Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/15/2002
Publication Date: 3/1/2003
Citation: DUIKER, S.W., RHOTON, F.E., TORRENT, J., SMECK, N.E., LAL, R. IRON OXIDE CRYSTALLINITY EFFECTS ON SOIL AGGREGATION. SOIL SCIENCE SOCIETY OF AMERICA JOURNAL. 67:606-611. 2003. Interpretive Summary: Iron oxides are an important soil component in terms of stabilizing soil aggregates and reducing erosion losses. Their importance, however, can vary greatly between soils on slope positions in the same field even though no differences exist in concentrations. This discrepancy was investigated by chemically characterizing several soil layers in terms of relative amounts of amorphous (gel-like) versus crystalline iron in their iron oxide fractions. When these data were compared with soil aggregate stability data, the amorphous iron oxides were found to be effective at forming water stable aggregates. This information will permit more precise mapping of soil erodibility on a field or watershed scale.
Technical Abstract: Differences in crystallinity may explain why total Fe oxide content has a variable effect on aggregate stability. This study was conducted to determine the contribution of the different crystalline Fe oxide phases to aggregation. Soil samples with a range of poorly crystalline Fe oxide contents were characterized for water stable aggregates >0.25 mm (WSA), mean weight diameter (MWD), soil organic carbon (SOC), particle size distribution, pH, exchangeable cations, citrate/bicarbonate/dithionite (d) and acid ammonium oxalate (o) extractable Fe, Al and Si. The WSA and MWD range from 23 to 95%, and 0.3 to 5.1 mm, respectively. The effects of Feo (1.1 to 6.8 g kg**- 1), Fed (3.2 to 19.6 g kg**-1), SOC (2.4 to 24.0 g kg**-1) and clay (141 to 467 g kg**-1) contents on WSA and MWD of both A and B horizons of these soils was studied using linear regression. The poorly crystalline Fe oxide (Feo) and SOC contents are significantly correlated with WSA in the A horizons (r = 0.97 and 0.96, respectively) and in the B horizons (r = 0.86 and 0.87, respectively). When regressed against MWD, Feo has an r of 0.95 in the A, and 0.99 in the B horizons. The linear correlation coefficients of MWD vs. SOC contents is 0.99 in the A and 0.88 in the B horizons. Clay and Fed contents are not significantly correlated to WSA or MWD. Apparently, the Feo component (poorly crystalline) is more effective than Fed at stabilizing soil aggregates, even though it is present in lower concentrations. The Feo component appears more important than SOC in terms of WSA and MWD for soils with relatively low soil organic matter contents.