Fig. 1.

Potentially mineralizable soil N before and after 51 yr of nitrogen–phosphorus–potassium fertilization of previously unfertilized Morrow Plots cropped to continuous corn [C-C], a corn-oats (1876–1966) or corn-soybean (since 1967) rotation [C-O(S)], or a corn-oats-alfalfa hay rotation [C-O-H]. Fertilizer N was applied as urea to corn (168 [1955–1966] or 224 [since 1967] kg N ha−1) and oats (28 kg N ha−1). Potentially mineralizable N was estimated by the Illinois soil N test. Every change was significant at α = 0.001 by the step-down Bonferroni procedure, and all were negative, except for the 0- to 15-cm depth in the C–O–H rotation.