Following the early start to this year’s almond harvest, it’s not too soon to start planning your post-harvest field work. That includes tending to the nutritional needs of your trees.
Chances are, after spending so much energy producing this year’s crop, they could be short of one or more nutrients needed to produce nuts next year. What’s more, by the end of harvest, you should have results of your July leaf tissue analysis to guide your fertilizing decisions.
Gurreet Brar, University of California Cooperative Extension farm advisor for Fresno and Madera Counties, lists the critical values of leaf nitrogen:
Deficient - below 2.0%
Adequate - 2.2% to 2.5%
Higher – above 2.5%
If leaf N is in the normal range, Brar recommends applying 20 percent of your total N budget after harvest. If your leaf tissues show N on the higher side (>2.5%), post-harvest application can be reduced slightly. However, if leaf N levels are very high (>3.0%), post-harvest N application may be skipped.
Make your application during flower bud development, which occurs about two weeks after Nonpareil harvest. In Nonpareil and other earlier-harvested varieties, N can be applied either at or around the time of the first post-harvest irrigation. In later varieties, like Monterey and Fritz, it can be put on between hull split and harvest time, Brar notes.
“University of California research shows that almonds remove about 68 pounds of N for every 1,000 kernel pounds produced,” he says. “In budgeting your nitrogen for the season, you should take into consideration your yield history and expected yield; Nitrate-N in your ground water, if any, and N from compost and manures. Also factor in the nitrogen use efficiency of about 70 percent.
Nitrogen can be applied by fertigation in the form of UAN-32 or CAN-17 liquid blends, he notes.
Brar suggests a post-harvest application of potassium to rebuild soil K reserves, if leaf tissue testing in July shows levels of this nutrient to be low, less than 1.4 percent. (Potassium levels below 1.0 percent indicate a deficiency.) July leaf tissue levels are the average of many trees across the orchard. So, aiming for an orchard goal of 1.6 to 1.8 percent K can be a safer strategy to prevent tree levels from falling below the critical point of 1.4 percent K, he advises.
In heavy soils, K can be soil-applied in a band. However, in light soils with low cation exchange capacity, Brar recommends making applications over a wider area to reduce concentration and any subsequent leaching.
“Make sure the applications are done within the wetting pattern of the irrigation system so that the applied K gets to the root zone with water,” he says. “Avoid heavy applications of K in sandy soils to prevent the nutrient from leaching out of the root zone.”
Both sulfate of potash and muriate of potash can be used for banding. However, Brar points out, muriate of potash (potassium chloride) poses a chloride risk to the tree.
For drip irrigated fields, K can be applied through the drip system, itself, as sulfate of potash, potassium thiosulfate, potassium nitrate and potassium chloride, he says.
To avoid a boron deficiency during next year’s growing season, Brar recommends collecting hull samples during this year’s harvest to analyze levels of this nutrient.
Boron is essential for growth of the pollen tube and fertilization of the flowers. Even just a moderate boron deficiency can reduce nut set on the trees, he notes.
Current critical values for boron are:
Deficient – 80 ppm or lower
Adequate – 80-150 ppm
Possibly toxic – over 200 ppm
Boron may be broadcast on the soil surface or sprayed on the leaves. University of California researchers have shown that in orchards with hull boron content of up to 120 ppm, a post-harvest foliar application of boron can increase nut set and yield by 20 percent to 30 percent.
“The best time for a foliar spray is after harvest but before the leaves become inactive in late October,” Brar says. “One to two pounds of Solubor or a similar product with 20 percent boron per 100 gallons of water may be applied for good results.”