Morphology, Genesis, Physico-chemical Properties, Classification and Potential of Soils Derived from Volcanic Parent Materials in Selected Districts of Mbeya Region, Tanzania

This study clarifies the morphology, genesis, physico-chemical properties and classification of soils developed from volcanic parent materials of Mbeya Region, Tanzania. Six typical pedons (MWK 01, IFIG 02, MKY 03, MWZ 04, KYE 01 and NDE 01), were identified, described and 33 soil samples analyzed for physical and chemical characteristics. Results indicate that all pedons were very deep, with textures ranging between fine and coarse. Soil moisture retention ranged between low and medium (78 - 101 mm/m). Some pedons had volcanic ash layers of varying thicknesses and buried 2BC, 2Bwb or 3Bwb horizons, typical of recent volcanic soils. Topsoils had low bulk and particle densities ranging between 0.70 to 1.26 g cm-3 and 1.95 to 2.55 g cm-3, respectively. Organic carbon (OC) content ranged from medium to very high (1.29 to 5.58%). The studied pedons had extremely acidic to very slightly acidic pH ranging from 4.02 to 6.58. Cation exchange capacity (CEC) ranged from medium to very high (16.8 - 41 cmol(+) kg-1 soil for topsoils and 21 - 42.6 cmol(+) kg-1 for subsoils). All studied pedons had pHNaF > 9.5, reflecting an exchange complex dominated by amorphous Fe and Al oxides and/or humus complexes. Phosphate retention capacity (PRC) ranged from 25 to 97% and one pedon (MWZ 04) met the andic properties requirement of PRC ≥ 85%. SiO2, Al2O3 and Fe2O3 ranged from 46.5 to 62.1%, 26.3 to 38.4% and 4.0 to 9.8%, respectively. On the basis of computed Chemical Index of Alteration (CIA), the degree of weathering of the studied pedons followed the trend NDE 01>MKY 03>MKW 01>KYE 01>IFIG 02>MWZ 04. According to USDA Soil Taxonomy, the studied pedons were classified as Alfisols (pedons MKW 01, MKY 03 and NDE 01), Inceptisol (pedon IFIG 02), Andisol(pedon MWZ 04) and Entisol (Pedon KYE 01), respectively correlating to Alisols, Cambisol, Andosol and Umbrisol of WRB for Soil Resources. The studied soils were generally rated as having low to medium fertility.