This study was conducted to determine the variability of reshaped spoil materials by characterizing samples collected within plant rooting zones for several chemical and physical parameters, and to determine the sampling intensity needed to describe this variability. With the exception of a recent study by Klages and Hopper (1982), clay mineralogy data for mine spoils in the Northern Great Plains are lacking. Therefore, the clay mineral composition of selected spoil materials and surrounding unmined soils was determined.