Fertilization of 1 × 4m plots of old‐field vegetation in Minnesota, USA, with various compounds resulted in increased plant tissue concentrations of N, P, K, Ca, Mg, Na, and Mn. Microtrus pennsylvanicus showed significantly greater activity, estimated by scat counts, on plots fertilized with sodium sulphate. Data also suggested that increased Microtus activity in response to elevated plant tissue sodium concentration resulted in greater soil nitrogen availability and higher levels of nitrogen in plant tissues.