An efficient sampling scheme for evaluating seasonal changes of inorganic nitrogen in a forest soil was designed. It was based on variances of ammonium‐ and nitrate‐nitrogen estimated from core samples from each of three horizons (A1, A2, B1) taken from 8 randomly selected sites in a three‐hectare study area. The scheme adopted was: At each sampling time a single composite sample for each horizon was made using 15 cores from randomly‐selected locations; duplicate subsamples from each composite were analyzed for ammonium‐ and nitrate‐nitrogen.