Improving plant productivity begins with efficient nutrient use. A large part of improving your nutrient use efficiency is by shifting your paradigm to view the start of next year’s crop at Postharvest, prior to dormancy. This is foundational for getting off to a strong, productive start with early spring development activities, especially the Pollination stage. The plant stored reserves (both carbohydrate and mineral nutrients) are the primary supply source for plants’ early growth and development. The plant reserve pool needs to be adequate to support plants from bud burst until the spring soil temperatures warm to 68°F when soil nutrient release begins.

There are numerous ways to improve plant stored reserves and they all occur prior to dormancy, while plants are still active. A few of the significant contributing activities for building up plant stored reserves are:

• Plant canopy nutrient content and it’s timely resorption

• Postharvest foliar nutrient applications

• Fall fertigations timed with fall rootflush activity

Each of these examples are foundational for preparing for dormancy and improving spring nutrient use efficiency. Once a nutrient is absorbed into a plant or a microbe, it is in a bio-available form. Bio-available forms of nutrients are highly efficient and stimulate a productive plant response. One example of good nutrient use efficiency at bloom-stage is when plants are able to bloom uniformly from top to bottom and on all sides simultaneously. Uniform growth and development are indicative of plants having adequate energy and nutrient supply available.

Let’s look at the critical roles of micronutrients and their support to plant health and productivity, and how to improve efficiency. Beginning with Boron, all plants need a good supply, but it’s very species specific as to how much each requires. It was once described to me, as a rule-of-thumb, like this; plants need Boron, but only when they are photosynthesizing, which is the majority of the growing season. Boron is mobile in some plant species and immobile in others, so knowing what your plant’s needs are is vital for plant productivity and yield potential.

Boron functions:

• Boron is a potent partner with Calcium and in fact is the catalyst that facilitates the release of Calcium out of the soil into the plant. But excess Calcium applied to soil will hinder Boron solubility, and uptake. [6]

• Potassium uptake is…improved with Boron, and…excess potash suppresses Boron solubility and its availability to plants. Sugar translocation, the synthesis of nucleic acids and plant hormones, and the activation of cell division depend upon Boron. [6]

• Important for assimilation of Nitrogen in plants as well as assisting plants in translocating accumulated carbohydrates from daytime photosynthesis in the leaf down into the root overnight. [6]

• Boron actively encourages uptake of Calcium, thereby helping reduce bitter pit. It is also widely used on pears to minimize storage disorders. [8]

• Boron and Zinc have a critical role to play in bud development, flowering and fruit set. [8]

• Responsible for pollen germination and pollen tube formation and activation. Thus, poor Boron supply results in poor flower and fruit formation. In severe cases it causes 'blossom blast' where flowers dry out and are shriveled at bloom. [8]

• Boron deficiency has been linked to numerous diseases, whether fungal, bacterial, or viral…Boron deficiency also affects the resistance of the plant to certain insect pests, such as mites…Conversely, applications of Boron to the soil or in the form of leaf sprays…stimulate the plant’s resistance to certain diseases. [9]

Boron facts:

• Care must be exercised in the use of Boron, since the difference between deficient and toxic levels is often small. [7]

• Boron actively encourages uptake of Calcium, thereby helping reduce bitter pit. It is also widely used on pears to minimize storage disorders. [8]

• Boron is crucial for plant reproductive success. It beneficially affects flower formation, pollen production and viability, whereas a deficiency causes premature flower and fruit drop. [1]

• Boron toxicity inhibits pollen germination and stops pollen tube growth in apples. At bloom stage, if a foliar Boron application is too highly concentrated, it is harmful to pollen tube growth. [2]

• Boric acid is a plant-available form of Boron. Additionally, Boric acid is toxic to cockroaches, termites and ants. [3, 4] Ants and bees are related and from the same insect taxonomic Order. 

• Bloom stage chemical thinning applications, with Boron included, hindered honeybee activities. “…Boron is slightly toxic at a biochemical level.” [5] 

Boron is essential for plant productivity. The downsides of deficiencies and  toxicities are equally serious. Applying Boron at timings and with methods which don’t hinder plant efficiency is of vital importance. At key stages of development, there isn’t room for mistakes despite your noble intentions.  I’ve met many successful growers who have improved their efficiency by modifying how or when they apply the needed nutrients. 

About ten years ago, I stopped advising growers  to make prebloom/bloom Boron foliar applications. I have since shifted micronutrient (i.e. Boron, Zinc, Manganese, etc.) applications  to the postharvest window to ensure better absorption and availability for the coming season. Multiple years of spring postbloom plant tissue analyses have confirmed the efficacy of fall micronutrient applications, with tissue levels at or above Sufficiency, which has proven to be adequate for the entire bloom period as well as early plant development activities. In addition, fall applications reduce the risk of interfering with pollinators and pollination activities during bloom stage thereby improving the likelihood of pollination and fertilization success. Moreover, having adequate stored plant reserves readily available facilitates a more productive early spring growth response from a healthy canopy. 

Growing crops is challenging. Improving the efficiency of your nutrient applications and pollinator activity are foundational for a productive growing season.

Here’s to your crops’ success!

References:

1. 0, 2023. ISBN: 978-0-12-819773-8

2. Boron Toxicity Causes Multiple Effects on Malus domestica Pollen Tube Growth. Frontiers in Plant Science, 2016,  doi: 10.3389/fpls.2016.00208

3. A Review of Boron Toxicity in Insects With an Emphasis on Termites. J. Agric. Urban Entomol. Vol. 23, No. 4 (2006). 

4. Asymmetrical Behavioral Response Towards Two Boron Toxicants Depends on the Ant Species (Hymenoptera: Formicidae). J. Econ. Entomol. 2013 doi: 10.1603/EC12246

5. Effects of Eco–Friendly Flower Thinning Formulations on a Pollination Insect, Apis Mellifera. J. Fac. Agr., Kyushu Univ., 65 (2) (2020).

6. The Farm as Ecosystem. Jerry Brunetti. 2014 ISBN: 978-1-30173-041-1

7. Western Fertilizer Handbook. – 9th Ed. California Plant Health Association 2002. ISBN: 0-8134-3210-3

8. YARA Pome Fruit PlantmasterTM; Yara International ASA Oslo, Norway

9. Healthy Crops. Francis Chaboussou, 2004 (trans.). ISBN: 1 897766 89 0Marschner’s Mineral Nutrition of Higher Plants – 4th Edition. Zed Rengel, Ismail Cakmak, Philip J. White, Eds., 336-35