A practical limitation of nitrogen soil tests is large spatial variability. The object of this research was to compare the variability of several forms of soil nitrogen: total N, nitrate, nitrate + ammonium, ammonium released during a two‐week anaerobic incubation at 25° C, and ammonium released by distillation with alkaline permanganate. A second objective was to compare sensitivity of these measurements to presence of green manure legume residues. Samples were collected from a 6.1‐ha area of a Nueva loam soil in transects with a 33.5‐m spacing and over a shorter distance at a 5‐m spacing. Plots with and without purple vetch (Vicia benqhalensis) were established on the site and at one other site with clay soil. Soil samples were collected in the spring just before and just after the legume was disced under. Coefficients of variability for nitrate, incub. ammonium, total N, and alk. perman. ammonium were 29–36%, 9–15%, 5–6%, and 2–4%, respectively. All variables showed similar spatial dependence except for incub. ammonium. Alk. perman. ammonium was not different in control and legume plots, while anaerobic incubation ammonium was higher in legume plots.