Ferrihydrite influence on infiltration, runoff, and soil loss

Soil aggregates low in organic matter and clay contents are generally susceptible to disintegration at low rainfall energies. This study was conducted to evaluate the effectiveness of ferrihydrite (Fe-5 HO8.4H(2)O) at stabilizing such aggregates, using five soils with a wide range of physical and chemical properties. The soils were amended with ferrihydrite at rates equivalent to 0, 0.34, 3.36, 16.80, and 33.60 Mg ha(-1), packed to a depth of 7.6 cm in plexiglass cylinders, and then exposed to simulated rainfall at an intensity of 64 mm h(-1) for 1.5 h. The erodibility data indicated that as ferrihydrite increased from 0 to 16.80 Mg ha(-1) on acid soils, infiltration increased an average of 21.5% while runoff and soil loss decreased 20 and 40%, respectively. Conversely, infiltration decreased 37% while runoff and soil loss increased 21 and 34%, respectively for alkaline soils. Further, sediment size distributions measured at these same ferrihydrite rates indicated that the >250-, and 250- to 53-mum fractions increased 24 and 22% for acid soils and decreased 15 and 14%, respectively in alkaline soils. The <53-mu m fraction decreased 21% in the acid soils and increased 46% in the alkaline soils. These results suggest ferrihydrite develops a net positive charge in acid soil environments that leads to formation of bonds with negatively charged soil particles and an increase in water stable aggregation. Conversely, in alkaline soils, ferrihydrite becomes negatively charged which results in dispersion and aggregate instability. Thus, ferrihydrite appears to be an effective amendment for reducing runoff and soil loss from acid pH soils at amendment rates between 3.36 and 16.80 Mg ha(-1).