Effects of agroecological land use succession on soil properties in Chemoga watershed, Blue Nile basin, Ethiopia

For the first time a method is presented to study the distribution of metals between humic and fulvic fractions in natural soils. Total organometallic complexes are separated by extraction with 0.1 M pyrophosphate. After acidification to pH 1.5 with H2SO4 the fraction associated with humic acids is separated by precipitation and the remaining solution contains the complexes associated with fulvic complexes. Concentrations of 11 elements (Al, Fe, Ca, Mg, K, Mn and the trace elements Cu, Pb, Zn, Cd, and Cr) were determined in the pyrophosphate and the fulvic acid extracts, and the fraction bound to humic acids was estimated by difference. Typical results from application of the method to andosols, cambisols, and podzols are presented. Fulvic acid is generally the main humic fraction reacting with metals, except for Ca where around 70% of the organic bound pool is found in the humic fraction. The corresponding figure is of the order of 30–40% for Al, Fe, Mn and Cr and even less for Pb, Cu, Zn, and Cd. For some elements the humic/fulvic distribution appears to be different between soil types. Some factors that may disturb the results obtained by the present method are discussed.