pamela Obura1, Darrell G. Schulze1, Cliff T. Johnston1, John R. Okalebo2, and Caleb Othieno2. (1) Purdue Univ, Agronomy Dept, 915 W State St, West Lafayette, IN 47907, (2) Moi Univ, Soil Science Dept, PO Box 1125, Eldoret, 30100, Kenya

Food production in many parts of the developing world is constrained by acid soils that are low in plant-available phosphorus but high in aluminum. Plant breeders are attempting to address these dual constraints by breeding crops with enhanced ability to exude organic acids from their roots, which appears to detoxify Al while releasing P into the soil solution for plant uptake. Crops bred on one type of soil may respond very differently, however, when grown on a different soil. Objectives of this study are; to characterize selected Kenyan acid soils, and compare their properties to acid soils of central Brazil (where plant breeding has occurred). Eleven pedons representing geomorphic provinces; (1) the highlands west of the Rift Valley, (2) the Plateau /upland plains west of the Rift Valley) and, (3) the volcanic foot ridges, east of the Rift Valley, were described and sampled. Characterization consists of wet chemical and x-ray diffraction analysis. Exchangeable cations, cation exchange capacity (CEC) and effective CEC were determined by the unbuffered salt solution method. CEC ranges from 8.5 to 22 cmolkg-1 in the topsoil. Soils east of the Rift Valley have higher exchangeable Al than soils west of the Rift Valley. Kaolinite and/or halloysite are the dominant clay minerals in all the pedons.  Mica and mixed layer minerals may be present in soils west of the Rift valley, which is consistent with the ~2 times greater exchangeable K and higher CEC of soils in the western highlands. Soils east of the Rift Valley contain more gibbsite than their western counterparts, consistent with their lower pH and higher Al saturation. We attribute the observed differences to the fact that soils in the eastern region developed from porous volcanic ash deposits associated with Mt. Kenya, while soils from the western region developed from less porous igneous rocks.