Using Mid-Summer Leaf Samples to Guide Fertility Programs, Part 1

By this time, mid-July leaf tissue sample results should be available. These tissue results are critical for any nutrient management plan. They should be taken annually to determine if the nutrient program is adequate for the orchard, and the values should be used to tune the orchard´s nutrient program. Research has shown that once an orchard reaches sufficiency of a specific nutrient, increasing it further will not improve yields. This has been demonstrated several times, both within nitrogen and potassium. Second-guessing and adding more fertilizer beyond sufficient levels to address existing yield problems will not improve the situation. This fact is relatively unpopular as it is easier to add more fertilizer than it is to fix the problems that limit orchard production, or assure oneself during a period of second guessing.  More on that can be found here (slide 17) and here (slide 20), and this article will focus on nitrogen The typical nitrogen program often budgets for 20% of the nitrogen to be applied during the post-harvest period. Mid-summer leaf samples should be used to determine if the post-harvest fertility plan should be changed. Within nitrogen, leaf tissue samples that exceed 2.5% can reduce or even eliminate the need for postharvest nitrogen applications. This is based on: Many orchards with leaf levels above 2.5% often have high residual nitrogen within the soil that will meet post-harvest needs, The reduced transpiration rates of the postharvest period reduce uptake of nitrogen, There is significant canopy loss from the harvest process, reducing nitrogen uptake, Nitrogen remobilization back into perennial tissues is happening earlier than previously thought – probably sometime between hull-split and harvest (or maybe even earlier!). Additionally, a long-term study in Arbuckle found that postharvest nitrogen applications did not improve yields in a reasonably high producing almond orchard. Nitrogen levels within

Read More

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.

Read More

Almond Postharvest Management: Nitrogen Considerations

Written by David Doll (UCCE Merced) and Franz Niederholzer (UCCE Yuba-Sutter-Colusa) Applications of nitrogen in the postharvest period have been promoted as a good farming practice.* Trees utilize nitrogen at this time to rebuild nitrogen stores and provide nutrients for bud development. Research from the Kern County nitrogen trials found that trees can absorb approximately 15% of annual nitrogen demand in the 8 weeks after harvest (30-45 lbs/acre, excluding application inefficiencies). This work was done in a highly productive orchard (3000+ lbs/acre) and applications were applied as multiple fertigation events through the entire year. Fertilizer nitrogen applications should be made as soon as possible after harvest to allow the trees the most time to absorb fertilizer nitrogen before the leaves drop, when root nutrient uptake essentially stops. Whole almond tree nitrogen need after harvest is much less than in the spring (no shoot growth or crop), therefore fall fertilizer nitrogen needs are relatively minor compared with leaf out to hull split.  Fall nitrogen fertilization need is further reduced by remobilization of leaf nitrogen into woody tissue storage as trees prepare to drop their leaves in November (if they are not burned off with zinc sulfate.) Interestingly, recent research suggests that late postharvest fertilization (October) can be skipped if mid-July leaf nitrogen levels are adequate (over 2.5% nitrogen). This work by Franz Niederholzer conducted at the Nickels Soil Lab in Colusa County has found no negative impact on yields when skipping postharvest nitrogen applications within orchards over the past two years. In this trial, trees had mid-July leaf nitrogen values of 2.55% or greater. Trees receive 0 or 30 lbs/acre of postharvest nitrogen in mid October. Trees didn’t receive late zinc sulfate foliar spray and maintained leaves up to or through Thanksgiving. Over the past two years, there were no differences

Read More