Boron and zinc are important for flowers to become seeds. Local dealers recommend boron foliar sprays at fall. It is said that boron applications in autumn make the buds store this nutrient for Bloom. I do not like to follow advices without thinking or testing, so I have done a trial in a mature almond orchard.
Boron is a nutrient that must be applied with care, because high levels are toxic for plants. Chemical analysis of leaves is important before deciding if and how much boron needs to be applied. Since I practice low yielding dry-farming, I do not carry on this test (irrigation is the main cause of boron accumulation). I do not know the boron content of my trees, so I decided to apply just the preventive dose.
Following the labels, I sprayed trees with the following mixture:
– Boron 11%. 1.7 ml/l
– Zn 5% with aminoacids (5% N). 2.5 ml/l
As usual, I checked pH before spraying. This mixture was slightly alcaline, so I reduced pH to 6. For most tanks I used an acidic product based on phosphorus, which contained an extra 15% P2O5 and 3% N (ammonium form). I added 1.7 ml/l of this acid to achieve that. Taking into account the nitrogen contained in the Zn solution, the mixture was equivalent to adding an extra 2.5 ml/l of a 7-10-0 foliar fertilizer. Phosphorus level was low in this soil, as shown in a previous test, so I consider this fall application is beneficial.
I sprayed the trees in mid September, 2015, leaving some untreated to check the effectiveness of this application. I used approximately 1 liter of this mixture per tree. I sprayed two rows without the phosphorus in order to test the beneficial effect of this nutrient included in the pH adjusting product.
I waited until next spring to see flower and fruit set. I observed no beneficial effect of this treatment. It is fair to say that fruit set may have been meagre because of heavy frost at bloom. In any case I see no reason to follow this practice. September is harvest time and that demands enough work and time.