Luke Milliron, UCCE Orchard Systems Farm Advisor, Butte, Tehama, and Glenn Counties & Joe Connell, UCCE Farm Advisor Emeritus, Butte County
As the bulk of your almond 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. The in-season corrective action that can be taken is to inform your last shot of nitrogen timed either just before or just after harvest. This last shot should account for no more than 20% of your total season’s nitrogen application and can be reduced if July nitrogen levels are excessive (see August nutrition bullet in this newsletter). Published July critical values established for almond 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 and help reduce hull rot by avoiding over fertilization.
July leaf sampling has physiological importance as leaf nutrient levels change through the growing season. Critical values have been developed for July when many nutrient levels in leaf tissue are stabilized. Concentrations of nitrogen, phosphorus, and zinc on a leaf dry-weight basis start very high early in the season and decline rapidly to a fairly steady state after mid-June, then drop off again from September to leaf fall. Potassium starts high in the spring then decreases, reaching a plateau about the same time as nitrogen, phosphorus and zinc. Concentrations of magnesium, manganese, boron and chloride remain fairly constant or increase slightly during the season. Boron, chloride, and sodium will increase steadily if excess amounts are present in the soil or water. Calcium is the one element that always starts low and increases steadily over the season as the leaves age.
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 woefully deficient. Hull samples at harvest are the most sensitive test for orchard boron status.
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 (including fungicides such as ziram, Manzate, and/or Ph-D), contaminated leaves will show excessively high levels of those nutrients and the reported levels should be disregarded. To help reduce this problem, check to ensure 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.
The relatively new protocol for April leaf analysis is used to give an advance estimate of July nitrogen levels that can be compared to the July critical value by entering the April nutrient level results into the N-predication model. To learn more about this prediction model please see this protocol for early-season sampling and in-season nitrogen budgeting.
When comparing lab results from one year to the next, or for an April and July sampling, it is important to consistently use the same sampling methods. The following methods should be followed:
- Define sampling block based on uniform soil type, variety, age, and management.
- Sample uniform, representative trees across the block at least 90 feet apart.
- Consider flagging the trees and going back to those same trees for annual sampling.
- To overcome tree to tree variability, collect a representative sample from a minimum of 18 to 28 trees.
- From each tree, collect all the leaves from 5 to 8 well exposed, non-fruiting spurs around the canopy located between 5 and 7 feet from the ground.
- A minimum of 100 leaves per sample should be combined in a single paper bag for analysis.
- 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.
Deficiencies that are most common in this area are nitrogen, potassium, and zinc. Zinc deficiency is most common in sandy or high pH soils and is easily identified in the field from leaf symptoms early in the season. Boron deficiency is more prevalent on sandy soils or on soils near the foothills. Manganese and iron deficiencies are sometimes seen on high pH soils or where soils are too wet or have areas with high water tables. Useful critical values are not established for iron or sulfur levels in almond leaf tissue.
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 26 in the Almond Production Manual, Publication 3364, or the CDFA Fertilizer Research and Education Program site for almond.
Interpreting results may be more nuanced than simply comparing your results to the critical levels. For instance, a July leaf nitrogen value of 2.2 or 2.3% appears to be adequate, however yield has been shown in certain cases to be reduced at these levels. This is because the critical levels were established by visual symptoms of single trees, while your leaf results represent an average of many trees. An average near the bottom end of adequate may therefore include an unacceptable number of deficient trees. Conversely, repeated results of 2.5% may indicate that at least a portion of the trees in the block are overfertilized. One key way to combat this problem is to follow the previously listed guidelines on reducing sampling variability.
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.
Table 1. Critical nutrient levels for almond leaves sampled in July (Almond Production Manual; UC ANR Pub. 3364).
*Fully expanded bearing spurs sampled in July.
** Use analysis results of hulls sampled at harvest to best assess almond boron status.
For help finding a laboratory in California, here is a list of laboratories for tissue/soil/water analysis.