An X‐ray Diffraction Study of Humification

Abstract

HUMIFICATION is the first stage of a slow process whereby vegetable debris is converted into peat, coal, anthracite, and finally into macrocrystalline raphite. The vegetable debris contains initially a diversity humus, which represents a ve early stage of humification, must also consist of a complex mixture o 7 chemical compounds. Since, however, the end-point of humification and coalification is a single chemical entity, viz. macrocrystalline graphite (certain meta-anthracites X-ray powder photographs which show a well-developed grap ite pattern), then it appears reasonable to suppose that during these changes there is a tendency towards the formation of chemical structures which become more and more alike as the slow process progresses: although intermediate stages may show a range of chemical composition, their classification together as humic acids and humins is justifiable on the grounds of certain specific characteristics and also is not contrary to the ideas of modem crystal chemistry. Waksman has pointed out that most of the investigations into the chemical properties of humus have been carried out on materials obtained from either peat or coals and, therefore, the results obtained should not be applied indiscriminate1 to the humus obtained from soils. Artificial humic acids can be prepare d by a variety of methods, and although the products show a certain similarity to those from natural sources, there is no proof of close physical and chemical similarity. According to their behaviour with aqueous alkali, humic substances are now usually classified as humic acids and humins; the former are easily soluble in cold aqueous alkali, whilst the latter dissolve only on heating. The different alkali solubility of the humins has been interpreted by Marcusson [2] in the sense that these substances are dehydration products or anhydrides of humic acid. The decrease in the solubility in aqueous ammonia which occurswhen the acids are dried at 105" C. has been ascribed to an alteration in the colloidal state, rather than to anhydride formation. The composition of humic acids obtained from peat and brown coal is not as constant as that of a well-defined chemical compound, but in many cases it a proximates to the empirical formula C,H,O,. In an investigation of of their degree of humification: it increases with the extent of the transformation of the original vegetable matter into humic substances and subsequently decreases radually with the process of coalification. Phenolic of the brown coals, and the proportion ecreases gradually duringthe humification process : they are still present in significant amounts in coals of rank intermediate between brown coals and bituminous coals. Ubaldini suggested the following approximate formula for humic acid : of chemica 7 compounds, and as Waksman [I] has pointed out, soil r y e Ita P ian brown coals Ubaldini [3] found that carboxyl acidity is a function B groups, on the other K and, are already resent in the parent organic matter /(COOH), ' ( W 2 C78H5205-(0H)7Journal of Soil Sci...