Characteristics of soil humic substances fractionated in relation to particle weight

Particle weight distribution of humic acids (A, B, Rp, Po, and P types) was estimated by applying the successive gel filtration method using five grades of Sephadex, and the fractions with different particle weights were characterized by analyzing their optical properties and infrared absorption (I.R.) spectra. With the progress of humification, the large particle components and polydis-persion of the particle weights decreased. The degree of humification in the A and B types was maximum for the middle particle weight fraction, while in the Rp, Po, and P types it was higher In the fractions with smaller particle weight. The differences in the optical properties among the fractions were more pronounced when the humification degree Increased. I.R. spectra of the fractions showed differences in Intensities at some bands; near 3,300, 2,920-2,840, 1,720, near 1,650, 1,600, near 1,520, 1,460-1,360, 1,180-1,030 cm-1 and a few specific bands. The large particle weight fractions contained colorless components and in the A, B, and Po types, additionally, highly humified components. All the I.R. spectra of these fractions showed absorption peaks arising from proteins, polysaccharides, aliphatic compounds and lignin-Iike substances. The I.R. spectra of the middle particle weight fraction and below, In the A type, did not show absorption peaks arising from the above components, but in the Rp type, the presence of proteins, polysaccharides and aliphatic compounds was confirmed.