Method for the Sequential Extraction of Organic Matter from Soils and Soil Fractions

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Abstract
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.