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Tips on postharvest nut crop nutrition

California Department of Food and Agriculture JM-TNFP1216-cdfa-postharvest.jpg
Energy and sugars from these nutrients are used in almond flowering and early shoot development for the upcoming season.
Potassium, boron among the nutrients that should be on the menu.

Not all nut crops need postharvest nutrition despite the fact that almonds, pistachios, and walnuts all lose significant amounts of potassium when crop is removed.

“Potassium is the first post-harvest nutrient I consider” for all three nut categories, says Phoebe Gordon, UCCE in Madera and Merced counties.  “Heavy banding on the side of a tree row is the traditional way of supplying this nutrient, allowing winter rains to dissolve fertilizer and move it into the soil.

“Potassium loosely binds to clay and soil organic matter which protects it from leaving the soil over the cool-weather months and banding works well on the eastern side of the San Joaquin Valley.”

Both potassium and nitrogen [K and N] are the biggest nutrient requirements for almond trees with the general recommendation that about a quarter of the total K and N requirements be applied post-harvest and be based on current season crop load.

Energy and sugars from these nutrients are used in almond flowering and early shoot development for the upcoming season. One caveat, however --- make sure soil moisture levels are optimal before applying fertilizers to avoid potential for root damage and to maintain fertilizer efficiency.

Conventional wisdom recommends banding high rates of sulfate of potash in late fall to build next season soil levels.  Grower reports indicate many almond orchards did not get a pre-bloom micronutrient application in spring 2021 because of wet weather, so applying these materials, post-harvest, may help avoid potential weather-related issues next year.

“Fertigation is another way to fertilize orchards using potassium to deliver nutrients directly to the area of nutrient uptake,” she says. "Fertigation also side-steps potassium-fixing soils as long as the soil doesn’t dry out completely.  Once that happens, potassium becomes fixed.”

Boron and nitrogen

Another relevant nutrient is boron, frequently found in excess on the west side of the San Joaquin Valley and in Yolo County, but often deficient on the eastern side of the county.

“All three nut crops need it, especially pistachios,” she adds.

“Boron is mobile in almonds, not so in pistachios and walnuts, so only almonds can re-mobilize and store fall-applied foliar boron, hence timing of application is dependent on the crop.  Soil boron applications should be made during the growing season and likely won’t affect tree boron status until the following year.

“Apply low-rates of boron to the leaves of almonds post-harvest in the ratio of a pound or two of 20% boron-containing product per 100 gallons of water.  Spring applications to trees is also acceptable, but should be applied at pink bud as later sprays may interfere with pollination.”

And although it comes at the end of the alphabet, Zinc is an upfront necessity because it is frequently found in deficient quantities in Central Valley soils.  While soil application is proving adequate in Arizona and New Mexico, the standard for California growers is foliar application, generally done in the spring.

And then there’s nitrogen.  “Why did I place it at the end of the list when it’s the nutrient almost everyone thinks about the most,” Gordon asks.

“There’s no evidence post-harvest fertilization in pistachios and walnuts is even necessary.  Research is documenting that nitrogen uptake in walnuts stops long before harvest and Kerman pistachios are showing much the same as walnuts where nitrogen uptake follows demand.

As new research is published, suggestions for almond post-harvest nutrition are changing, i.e., little nitrogen uptake after harvest, like 8% of total annual needs.  Gordon’s conclusion — post-harvest nitrogen in almonds is likely not necessary except in cases of deficient orchards.

In fact, where nutritional needs are concerned, “There is no general time for nutrient applications as that aspect depends on the time of greatest demand for N and K…and that is when the material will best be absorbed via foliar sprays and/or soil mobility,” she says.

Further information on the subject — including registration for a nitrogen training course — can be found at San Joaquin Valley Trees and Vines []

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