Nature of Plant Components Identified in Soil Humic Acids

INTRODUCTION The origin of the humic acid fraction of soils is still unclear in spite of extensive investigations accomplished over the last years (ref. 1). Different origins have been proposed, based on the most recent theory or on the applica­ tion of the latest analytical method. Thus, it has been proposed that soil humic acids are derived from lignins, microbial activity and synthesis of melanins, reaction between sugars and ami no acids, etc. (ref. 1). Although there is no doubt that the components mentioned are involved in the formation of humus, recent investigations have demonstrated that lignin, sugar and protein moieties are peripheral or loosely attached to the so-called core of soil humic acid and can easily be removed by acid hydrolysis (ref. 2). The question arises what the structure and origin of the more resistant part is. This core is not or hardly affected by acid hydrolysis or mild oxidations but is extensively degraded upon drastic oxidation with permanganate (ref. 3). This paper describes the chemical nature of the most resistant part of soil humic acids based on Curie-point pyrolysis-gas chromatography-mass spectrometric analyses. This technique has proved to be useful in the characterization of biopolymers and geopolymers (ref. 2, 4, 5).