supplement any deficiency. As you can see in Figure 1 , this drip irrigation orchard has a sulfate of potash (SOP) application nowhere near the wetting zone, which will lead to, at best, an inefficient uptake. This is an example of how not to apply SOP. If tissue samples are low, a liquid injection of K is recommended to get a quick response within the plant. I have always thought of K in the soil as a savings account and K levels in tissue as a checking account. You can remove and add money to a checking account easily. However, if you deplete your savings account, it can take years to replenish it. The last sample that should be taken this time of year is a hull sample. Hull samples give a better representation of Boron levels, since boron accumulates in the hull. According to the CDFA Fertilization Guidelines, a sample with less than 80ppm is considered deficient. Growers can apply Boron to the orchard soil in the dormant season, which can raise levels in deficient orchards. Samples that are between 80ppm-120ppm would benefit from a post- harvest foliar application of boron. Always discuss boron applications with your PCA/CCA, since over-application can lead to toxicity. As I mentioned above, post-harvest foliar nutrient sprays can be beneficial. Applying them soon after harvest is finished allows the plant to utilize that energy and put it to use for next year’s crop. Tasks may pile up after harvest, so be diligent in making this application. If it is applied too late in the season and leaves are falling, the benefit of the application will be lost. Many of the available nutrient sprays include material such as boron, phosphite and zinc. When these nutrients are applied, they enter through the leaf. The tree then goes through the senescence period, where the tree will pull all the available nutrients from the leaves to store for the dormant period. Applications of zinc sulfate used to be popular to “put the trees to sleep”. Nowadays, it is used to reduce inoculum of diseases like scab and rust by removing the leaves from the infected trees. In an orchard that does not have these issues, you would not want to prematurely remove the leaves for a couple of reasons:
1. Trees continue to go through the photosynthesis process after harvest and are producing carbohydrates, which will be stored for next year’s bloom. 2. If an application of zinc sulfate is made before the plant has entered the leaf senescence period and is still trying to perform photosynthesis, the tree may produce new leaves. These new leaves will remove the stored energy from the plant and most likely lead to a reduced bloom the following year.
Gypsum Pile Ready for Application. Photo by Trent Voss
Soil amendments are another topic that needs to be discussed for the post-harvest timing. In reviewing soil samples with your PCA/CCA, you can decide which product you may or may not need. Ag Lime is used to raise soil pH in acidic situations, and if your soil is low in magnesium, dolomite may be a better fit. For soils that already have a high pH, you will need to look at applying sulfur to lower the pH. Gypsum is pH neutral and can be used to help reduce soil crusting. The reduced soil crusting will increase water infiltration. Winter weeds will start showing up soon after harvest. The UC Davis IPM website Introduction to weed photo gallery — UC Statewide IPM Program (ucanr.edu) has an excellent photo gallery to help you identify what weeds you have. This, along with talks with your PCA, will give
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SEPTEMBER–OCTOBER 2023
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