Postharvest Nitrogen Management 2018

Many operations often apply nitrogen postharvest. Application rates vary from orchard to orchard, but typically I hear applications between 30 and 50 lbs/acre. This nitrogen is used to rebuild the nitrogen reserves within the tree and help with a variety of tree processes. Being applied late in the season, however, it is susceptible to leaching from over-irrigation and early rains.While it is critical to make sure there is nitrogen available for the tree, postharvest nitrogen budgets should be evaluated to determine if they are too high or not needed. Work at UC Davis has indicated that 15-20% of the nitrogen demand occurs from harvest through the post-harvest period. This suggests that somewhere between 20-30 lbs of nitrogen is utilized during this period, which could be sourced from residual nitrate in the soil or post-harvest applications. A further breakdown of the uptake curve shows that less than 8% of the annual need occurs after September, suggesting the importance of timeliness in application (Figure 1). Applications can begin when the abscission layer begins to form on the later harvesting varieties, but consider lowering rates to reduce the risk of hullrot flare ups (e.g. apply 10-15 lbs/acre instead of 25-30 lbs/acre).Applications of nitrogen in the post-harvest tend to be relatively inefficient due to several factors, including tree defoliation, leaching by early rains or long irrigations, shorter days and lower water use. This suggests that more nitrogen may be needed to achieve a 20-30 lb uptake, but studies in Kern County failed to see a response from applications greater than 50 lbs/acre of nitrogen applied after Nonpareil harvest (mid-August). Rates should not exceed this amount and applications made in September and October should be significantly reduced.Mid-July leaf nitrogen and residual soil nitrate should be taken into consideration when determining the amount. Nitrogen applications to trees with adequate or excessive mid-July leaf nitrogen levels (>2.5%) are most likely not needed. This is supported by research conducted by Franz Niederholzer (UCCE Yuba/Sutter/Colusa) who has shown that mid-September or later postharvest nitrogen applications to trees with mid-July leaf nitrogen at or over 2.5% did not improve crop yields the following year. Trees were followed for four years within this study and nitrogen applied in these situations was lost to leaching. This suggests, especially with later harvesting areas or varieties, that postharvest nitrogen applications can be eliminated with the  entire nitrogen budget applied in the spring during the periods of rapid uptake.