Post harvest foliar fertilization

I was taught at university that there is a treatment which is worth applying every year: postharvest foliar fertilization. My experience shows that this application is useless. 
20150908_192809

Under dry Mediterranean conditions, September and October bring the first rains and mild temperatures, which are good for young trees to grow and for older trees to save nutrients for the coming year. At this time, a foliar spray is said to help the plant to harness the most out of this spell.

In second and third leaf almond trees I have provided the nutrients that are limited in the orchard. Soil test reveals that phosphorus is the limiting macronutrient. Moreover, it has a pH of 8,5, which makes key micronutrients such as Fe, Mn, Zn and B less available to the plant. High levels of calcium may also have an antagonism with Magnesium. I do not consider important to incorporate Boron for young trees before they become productive.

The foliar spray that I have tailored for this orchard is a simple mixture of two fertilizers:

NPK 5-20-5, Fe (1%), Mg (1%) at the recommended dose (2.5 cc/l).

Zn (5%) + Mn (5%), at half the recommended dose (1.25 cc/l)

20150908_193205Before using any spray, I like to check the pH of the mixture in order not to harm plants and make the fertilizer effective. The mixture had a pH around 5, which is acceptable.

I applied this fertilizer with a manual sprayer last September, 8, 2015. I used 25 liters of mixture per 100 trees. The cost of it is extremely cheap, around 0.01 € per tree. This application has not shown either beneficial or detrimental effects, so I have no reason to do it any more.

In previous years I applied Potassium Phosphite as the basis for the fall foliar spray in September. It was wonderful and made the trees healthier. Unfortunately, this fertilizer is no longer provided by suppliers in Spain.  That reminds me that you can not rely on any particular product because you may lose it.

For mature trees, 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

B_Zn_2015As 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.

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