Method for the Sequential Extraction of Organic Matter from Soils and Soil Fractions
1989
Schnitzer, M. | Schuppli, P.
The extraction of organic matter (OM) from soils is one of the unresolved problems in soil chemistry and biochemistry. To contribute to the solution of this problem, we developed a procedure for the extraction of OM in which the following sequence of extractants is used: (i) n-hexane; (ii) chloroform; (iii) 0.1 M Na₄P₂O₇ solution under N₂; (iv) 0.5 M NaOH solution under N₂; and (v) distilled H₂O. The n-hexane extracts alkanes and fatty acids; chloroform removes fatty acids, long-chain alcohols, and wax esters; 0.1 M Na₄P₂O₇ solution extracts OM “complexed” to metals and clays; whereas 0.5 M NaOH and H₂O remove “free” OM, that includes less decomposed materials. The method was tested on three soils and 25 particle-size fractions separated from these soils, which varied in mineralogical composition and OM content. Our data show that 0.1 M Na₄P₂O₇ solution extracts relatively more OM from finer soil particles than from coarser ones while the reverse is true for 0.5 M NaOH solution. In general, OM is more readily extracted from soils low in clays than from those rich in clays. Between 35.4 and 80.9% of the soil-C and between 21.0 and 75.6% of the soil-N are solubilized. Determinations can be made at each step of the procedure of how much of the initial OM and which major components are extracted. Humic acids isolated from the 0.1 M Na₄P₂O₇, 0.5 M NaOH and H₂O extracts have analytical characteristics which are very similar to those of typical soil humic acids. Contr. no. 88-68.
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