Acala Cotton Nitrogen Management and Residual Soil N

The drive for greater efficiency in fertilization practices in cotton requires improved evaluations of: (a) soil fertility level on a field-by-field basis; (b) a means to evaluate and deal with field-by-field variation in crop growth and nutrient status conditions (some measure of plant N status, plant vigor and  fruit retention that is adjusted for stage of growth); and (c) an understanding of the required timing for split fertilizer applications in meeting critical plant needs.  In this type of system, adjustments in nutrient applications are made depending on levels of residual soil N, irrigation water N, and crop condition, which has been referred to as a “feedback” approach to fertilizer N management.  A three year study with three to four field sites per years was conducted to evaluate a proposed feed-back approach to improve nitrogen management decision-making in Acala cotton production in the San Joaquin Valley.  Under conditions where soil nitrate-N levels in the upper two to four feet of the soil profile were in the low to moderate range as determined from prior cotton nitrogen management studies, treatments were established to supply a total of residual N plus applied N of either 115 or 180 lbs of N./acre, with supplemental applications of an additional 55 to 60 lbs N/acre made during early bloom.  Plant petiole nitrate-n was monitored during bloom, and limited plant mapping was done during the same period to assess crop growth vigor and fruit retention.  Methods were proposed in which these plant measurements could be used in combination with soil nitrate-N measurements to assess likelihood of positive yield response to supplemental N.  Although this approach would require a targeted number of soil nitrate, petiole nitrate and plant mapping measurements and associated costs, this feedback management approach could reduce occurrences of unnecessary fertilizer applications, or conversely, N deficiencies damaging to yield potential.