A tracer technique was used to evaluate crop recovery of residual inorganic nitrogen from several soil depths. 15 N‐labeled nitrate was placed at depths of 9, 60, 120, and 180 cm in field plots planted to corn ( Zea mays L.) and containing 409, 468, and 582 kg inorganic N/ha in the 180‐cm profile. Flag leaf samples were collected at three growth stages and their 15 N and total N contents determined. At early silk stage, no labeled nitrate was extracted from soil at depths below 120 cm, regardless of residual N level. Nitrate was absorbed at the blister stage from a depth of 180 cm in soil containing the lowest amount of residual N. In soil at the highest level of residual N, little labeled nitrate was recovered at any growth stage at depths below 120 cm. At harvest, more 15 N derived from the upper soil profile was found in the stover, while the grain contained considerable 15 N obtained from the middle and lower profiles, presumably related to the later period of uptake.
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Gass et al. (1971) studied this question.