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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
-1
exchange capacity (CEC) ranged from medium to very high (16.8 - 41 cmol(+) kg soil for
-1
topsoils and 21 - 42.6 cmol(+) kg for subsoils). All studied pedons had pH NaF > 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%. SiO 2, Al 2 O 3 and Fe 2 O 3 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. |
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