Nitrogen Management in Mature Almonds, Part II

In a recent post, I talked about the 4 R’s – Right Rate, Right Timing, Right Place, and Right Material.  How do all these R’s fit together to help produce a profitable, sustainable almond crop?  I’ll use an example familiar to me: the Nickels Soil Lab in Arbuckle in the southern Sacramento Valley. 

Step 1.  In January, as a starting point, estimate a total N budget for the year.  At Nickels our ten-year average annual crop production is 2500 lbs kernels per acre.  So, 2013 initial N budget is calculated thus:  65 lbs N per 1000 lbs kernels x 2500 lbs kernels per acre = 162.5 lbs N per acre per year.  Add 30 lbs N per acre per year for N required to support vegetative growth and the total annual N budget for 2013 is 192.5 lbs N per acre.  Assume 90% efficiency due to our heavier soils and subsoil clay layer (less leaching potential than on lighter soils with excellent drainage) and our total N budget is 214 lbs N per acre per year.  Round up and get 220 lbs N per acre per year.  Our water district uses surface water, only, so there is little to no N in irrigation water.  No compost was applied or legume cover crop grown, either, so there are no “deductions” from the fertilizer N budget.  The initial 2013 fertilizer N budget is 220 lbs. N per acre per year.

Step 2.  Determine rate and deliver first — early spring — N injection with irrigation water.  The new guidelines suggest application of 20% of total N budget before full leaf out.  So, 20% of 220 lbs. N per acre is 44 lbs N per acre.  We’ll be putting that material (UN32) out in the next week (week of March 10) at Nickels.  That material will go into the irrigation water in the middle third of the irrigation set.  Clean water first, then inject N, followed by more clean water.

Step 3.  Take early season leaf sample and estimate cropload to determine future rates of N fertilizer.  Follow the UC Early Season Protocol (UC ESP) for leaf sampling (details below).  So far, this program has only been tested on Non-pareil, but it should work well for other varieties.  (Dr. Brown’s lab is working up the details for a Monterey variety sampling protocol.  For now, here’s how to take leaf samples using the new UC protocol worked up for Non-pareil:

• Sample leaves approximately 43 days (plus or minus 6 days) after full bloom.  It is very important to hit this range of sampling dates, as leaf nutrient levels change with age and too old or too new leaves in the sample could result in an inaccurate report.  If you sample outside the sampling “window”, mark the date on the sample bag.  At Nickels this year, Non-pareil full bloom was the last week of February.  Call it Feb 26.  So we will target early leaf sampling the week of April 8 at Nickels Soil Lab.
• Collect leaves from 18-28 trees per orchard.  Target uniform, representative trees from the block to sample.  Sample trees must be at least 90 feet apart.  From each tree, sample all the leaves of 5–8 non-fruiting, well-exposed spurs.  Combine all the leaves in a single sample bag.  A minimum of 100 leaves per sample bag is required.
• Send the samples to a reputable lab – check with your PCA and/or ask around for a good, reliable lab if you don’t currently use one — and ask for a FULL NUTRIENT ANALYSIS (N, P, K, B, Ca, Zn, Cu, Fe, Mg, Mn, S) and application of the UCD-ESP program.  A good lab should have the results back to you in a week, maybe two.

Step 4:  Review results from UCD-ESP program and your cropload estimates.  Use Table 1 options to adjust remaining fertilizer input rates to assure adequate N availability to the trees.  Sample leaves again in July to check these new practices.  Dr. Brown suggests following the same protocol for sampling leaves in July as in April:  18-28 trees, each tree at least 90’ apart, all the leaves of 5–8 non-fruiting, well-exposed spurs from each tree, all the leaves in a single sample bag and a minimum of 100 leaves per sample bag.

Table 1.  Suggested options for adjusting almond fertilizer input rates for fruit growth (late April/May) and kernel fill (shell hardening to early hull split) applications and based on examples of current year cropload estimate and UCD-ESP leaf analysis results.

*Cropload correction factor is the ratio of the current estimated cropload to average cropload.

** If UCD-ESP leaf levels are “Adequate to excessive” and actual crop yield at harvest is significantly less than estimated earlier in the year, the final N application (post hull split) is probably not needed and can be omitted. [For detailed explanation of the UC-ESP and N in almonds, please see the on-line publication from Dr. Patrick Brown and others on the Almond Board of California’s web site ]

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