The Use of Chelating Reagents and Alkaline Solutions in Soil Organic‐matter Extraction

Abstract

The extent of organic-matter extraction tended to rise with the pH of the extractant solution, as did the proportion of the humic-acid fraction in the extracts. Chelating reagents extracted more organic matter than other extractants of comparable pH, but extraction by all reagents rose with increasing time and temperature of extraction. The equivalent weights of both the humic and fulvic fractions of alkaline extracts rose with the temperature of extraction, as did the extent of conversion of the humic-acid fraction to acid-soluble compounds. The humicacid fraction was flocculated by various salts to a much greater extent than the fulvic fraction. The nature of soil organic-matter extraction is considered in the light of these results.ALTHOUGH the classical method of extracting soil organic matter with caustic soda still remains quantitative1 the most efficient available, there is evidence that the materials extracte B may have undergone considerable modification in the process, by oxidation and hydrolysis. Hobson and Page ( I 32) found 0.2 N Na,CO, to be rather less than half as effective as 0-5 d NaOH in extraction, but little is known of the effect of pH on the efficiency of extraction or on the degree of modification of the organic matter extracted.Extraction of organic matter by neutral solutions is generally slight, but Simon (1929) found sodium fluoride to be an efficient extractant. Subse uently Bremner et al. (1946, 1949) and Hamy and Leroy (1952) form chelates or co-ordination complexes with calcium an the trivalent metal cations, which often prevent the ready deflocculation of much of the organic matter in soils, are ve efficient in the extraction of humus. of extraction, and pretreatment of the soil with acid, did not great y affect the efficiency of extractions by pyrophosphate, whereas they did influence the efficiency of extractions by NaOH. Such results may imply that, unlike NaOH, the chelating reagents extract only a s ecific fraction of cations which can be chelated by the extractant anion. If such reagents are fairly specific a procedure, more rapid and efficient than that of Tiulin (1938), for the fractionation of soil or anic matter according to purpose of this paper to examine this possibility further.x have s x own that neutral solutions of anions which can recipitate or Bremner and Lees (1949) found x t at time, temperature, and re etition the organic matter present in a soil, namely that floccu H ated by the metal the dominant cation rendering it insoluble cou 'i d be devised, and it is the P Experimental MethodsThe soil used in all extractions was a sample of neutral topsoil from the pasture in the University Parks, Oxford, prepared to pass the I-mm.

Micro‐scale variation of solid‐phase properties and soil solution chemistry in a forest podzol and its relation to soil horizons

“…This extraction was followed by a citrate-bicarbonate extraction (Me,b), by an oxalate treatment (Me, ; mainly fenihydrite) (Schwertmann, 1964), and finally by a dithionite-citratebicarbonate extraction (Med) representing well-crystallized iron (hydr)oxides (Mehra & Jackson, 1960). The citratebicarbonate solutions were yellow to brown due to the extraction of soil organic matter similar to alkaline bicarbonate solutions (Evans, 1959;Posner, 1966); their absorption at 400 nm was measured after diluting them 1 : 50 (called uv&). After these sequential extractions, all samples were analysed for their particle size distribution with a laser scattering device.…”

Experimental Conditions Concerning Potentiometric Titration of Humic Acid

“…. EDTA was chosen as extractant as it is assumed that strong ligands extract organic matter precipitated b metal ions (Evans, 1959; Morextraction would result in a more definite fraction than obtained by NaOH. The extraction procedure did not give a reat yield but the A low ash content was referred as the subject of the present investigainorganic constituents which are known (Martin and Reeve, 1958) to interact protolytically.…”

Acid pyrophosphate extraction of soil fulvic acids

“…Bremner & Lees (1949) found 0.1 M Na,P,O, (pH 10) the most effective extractant out of 80 electrolytes examined. Evans (1959) noted that for a given soil the ratio of humic acid to fulvic acid extracted decreases with decreasing pH.…”

“…Several workers have used neutral or alkaline pyrophosphate for the co-extraction of fulvic and humic acids from soils (Evans, 1959;Posner, 1966;Pierce & Felbeck, 1972;Goh & Reid, 1975). Stevenson (1982) noted that it is a more selective and milder reagent than NaOH, while Posner (1966) reported that pyrophosphate at pH 7 gave a humic acid with lower ash content (0.2%; cf.…”