Plants need several essential nutrients to flourish. One of these nutrients is boron, a micronutrient that is essential for cell formation and stability, and pollination. Boron deficiency can make it extremely difficult for a plant to grow to its full potential. Boron deficiency symptoms first appear in newly developed plant tissues due to boron’s low mobility in the plant’s phloem. In other words, most plants are not able to remobilize boron from old to new tissue as with other nutrients, such as nitrogen. Understanding whether a nutrient is mobile or immobile in plants and the available solutions to correct boron deficiency can help you overcome this problem and enjoy a fruitful harvest.
Understand nutrient mobility
Once inside plants, nutrients are transported via the xylem vascular system to plant tissues. Once in those tissues, some nutrients can be remobilized/translocated through the phloem vascular system to growing points in order to compensate for the lack of supply by the soil. Those nutrients that can be remobilized by the phloem are classified as mobile and those that cannot as immobile. Immobile nutrients get “locked” in old plant tissue, while those that can be remobilized move to areas with greater need.
Of all the plant nutrients, boron is somewhat unique because it has limited mobility in most plant species but is freely mobile in others. Boron is mobile in plant species that produce high amounts of the sugar sorbitol such as almonds
, and nectarines. In plants that do not produce significant quantities of sorbitol, boron cannot be remobilized by the plant and will accumulate in old leaves, where it stays.
Because of its low plant mobility in most plants, boron must be uptaken continuously via the root system throughout the growing season. This creates certain challenges to manage boron fertilization. In many circumstances, fertilizer is applied at the beginning of the growing season and not reapplied. For those crops where boron is immobile, when the soil runs out of boron, the farmer needs to apply more boron whenever there is new growth to ensure adequate supply since it cannot the remobilized from old leaves.
Diagnosing and treating boron deficiency
Tissue testing is the best way to determine whether your crops have a boron deficiency. Because boron accumulates in the older leaves of boron-immobile plant species, you do not want to sample mature leaves because they may not reflect the boron status of the growing tissues, for which a constant boron supply is critical. For boron-immobile species, it is appropriate to sample new tissues (leaves).
When it comes to correcting boron deficiency, there is no one-size-fits-all approach. Supplementation approaches will differ depending on management practices and also on the boron mobility of a particular plant species. Some growers may prefer to deliver some or all of their boron needs using foliar applications due to its convenience (eg, tank mix). Others may prefer soil applications, which enable higher application rates and can be more economical, as only one application is usually needed.
The critical success factor is understanding the crop, and then applying nutrients from the right source, at the right rate, right time, and right place. If you only applied foliar boron, you will likely need multiple applications to meet crop demand since foliar application rates are lower than soil application. And, if a single soil application is used at the beginning of the season, it may not be enough to meet the crop demand late in the growing season due to loss mechanisms (leaching
) and limited boron mobility in most plants. In some cases, the best practice is the combination of soil and foliar applications.
It’s important to keep in mind that there is a fine line between boron deficiency and toxicity. For this reason, it is always important to consult with an agronomist.
Fortunately, U.S. Borax has the solutions you need to address boron deficiency in any crop. Granubor®
is ideal for bulk blending for soil application. When you need a foliar application, Solubor®
is the best solution to supply boron to your plants.
For a more comprehensive look at boron mobility in different plant species, including tables of boron concentrations, download Agronomy Note: Boron Mobility in Various Plant Species (PDF)