USING COTTON PETIOLE NITRATE-NITROGEN CONCENTRATION FOR PREDICTION OFCOTTON NITROGEN NUTRITIONAL-STATUS ON A CLAYEY SOIL

Soil and petiole nitrate-nitrogen (NO3-N) analyses have been used to m onitor the nitrogen (N) nutritional status of cotton (Gossypium hirsut um L.) in Arkansas. The basis for in-season N fertilizer additions has been NO3-N monitoring for the most recently matured leaf petiole. In this manner N fertilizer can be adjusted so that N is not limiting dur ing growing seasons conducive to high yields. The objective of studies reported herein was to determine petiole NO3-N levels that define def icient, adequate, and excessive N nutritional status of irrigated cott on growing on a clayey soil. Experiments were conducted on a Sharkey s ilty clay (very fine, montmorillonitic, nonacid, thermic, Vertic Hapla quept) located on the Northeast Research and Extension Center at Keise r, AR. The experimental design was a 5 by 5 Latin Square with a split plot arrangement of treatments. Subplots were cultivars of cotton, and main plots were N rates of 0, 56, 112, 168, and 224 kg/ha applied as 56 kg/ha splits beginning just before planting, again at pinhead squar e, and again at 56 or 110 kg/ha at early bloom until all N for a treat ment was applied. The N fertilizer was 32% N solution (16% urea-N and 16% ammonium nitrate-N). Petioles were collected weekly beginning the week before first bloom and continued for eight weeks. Cotton was harv ested and lint yield calculated. Subsequent analysis of petiole NO3-N and lint yield were performed using (a) X(2) and (b) percent correct p redictions on both the actual lint yield versus petiole NO3-N content and the lint yield versus the slope of the petiole NO3-N content with time. The results indicated that (a) the percent correct predictions w ere the best predictor, (b) petiole NO3-N predicted lint yield losses from the week before first bloom until three weeks later at approximat ely 80% correct estimates and falls to 50% correct estimates for subse quent weeks, and (c) slope of NO3-N with time gives percent correct es timates approximately 50% of the time. These results indicate that pet iole NO3-N content by itself is useful for determining the N nutrition al status of cotton until the third week of bloom.