Additional thoughts on bloom spray additives

I received a question from a community member (slightly edited):

"What other nutrients are good at the pre-bloom, pink bud stage. Everyone seems to want a zinc/boron mix with some kind with seaweed. Is any of that helpful? What about an air application of foliars? Seems like with a dry period the fertilizer folk want to find a way to apply some material."

Boron is the only nutrient that has been shown to increase yield with pre-bloom or early bloom sprays. This is why nearly all bloom spray products contain boron. Nearly all sources of boron are effective, with the simple standard being sodium tetraborate (20-21% boron) applied at 2-4 lbs/acre (2.2-4.4 kg/ha). 

Even though a lot of operations apply zinc at this time, this is not a critical element for flowering, but rather a critical micronutrient for leaf expansion. There have been no replicated studies showing consistent increases in yield from an application of zinc at flowering. Studies have shown that zinc applications through the season do not provide an increase in growth or yield in orchards with zinc sufficient orchards (Ellis, 2016). Furthermore, combination of zinc with other micronutrients may impede uptake of both micronutrients due to competition for plant available uptake channels(Canesin, et al, 2010). Due to this, it is best to apply a single micronutrient for maximum effect. 

Multiple field trials conducted within almonds over 15 years have shown NO effect of seaweed suspensions, solution, or products. Seaweed/algal extracts have been shown to improve performance in plants that are under stress, which is thought to be due to increased stomatal conductance. This hasn't been consistently demonstrated on a year-to-year basis, and mostly has been demonstrated in annual crops and cereals. Gains are thought to be due to hormonal influences within the plant. Based on the field studies, the mechanisms hypothesized to be responsible for yield, and the sensitivity of flower organs during flowering, I do not consider the addition of any seaweed products.

There is some field evidence that applications of calcium at petal fall/shortly after petal fall may help increase kernel size. Although research hasn't been conducted by the University, a large operation shared trial data with me showing consistent increases (statistically significant)  in kernel sizes with blocks sprayed with calcium at petal fall versus an untreated control. All other fertility activities were the same. Based on this multiple year, multiple region trial, and the aspect that calcium is important for cell wall expansion, I think there is merit in applying a calcium material with the petal fall spray. We utilize an amino acid chelated calcium.

Additional application of nutrients at flowering dont make much sense. The tree is running on stored reserves of carbohydrates, and the tissues have already been formed within the buds. Making a special application just to put on 1-2 lbs/nitrogen per acre seems like a very cost inefficient way to boost fertility. This is especially true since work within CA has shown very little nitrogen increases within plant tissues from external sources until 3-5 weeks post bloom.  

Personally, I know just as many farmers who do the minimal applications at bloom and yield just as much as the ones who spray everything, including the kitchen sink. This is most likely because nearly all of the yield determinates were established in the fall of the previous year.