Submitted to: Soil Science Society of America Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/2/2003
Publication Date: 6/1/2003
Citation: Soil Science Society of America Journal. 2003. V. 67, p. 1220-1226 Interpretive Summary: Soils low in organic matter and clay have aggregates that are easily disrupted by raindrop impact which causes the soil surface to seal, thus reducing infiltration, and resulting in excessive runoff and erosion losses. Even at low concentrations, iron oxides have the ability to increase soil aggregate resistance to disruption by rainfall. Ferrihydrite, a naturally occurring iron oxide byproduct of water treatment plants, was added as an amendment to several soils to evaluate its ability to improve aggregation. Our results show that ferrihydrite applied at rates approximating 3 tons/acre can effectively stabilize soil aggregates, increase infiltration, and lower erosion losses on acidic soils (below pH 7.0). Basic soils (above pH 7.0) did not respond to the amendments as well, because ferrihydrite acted as a dispersing agent above pH 7.0. These results clearly indicate that ferrihydrite is an effective aggregating agent that can improve infiltration for acidic soils.
Technical Abstract: Soil aggregates low in organic matter and clay content are generally susceptible to disintegration at low rainfall energies. This study was conducted to evaluate the effectiveness of ferrihydrite (Fe5 HO8 . 4H2O) at stabilizing soil aggregates. Ferrihydrite was added in slurry form to five different soils with textures ranging from clay loam to loamy fine sand. Soil pH ranged from 5.1 to 7.6; clay and organic matter contents ranged from 99 to 310 g kg-1 and 2.6 to 46.6 g kg-1, respectively. The soils were amended with ferrihydrite at rates equivalent to 0, 0.34, 3.36, 16.80, and 33.60 Mg ha-1, then packed to a depth of 7.6 cm in plexiglass cylinders. Simulated rainfall was applied to these cylinders at an intensity of 64 mm h-1 for 1.5 h after which estimates were made of infiltration, total runoff, soil loss, and sediment size distributions. As ferrihydrite increased from 0 to 16.80 Mg ha-1 on the acid soils, infiltration increased an average of 21.5% while runoff and soil loss decreased 20 and 40%, respectively. For the alkaline soils, infiltration decreased an average of 37% in conjunction with a 21% increase in runoff and a 34% increase in soil loss. Sediment size distributions as a function of increasing amendment rates for the acid soils showed an average increase of 24% for the >250 µm fraction, 22% for the 250 to 53 µm fraction, and a decrease of 21% for the <53 µm fraction. The alkaline soils had an average decrease of 15% for the >250 µm fraction, 14% for the 250 to 53 µm fraction, and an increase of 46% in the <53 µm fraction. These results indicate that when equilibrated in acid soils, ferrihydrite develops a net positive charge and forms bonds with negatively charged soil particles to increase water stable aggregation. Conversely, under alkaline soil conditions, ferrihydrite develops a net negative charge which creates a dispersive condition that leads to aggregate instability.