Alfalfa (Medicago spp.) interseeded into rangeland has been shown to improve the quantity and quality of forage available for grazing. Information regarding soil responses to interseeded alfalfa in rangeland, however, is lacking. A study was conducted to investigate the effects of alfalfa transplanted into native rangeland on soil organic carbon and total nitrogen. The study was located approximately 5 km south of Mandan, ND USA on a Temvik silt loam soil (USDA: Fine-silty, mixed, superactive frigid Typic Haplustoll). Treatments included three alfalfa cultivars transplanted into rangeland and a native vegetation control. Treatments were applied using a randomized block design with five replications. Soil properties measured during the study included soil bulk density, soil organic carbon, and total soil nitrogen. Measurements were made in 2001 (baseline) and again 2005 following four years of alfalfa growth. Samples were collected using a step-down probe in depth increments of 0-10, 10-20, 20-30, and 30-40 cm. Duplicate cores from each treatment were composited by depth. Soil carbon and nitrogen were quantified by the dry combustion method. Data may be used to understand soil property responses to interseeded alfalfa in rangeland. Data are generally applicable to rangelands under a semiarid Continental climate for the following soil types: Grassna, Linton, Mandan, Temvik, Williams, and Wilton.