Soil inorganic nitrogen content and indices of red raspberry yield, vigor, and nitrogen status as affected by rate and source of nitrogen fertilizer

The single-year response of soil inorganic nitrogen (N) content and indices of red raspberry (Rubus ideaus L.) yield, vigor, and N status to rate and source of fertilizer N were determined. Twenty-nine trials were conducted in commercial plantings from 1994 to 1996. Treatments were 0, 55, or 110 kg N ha(-1) as ammonium nitrate or 55 kg N ha(-1) as a slow-release fertilizer product containing 60% polycoated sulfur-coated urea and 40% urea. Soil nitrate (NO3) content frequently increased during the growing season, indicating that soil N supply was nonlimiting. The plant indices were generally insensitive to fertilizer-N rate under these high-N fertility conditions. Soil nitrate content measured after berry harvest was frequently excessive even at the recommended N rate and can be used to identify fields with excess N fertility. The slow-release N fertilizer provided limited benefits compared with use of ammonium nitrate.