The Bremner Contribution to the Isolation of Soil Humic Materials and to Their Nitrogenous Components.

Aqueous sodium or potassium hydroxide solutions were the solvents of choice for isolating humic substances (HS) since 1786. In the early 1950s, when at Rothamsted, Jack Bremner recognised that aqueous base gives artefacts from the oxidation of some components of HS. That led him to use neutral salt solutions, and he introduced dilute neutralized sodium pyrophosphate which complexes the divalent and polyvalent cations that render HS largely insoluble in water. He showed how pyrophosphate solutions at pH 7 complex humate insolubilizing divalent and polyvalent cations, allowing the more highly oxidized and negatively charged humic components to solubilize. His use of HF for the release of HS complexed with clays was, like much of his work, ‘ahead of the field’. However, he saw that the bulk of the humic materials are not dissolved in aqueous base or neutral salt solutions. We now know that significantly greater amounts of these are released when base + 6 M urea follows exhaustive extractions with base or pyrophosphate. Using DMSO + 6% concentrated H₂SO₄, following the base/urea extraction, almost all of the humic components can now be isolated and studied. In the ‘days of sail’, in so far as modern analytical instrumentation is concerned, Jack Bremner pioneered studies of amino nitrogen, amino sugars, and amino acids in HS using paper chromatography, and the Van Slyke procedure for determinations of α-amino N. He was first to observe degradations of tyrosine and tryptophane in alkaline digests. HPLC and modern analytical instrumentation have advanced the quantitative leads given by Jack Bremner, and ¹⁵N NMR has shown that the amino sugars and amino acids are, overwhelmingly, the major N-containing components in HS. His pioneering HF work has led to an awareness of how biological molecules such as peptidoglycans, peptides, and polysaccharides, waxes, and lipids sorbed on soil clays are protected from microbial degradation.