Really, how exciting is an element like boron? Yeah, it's some chemical from the periodic table that plants need, but you might be thinking, "I only need to know the N-P-K plus a few others, right?" You could be asking yourself, "why talk about boron when there are more than a dozen other required and even more recognized beneficial elements and compounds?"
Simply put, boron is that little known element that is often masked in obscurity. Whether you fall victim to this view point or not, strap in because we are going to navigate through the primary role boron has on plants and just maybe strengthen your expertise on this little-known, but very important, micronutrient.
Uptake and Movement of Boron in Plants
A form of boron that is readily available and water soluble is boric acid. This weak acid is added to most commercially made fertilizers or can be individually supplemented via foliar application. The primary mode of uptake occurs via transporters in the root cells. Once inside the root cells, boron will move with the transpiration stream, loading first into the active growing sites of the stem and leaves.
For foliar application, the movement of boron is fixed (immobile) within the leaf itself. Therefore, foliar application for most plants will only have a localized (leaf) effect. For some members of the rose family, boron can be transported down the phloem, attached to the sugar alcohol complex and utilized by the stem and roots.
Fortifying the Cell Wall
Boron, like calcium or the beneficial element silicon, has an important structural role in the development and growth of most plants. In fact, 90% of total boron content within the plant is located in the cell walls. Boron helps with the cross-linking of the complex carbohydrates (polysaccharides) that comprise the cell wall. Imagine several metal fences in close proximity to one another. Among its structural roles, boron helps link these fences together further promoting the rigidity and structural support for each cell, culminating the plant's overall strength. While boron has several other additional roles, cell wall fortification is the most visible function; a deficiency can lead to structural issues.
Problems of Toxicity and Deficiency
Boron accumulates within the leaf margin when immobile (species specific); therefore, excess accumulation will cause discoloration and subsequent death of this area of the leaf. Look for browning or blackening of the leaf tip and margin as clear diagnostic signs of boron toxicity.
In plants where boron is mobile, death of young shoots and leaves are likely signs of boron toxicity. Both boron deficiency and toxicity will reduce or cease plant growth causing stunting. The root response to an absence of boron, especially in hydroponics, can be fast. Inhibition of root growth can occur within three hours when boron is totally removed from the nutrient solution and completely stopped after 24 hours.
The good news is rectifying the situation by adding boron to the root zone is equally quick, occurring within 12 hours of application. Since boron is affected by leaching, avoid overwatering and flushing out boron, or consider foliar supplementation if necessary.
Like several nutrients, the optimal range for boron separating deficiency from toxicity is relatively small. Boron, at concentrations below 0.2 ppm, is likely to result in signs of deficiencies just as concentrations above 2 ppm can result in symptoms of toxicity.
Boron toxicity will greatly impact root development as it has been shown to stop cellular division of the root meristem, which can subsequently cause a myriad of problems downstream.
As one group of authors best described it, "boric acid toxicity is more difficult to manage than boric acid deficiency, which can be avoided by fertilization." Conceptually this makes sense as there is probably more leeway for correcting the boron concentration when it is needed versus when it is in excess and the damage has irreversibly occurred.
Therefore, a little amount goes a long way, but careful monitoring is needed to ensure levels do not fall below a critical threshold. One final word of caution: the above-mentioned guidelines will not match all plants' needs. What is toxic for one plant will be the optimal requirement for another, so do your own homework prior to supplementation above the recommended dose.
Commercially prepared fertilizers will provide the necessary boron requirement for the vast majority of plants.
Benefits of Boron Supplementation in Plants
Besides its benefits to an established plant, boron may also aid in the health of plant cuttings. Sunflower cuttings have shown a positive response to the addition of boron. In this study, 0.1 mM boric acid or 1.1 ppm boron caused the highest amount of adventitious roots (formation of roots from a non-traditional area such as the stem, leaves, etc.) per cutting relative to the control group.
While this study did not concretely state the direct role boron may have on adventitious root formation, it was speculated that it may play a role in promotion of endogenous (internal) auxin production.
In addition, since one of boron's roles is structural enhancement, foliar supplementation of boron may help reduce leaf drop in cuttings. I base this on personal experience but cannot conclude that the results will be uniform across all plants.
For calculations, remember boron constitutes only 17.7% of the weight of boric acid, so if you want 0.5 ppm boron, you will need to add 9.87 oz. per 0.26 gal. of boric acid. Always start small, and slowly work your way up until the deficiency has been rectified and normal growth resumes. In addition, make sure you have your water source tested prior to use. Boron levels in the water should not exceed 1 ppm for general hydroponic use.
Boron, while not often discussed with the big boys (N-P-K), is an integral structural micronutrient whose role within plants is still being investigated today. Unfortunately, the need of boron and the sensitivity to it is often quite variable within the plant community.
Some plants, such as the tomato, flourish with a constant supply and will elicit a quick response with its removal, while other plants, such as peas, are more tolerant and have a delayed response.
In hydroponics, it is prudent to maintain optimal levels of boron as the plant's response and subsequent decline in growth is often quick. Remember, the line between deficiency and toxicity for most plants is minute, so don't overdo it!
While not glamorous, now you have a better appreciation for this lesser-known micronutrient, boron.