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This study was done on soils developed from volcanic parent materials including pumice and ashes originating from various volcanoes in and around the Rift Valley of South Western Tanzania. The soil moisture and temperature regimes in the area are udic and isothermic respectively. One pedon close to Lake Nyasa (Mwaya) had aquic SMR and isohyperthermic STR. Soil OC content varied widely among and within pedons, with surface horizons generally having higher values (high to very high). N levels also varied similarly and correlated very well with OC. C:N ratios were very variable and increased with increasing OC. Based on the distribution of OC, N, C:N ratios and the relationship among these parameters, the Tanzanian volcanic ash soils compare quite well with known Japanese volcanic ash soils, although the absolute values of OC were much higher for Japanese than for Tanzanian soils. Results on humic acid studies indicate that the soils predominantly contain A type humic acids particularly in the surface A horizon, buried A, AB and BA horizons. These horizons are characteristically humusrich with intense dark colors. UV-VIS absorption spectra revealed in some soils the presence of Pg fraction with absorption bands around (280), 450, 570 and 615 nm, which are normally associated with the presence of dihydroxyperylenequinone (DHPQ) nuclei responsible for the absorption bands at those wavelengths. On the basis of ∆log K, RF, HE, PQ values and the UVVIS absorption spectra, the characteristics of HAs of the Tanzanian volcanic ash soils compare very well with Japanese volcanic soils |
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