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Water-Dispersible Soil Organic-Mineral Particles: II. Inorganic Amorphous and Crystalline Phases in Density Fractions of Clay-Size Particles¹

ABSTRACT

Water-dispersible clay-organic particles from six epipedons and one Bh horizon were fractionated by density. After oxidation of organic matter in hydrogen peroxide, the amorphous particle phase was removed using a kinetic method and analyzed for Si, Al, and Fe. Light particles contained high levels of amorphous oxides and a poorly ordered phyllosilicate matrix. Heavy particles had low levels of amorphous oxides and a more crystalline matrix. A continuum in particle compositions and properties between the density extremes was observed and explained in terms of an aggregate model proposed in part I of the study.

Molar silica to alumina ratios of the amorphous phase in particles ranged from 0.2 to 6.7. Low ratios (<1) occurred in particles from an Aquod Bh horizon and were attributed to organically complexed Al. The amorphous particle phase, mineral matter dissolving at a fast rate, was allophanic (low ratio) in two Andepts, and highly siliceous in an Argid. Intermediate ratios in other soils may have resulted in part from disordered phyllosilicates.

¹ Contribution from the USDA, Science and Education Administration, Federal Research, in cooperation with the Oregon Agric. Exp. Stn. Corvallis, OR 97331. Oregon Agric. Exp. Stn. Tech. Paper No. 4833. Presented in part at 1977 SSSA Meetings, Los Angeles, Calif.

² Research Assistant, Soil Science Dept., Oregon State Univ., and Research Chemist, Western Region of SEA-AR, USDA, Corvallis, Oregon, respectively. Senior author now Research Associate, Forest Science Dept., Oregon State Univ.

Received for publication August 14, 1978. Accepted for publication November 15, 1978.