The distribution and genesis of eroded phase soils in the conterminous United States

The delineation and mapping of eroded phases of existing soil series has been an important activity throughout the history of soil survey activities in the United States, with implications for land management, crop production and the estimation of historical sediment losses and fluxes. An analysis of the SSURGO database shows that 462,979 km² of eroded phase soils (16% of which were classified as severely eroded) are mapped in the conterminous United States, with 9% of 2013 cultivated lands occurring on eroded phase soils. Eroded phases of 2265 soil series in 9 soil orders (excluding Gelisols, Oxisols and Histosols) have been identified and mapped. Examining the distribution of eroded phase soils within survey-independent large-scale physiographic (EPA Level III Ecoregion) boundaries reveals consistent patterns in land-use histories and eroded soils, and that the pattern and distribution of eroded phase soils at regional to continental scales are responsive to the five soil forming factors. The proportion of ecoregion land area mapped as eroded phase was significantly affected by topography, with eroded phase soils peaking in ecoregions with topographic ruggedness indices (TRI, a normalized elevation difference index) between 1 and 2. Among TRI groups, the proportion of total ecoregion land area mapped as eroded phase was significantly related to the historical maximum cultivation intensity, while among a subset of ecoregions with significant histories of cultivation, eroded phase proportion was related to the rainfall-runoff erosivity (RUSLE R-Factor). Of the 2265 named soil series with mapped eroded phases, 73% had family particle size classes of fine, fine-loamy and fine-silty which corresponds with the peak in the RUSLE erodibility factor (K) with respect to particle size. Lastly, ecoregions with histories of significant cultivation for more than 100 years had greater proportions of their land area mapped as eroded phase. These results suggest that despite morphological constraints and subjective factors in the delineation of eroded phases, these soils should be viewed as unique pedological entities that hold lasting value for understanding the effects of accelerated erosion on soil morphology, crop production, and ecosystem services.