Fall Foliar Fertilizers: Targeted v/s General Approach

A PCA asking me the other day what I thought of a general micronutrient (iron + molybdenum + zinc + copper + manganese + boron +?) compared to a standard zinc + boron spray program that is common in postharvest almonds in the areas where I work. This question got me thinking about a review of fall fertilizer programs in general, with a focus on foliar nutrients.There are more than a dozen mineral nutrients essential for plant growth. However, only a handful of those nutrients have been shown to be important – in general -- to profitable almond production. Theseinclude nitrogen (N), potassium (K), boron (B), and zinc (Zn)*. I haven’t seen any data that supports the widespread need of other nutrients in almond production. Note: Under certain conditions and locations, not all of these nutrients (N, K, B, and Zn) are required for high yield almond production. For example, in areas where B is plentiful in soil and irrigation water, there is no need for B fertilizer. Hull boron levels should be checked every year at harvest. Click here to read recent post on boron and hull analysis.High yielding almond orchards require hundreds of pounds, each, of N and K per year, much more that can be economically delivered with a foliar-only approach. So, soil applied N and K fertilizer are the primary method of delivering these key macro-nutrients. Click here to see a recent post on fall N application and rates. Fall N is applied with the intent of reloading the tree’s woody tissue with N following nut harvest and ahead of bloom next year. Any soil N in nitrate form – a negatively charge ion (anion) -- left in the root zone after leaf drop probably won’t be there at bloom after leaching by winter rains and/or irrigation. Fall K fertilizer applied to the soil is intended to reload the soil exchange sites with K ahead of next growing season, as the soil holds the positively charged potassium ion (cation) and limits leaching of this important and expensive nutrient. Some soils will fix K and reduce the plant availability of that nutrient. Click here to see an article on K fixing vineyard soils.Zinc and boron are micro-nutrients, needed in very small to relatively small amounts by high yielding orchards. Annual almond orchard zinc and boron demand, when fertilizer is required, can often be met with one or perhaps two sprays. Fall is an excellent time to apply Zn and/or B to healthy almond canopies full of leaves as the crucial time for adequate levels of these nutrients in the tree is at bloom. Both Zn and B can be absorbed through the leaves, stored overwinter, and moved to the buds for use at almond bloom. [Boron is not mobile in pistachio or walnut, so boron foliar sprays must be applied in season for those crops.] Fall application of soil applied boron doesn’t increase flower B levels the following spring, but can increase hull B levels the following year if appropriate rates are used.What about a general fall micro-nutrient spray instead of Zn/B spray? There is limited value, in my opinion, of a general spray of several micronutrients when you only need one or two of those nutrients. This is especially true if the rate of the actually needed nutrients -- established by tissue analysis, often Zn and/or B -- is reduced so the other nutrients fit in the spray tank or the budget. Why is that? A foliar Zn spray is absorbed into the leaf by diffusion across the leaf cuticle. Diffusion is concentration driven, so the higher or lower the Zn concentration on the leaf surface, the greater or lesser the movement of the Zn into the leaf. For the best results, use a high, labeled rate of zinc to get the job done. In the case of B, a significant amount of B is moved out of the orchard in the crop (0.4 lbs actual B in 2000 lbs kernel crop), so even though most B foliar fertilizers are readily absorbed into leaves, you need to put enough on to do the job. For example, 2 lbs of Solubor® contains 0.4 lbs of actual B, whereas a half gallon of 4% liquid B boron fertilizer delivers 0.19 lbs of B. Foliar B rates higher than that equivalent to 2-3 lbs of Solubor® in 100 gallons of water may reduce set arnd are not recommended. If the documented need is for a specific nutrient, make sure you are meeting that need before you add any other nutrients “just in case” they might be of value.Check with your PCA when tank mixing Zn and B to make sure the mixture of the particular products you chose are compatible. A buffer/acidifier may be needed to make high rates of zinc sulfate (20 lbs 36% zinc sulfate/acre) compatible with sodium polyborate materials (2 lbs Solubor®, etc.)*************************************************************************It is possible that other nutrients may be shown to be important in the high yielding almond orchard systems becoming more common in California, but excellent almond production can be acheived with a fertility focus on those nutrients (N, K, B, and Zn) and attention to detail in all other aspects of almond production (variety/rootstock selection, irrigation, pest management, etc.).