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

Abstract

This study clarifies the morphology, genesis, physico-chemical properties and classification of
\nsoils developed from volcanic parent materials of Mbeya Region, Tanzania. Six typical pedons
\n(MWK 01, IFIG 02, MKY 03, MWZ 04, KYE 01 and NDE 01), were identified, described and 33 soil
\nsamples analyzed for physical and chemical characteristics. Results indicate that all pedons were
\nvery deep, with textures ranging between fine and coarse. Soil moisture retention ranged between
\nlow and medium (78 - 101 mm/m). Some pedons had volcanic ash layers of varying thicknesses
\nand buried 2BC, 2Bwb or 3Bwb horizons, typical of recent volcanic soils. Topsoils had low bulk
\nand particle densities ranging between 0.70 to 1.26 g cm -3 and 1.95 to 2.55 g cm -3 , respectively.
\nOrganic carbon (OC) content ranged from medium to very high (1.29 to 5.58%). The studied
\npedons had extremely acidic to very slightly acidic pH ranging from 4.02 to 6.58. Cation
\n-1
\nexchange capacity (CEC) ranged from medium to very high (16.8 - 41 cmol(+) kg soil for
\n-1
\ntopsoils and 21 - 42.6 cmol(+) kg for subsoils). All studied pedons had pH NaF > 9.5, reflecting an
\nexchange complex dominated by amorphous Fe and Al oxides and/or humus complexes.
\nPhosphate retention capacity (PRC) ranged from 25 to 97% and one pedon (MWZ 04) met the
\nandic properties requirement of PRC ≥ 85%. SiO 2, Al 2 O 3 and Fe 2 O 3 ranged from 46.5 to 62.1%,
\n26.3 to 38.4% and 4.0 to 9.8%, respectively. On the basis of computed Chemical Index of
\nAlteration (CIA), the degree of weathering of the studied pedons followed the trend NDE
\n01>MKY 03>MKW 01>KYE 01>IFIG 02>MWZ 04. According to USDA Soil Taxonomy, the studied
\npedons were classified as Alfisols (pedons MKW 01, MKY 03 and NDE 01), Inceptisol (pedon IFIG
\n02), Andisol (pedon MWZ 04) and Entisol (Pedon KYE 01), respectively correlating to Alisols,
\nCambisol, Andosol and Umbrisol of WRB for Soil Resources. The studied soils were generally
\nrated as having low to medium fertility.