July Leaf Sampling: A Critical Task in Prune Production

Luke Milliron, UCCE Orchard Systems Farm Advisor, Butte, Tehama, and Glenn Counties; Franz Niederholzer, UCCE Orchard Systems Farm Advisor, Sutter/Yuba, and Colusa Counties; & Joe Connell, UCCE Farm Advisor Emeritus, Butte County

As the bulk of your prune fertility program for the season comes to an end, it’s time to get the report card on how you did. Although little corrective action can be taken this season, this report card will help inform next year’s program. Published July critical values established for prune by UC researchers can help guide you in your fertilization practice. Analysis reveals specific nutrient deficiencies and alerts you to developing trends when results are compared from one year to another. Keeping mature trees below excessive levels for nitrogen can save on fertilizer costs, reduce brown rot risk and avoid excessive vegetative growth. Potassium (K) fertilizer is expensive, but K deficiency in a single season can damage orchard health for years to come. Leaf analysis results help keep an orchard between excess and deficiency, which means the most efficient use of your fertilizer dollar!

Excessive amounts of chloride and sodium should be monitored if water quality is poor and/or chloride is a component of the fertilizers frequently used in the orchard. Depending on your location and water source in the Sacramento Valley, your boron levels could be toxically excessive or deficient.

Most laboratories group several key macro and micronutrients together in one easily requested analysis. Note that if micronutrients have been applied in a foliar spray, contaminated leaves will show excessively high levels of those nutrients and the reported levels should be disregarded. Check that the laboratory you use washes leaf samples before analysis and that you promptly send in leaf samples (i.e. desiccated leaves cannot be washed). Although micronutrient spray residues may not be possible to wash off, laboratory washing of the leaves has value because micronutrients in dust can also skew results.

When comparing lab results from one year to the next, it is important to consistently use the same sampling methods. The following methods should be followed:

  • Define sampling block based on uniform soil type, age, and management.
  • Sample uniform, representative trees across the block.
    • Consider flagging the trees and going back to those same trees for annual sampling.
    • For each sample, collect a minimum of 50-75 fully expanded leaves on well exposed, non-fruiting spurs at 5 to 7 feet above the ground. Sample 1-2 leaves per tree.
    • Collecting leaves from fruiting spurs could show erroneously low nutrient (e.g. potassium) levels, which may result in an incredibly expensive and wasteful fertilizer response.
  • Ignore interplants or sample separately.
  • Avoid sampling leaves that are regularly “hit” by irrigation water, as irrigation water can leach some nutrients like potassium and leave salt deposits that can skew the analysis report.
  • Take notes while sampling, noting relative vigor and other observations between blocks, to better inform the analytical results.

Leaves selected for analysis should be free of obvious tip burn, insect or disease injury, mechanical damage, etc., and should be from normal, healthy trees. If you have a weak area and you’d like to diagnose the problem, sample that area and compare the results with those of a sample from your best area to see if tree nutrition might be involved. This type of troubleshooting analysis can be done at any time during the season. Keep in mind that nutrient deficiency might be a symptom of another problem, like compromised root health.

Critical values for July leaf samples are shown in Table 1. Keep the results with your fertilizer application and yield records to better evaluate and estimate future fertilization needs. For more information on nutrient deficiencies and toxicities, sampling procedures, and critical values, see Chapter 16 in the Prune Production Manual, Publication 3507, or the CDFA Fertilizer Research and Education Program site for prune and plum.

Table 1. Critical nutrient levels for prune leaves* sampled in July.


*Fully expanded leaves from non-bearing spurs sampled in July

Leaf analysis is one of the many helpful report cards we receive in orchard management. When responding to leaf levels, include orchard appearance and growth before corrective action is taken. Visual observation is an excellent complement to any lab analysis. Make sure that a deficient element is really the problem before you seek fertilizer applications as a solution. Learning from the report card of July leaf samples is one more way of taking an analytical approach to farming and continually improving your production practices each year.

How can maximum value at minimum cost be achieved from leaf analysis?  If money is tight, reduce the number of nutrients requested for analysis, but don’t abandon leaf sampling/nutrient analysis for the year. Key nutrients for analysis in every sample, every year, are nitrogen, potassium, and zinc.

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