An attempt was made to evaluate the N supplied by crop residues through crop rotation experiments on Oxisols and Ultisols. Field experiments were conducted on three typical soils following a split-plot design. Main plots were three rotations: soybeans, corn , corn; fallow, corn, corn; and continuous corn. Subplots included two treatments: 0 and 110 kg/ha of fertilizer N. Fair yields of soybeans were obtained while corn yields were good, especially on the Humatas soil (Ultisol), 6240 kg/ha. Substantially higher yields were obtained in the Bayamón (Oxisol) and Humatas soils from the first corn crop following soybeans or corn than when following fallow. Although the second corn crop yield following soybeans was slightly higher than the first, the second corn crop after initial corn and fallow were substantially higher. The effect of applied N at all sites was striking, regardless of the previous crop. There was no apparent relationship between the amount of N returned to the soil and yields of subsequent corn crops.