Chemical characteristics of humic acids from five soils in Kenya

Humic acids (HAs) isolated from five soils (Luvisols, with 460–600 mm rainfall, Nitosols and Ferralsol, with 1060–1950 mm rainfall) from different ecological regions of Kenya were characterized by ¹³C NMR and IR spectroscopy, elemental analyses and optical measurements. The data suggest that excessive rainfall in sub-humid regions may lower the molecular weights of the HAs as well as their aromaticities. Concentrations of aliphatic components, including aminoacids and carbohydrates, in the HAs from the sub-humid regions were higher (40–42 wt/wt %) than those in the HAs isolated from soils of the semi-arid regions (29–32 wt/wt %). Chemical characteristics of the soils from the sub-humid regions of Kenya resembled those of the HAs from temperate regions but differed from those of HAs originating from arid regions. Annual rainfall and vegetation appeared to affect the composition of HAs but effects of elevation and temperature were less clear.