The sodium hypothesis of Aumann and Emlen (1965) states that peak population densities of microtine rodents are strongly correlated with soil sodium levels. We tested this in southern Indiana with snap—trapping data from 21 grassland areas occupied by Microtus pennsylvanicus and M. ochrogaster. A total of 128,052 trap—nights were expended from 1965 to 1970, and soils from area were analyzed for seven soil parameters including sodium. Soil sodium levels did not correlate with either peak density or mean density levels of either Microtus species. In M. pennsylvanicus 38% of the variation in peak densities could be explained by differences in soil pH and potassium levels. Cover may be an important predictor of peak densities but this was not measured. Microtus pennsylvanicus is not confined to bottomland soils in the Bloomington, Indiana, area and both species of Microtus coexist over a broad range of soil types. See full-text article at JSTOR
