Luke Milliron, UCCE Farm Advisor, Butte, Glenn and Tehama Counties
Franz Niederholzer, UCCE Farm Advisor, Colusa and Sutter/Yuba Counties
Sebastian Saa Silva, Associate Director, Agricultural Research at Almond Board of California

Almond orchard nitrogen (N) deficiency, measured as July leaf N%, is linked to reduced crop yield compared to orchards with adequate leaf N levels. Excess leaf N increases the risk of hull rot infections that reduce yield and increase orchard sanitation costs. What July leaf N level is the target for best net grower return?

First, a quick review of how research and commercial practices differ and are used together may help explain what follows. Leaf nutrient levels vary from tree to tree in an orchard. Fertilizer trials are usually done using individual trees to eliminate this variability. Orchard or block leaf samples are taken from numerous trees and one value is presented after analysis. 

UC recommends 2.2 – 2.5% as adequate July leaf N%. However, due to the natural nutrient variation across an orchard, potential yield might not be maximized at a lab reported 2.2 or 2.3% leaf N for the orchard. Specifically with an average of 2.2% on the lab result about half your trees could be experiencing a yield reducing nitrogen deficit (below 2.2%). Conversely if your samples are at 2.5%, most of the trees are likely being over-fertilized and are at an increased risk of hull rot and delayed nut maturity. The sweet spot in July is 2.4%, which ensures most trees are above the critical value of 2.2%. 

This leaf Nitrogen report card is mainly useful in helping inform next year’s program. However, it can inform your late season nitrogen timed either just before or just after harvest (or split between the two). This last shot(s) should account for no more than 20% of your total season’s nitrogen application and can be reduced or skipped if July nitrogen levels are above 2.4% as research shows no increase in yield next year if late season N inputs are skipped in orchards with at least 2.4% July leaf N levels. Some growers elect for a small rate of late season N if July leaf N levels are on target.  As an example, given a 2.4% July leaf N result, the Nickels Soil Lab will still add 10 lbs low biuret nitrogen per acre to their fall foliar spray along with boron and zinc, to “top off the tank”. (This is an example, NOT a fertilizer recommendation)

Resources: More information on July leaf sampling / more information on nitrogen applications