Genesis of podzols on coastal dunes in southern Queensland .III. The role of aluminum organic complexes in profile development

Abstract

Organic matter was extracted sequentially with 0.1 M HC1, 0.5 M HCl and 0.5 M NaOH from three soil samples consisting of the dark brown organic nodules and matrix material from the Bhs2 horizon and the untreated Bhs3 horizon material of a freely drained podzol from Cooloola. The NaOH extract was further divided into fulvic and humic acid. After extracts were separated on G-50 Sephadex gel, titration data demonstrated that carboxyl groups in the collected fractions ranged from 8.8% to 61.3% of the total carbon although some overestimate of the carboxyl content may result from the chromic acid method used. Fractions excluded by the gel made up >95% of each extract and the first fraction collected in each case was the lowest in carboxyl content. These fractions from the 0-1 M HC1 extracts were generally low in carboxyl content (8.8%-24.9%) as were those from the humic acids (10.7-11.2%) although the major remaining humic acid fractions were extremely high (58.6-61.3510). The 0.5 M HCl and fulvic acid extracts appeared similar in chemical properties with carboxyl content of the separated fractions ranging from 16.4 to 44.0%. Molar absorptivities were in the order 0.1 M HCl < 0.5 M HC1< fulvic acid < humic acid. The pKa values of the acids were found to increase in the order humic acid < fulvic acid = 0.5 M HCl < 0.1 M HCl as well as in the order Bhs2 (dark brown nodules) < Bhs2 (yellow brown bulk) < Bhs3 for each fraction. Ease of flocculation of the extracts by aluminium counter ions decreased in the order humic acid > fulvic acid > 0.5 M HCl > 0.1 M HCl. It was demonstrated that other organic extracts low in aluminium content could be used to remobilize flocculated humic acid through redistribution of the aluminium bound to the precipitated phase into the solution phase. From these and other data, a hypothesis explaining the specific manner in which organic matter is arrested during podzolization is proposed. Some major factors appear to be (a) the pH of the horizon, (6) pKa and aromaticity of the organic acids and (c) availability of aluminium to the organic fractions. Remobilization of precipitated organic matter requires the presence of organic acids of low aluminium content in the percolating soil solution. This hypothesis adequately describes the process by which distinct Bh, Bhs and Bs horizons are formed and evolve during profile genesis.