EFFECTS OF RESIDUAL SOIL-NITROGEN AND UREA ON YIELD AND PETIOLE NITRATE OF COTTON

Little information is available on the effects of residual soil N fertilizer requirements or the effects of "standard" vs. slow-release N fertilizers on irrigated cotton (Gossypium hirsutum L.) production and resulting soil N carryover in the semi-arid Southwestern USA. Deep soil sampling for profile NO3--N or petiole analysis for NO3--N have not yet been widely accepted in this region. Field studies conducted in 1980 and 1981 in the Pecos River Valley of southeastern New Mexico evaluated (i) effects of residual soil N and varying rates of urea or sulfur-coated urea (SCU) on cotton yield, (ii) effects of urea and SCU on petiole NO3--N, and (iii) effects of urea and SCU on residual soil N. Neither fertilizer nor N rate significantly affected seed cotton yield. Residual soil N was lower in 1981 than 1980 but N mineralization potential was greater, thus reducing the effects of N fertilization. Early season petiole NO3--N was adequate in both years with no significant differences between either N rate or fertilizer material. Significant differences were observed between N rates but not fertilizer materials in late season petiole NO3--N. Late season petiole NO3--N levels in 1981 were or near deficiency levels but, since most of the cotton crop is formed early in the season, low petiole NO3--N did not affect yields. Significant differences in profile NO3--N at harvest appeared to be related to distribution of growing season rainfall. Although there was no distinct advantage of a slow-release N fertilizer, the amount of NO3--N remaining in a 3-ft soil profile at harvest was reduced by delaying fertilizer N availability. Residual soil N may be an important factor in meeting cotton N fertilizer requirements in this region.