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Iron Distribution in a Developmental Sequence of Soils from Mica Gneiss and Schist¹

ABSTRACT

Upland residual soils formed from mica gneiss and schist in the Piedmont and Blue Ridge provinces of North Carolina represent developmental sequences where particle-size class, profile development, and mineralogy are related to landscape position and slope. In this study the distribution of total and free iron in 10 Hapludult and Dystrochrept pedons is examined and interpreted with respect to degree of profile development. Primary mineral alteration and B horizon free iron/total iron ratios vary nearly systematically with particle-size class and solum thickness. The latter two parameters were found to be good indicators of degree of profile development and hence of relative soil age. The free iron/total iron ratio was found to be a particularly sensitive indicator of relative soil development. The ratio reflects the degree of primary mineral alteration, the amount of clay formed, as well as the pedogenic process of illuviation. The B horizon ratios are much better indicators of relative soil development than those from either C or A horizons. Ratios of free iron/total iron in A horizons are influenced by downslope movement of surficial material, the low extractability of iron oxides of large particle size, and clay content differences. Free iron/total iron ratios of C horizons are dependent on sampling depth, the amount of clay formed, and the degree of illuviation that has occurred below the solum. All soils had oxidic ratios (%Fe₂O₃ + %gibbsite/%clay) > 0.2; however, all but one was excluded from the oxidic mineralogy class due to having >40% mica. A simple free iron/total iron ratio in B horizons, which is systematically related to the degree of weathering of primary minerals and the degree of morphological soil development, better serves as soil family criteria than the present % Fe₂O₃ + % gibbsite/% clay.

¹ Paper no. 9261 of the Journal Series of the North Carolina Agric. Res. Serv., Raleigh 27695.

² Research Associate and Professor of Soil Science, Dep. of Soil Science, North Carolina State Univ., Raleigh 27695.

Received for publication April 9, 1984. Accepted for publication November 16, 1984.