To the best of my knowledge, chemists and ag scientists doing research on the mechanism of boron’s working in plants has been few. It has been widely observed, discussed and research that boron is necessary for the uptake of calcium. I myself spent 30 years figuring out how boron worked, and I believe the problem has been obscured by the famous disregard of, or lack of interest in, silicon. This trend started with Liebig’s rule that to prove an element essential it must be excluded from the growing medium, and this has proven impossible with silicon. Thus by Liebig’s rule silicon has been ignored as non-essential. As far back as 1924, when Rudolf Steiner gave his Agriculture Course, the biodynamic movement has identified silicon as one of the two most important elements in the chemistry of plants and animals. Steiner was a chemist by training, and one of the most forward thinking of his time. But because he was far more famous for his teaching of the science of the invisible—i.e. spiritual science—his contributions to agricultural and medicinal chemistry have tended to be ignored.
Change has been slow, but silicon has increasingly been ‘coming out of the closet’ lately. One of the best agricultural biochemists of our times, Horst Marschner, discusses the role of silicon, the nature of its essentiality and its interactions with boron on pages 417-422 in his maximum opus, Mineral Nutrition of Higher Plants, first published in 1995. Marschner points out the deposition of silicic acid (as well as silica’s sibling, germanic acid) and traces of boric acid in the cell walls and linings of xylem cells, but even though he acknowledges the interaction of these acids on ortho diphenols, he considers these silicic and boric acid complexes inert. In short the mechanism by which boron works to provide calcium uptake is poorly understood.
From my own studies over 30 years of research in actual farming situations involving over 100 cultivars of more than 40 kinds of fruits and vegetables I found that with grasses, particularly, ginger, turmeric, sweet corn, maize and sorghum, that demand for boron was lower than in broadleaf plants such as clover, alfalfa (lucerne), garden beans, soybeans, potatoes, tomatoes, pecans, apples, stone fruits, etc. Over the course of several years, starting with the initial advice from my local coop agronomist that legumes would fail to nodulate on my soil (B = 0.2 ppm) without a 5 lb./acre supplement of borax (which I applied and got beautiful nodulation in clover and soybean covers, I investigated the mechanism of boron’s working over the following 30+ years. Boron certainly was not directly involved in nitrogen fixation, so why was it essential for effective nodulation? And why did legumes like clover and soybeans require 5 times the levels of grasses such as maize? One of the clues was that nitrogen fixation is a VERY energy intensive process, requiring at least 10 units of sugar for every unit of amino acid produced. Another was that leguminous crops such as lucerne and beans, peanuts, etc., to say nothing of potatoes, and brassicas in general, were obviously hollow stemmed when lacking in boron, but with sufficient boron the stems were solid. All manner of visual signs showed improvement in water uptake with solid stems—lack of wilting in noon day sun being one of the most obvious. Other signs, in other vegetables clearly pointed to boron’s involvement in sap flow. Boron deficient sweet corn and maize would not fill cobs, strawberry hearts were hollow, citrus (where sap flow moved down the central pithy core to the stylar end and then filters back in a network of capillaries under the skin to fill out the fruit) would be insufficiently juicy toward the stem end, pecans had similar problems with shrivelled butts, potatoes developed hollow heart, tomatoes suffered from blossom end rot, and, of course, legumes failed to receive enough energy in nodules, fed by sap from the phloem carrying the photosynthetic products needed for nitrogen fixation.
While continuing my research in the tropics it soon was apparent that bananas set more hands of fruit with higher boron levels. And when available mono and polysilicic acids increased bananas were both more productive and sweeter, but this also depended on increasing boron availability along with the extra available silicon. In instances where only diatomaceous earth was applied without supplemental boron there was little change. However, one of the most dramatic occurrences was with mangoes. Nutri-Tech Solutions had spent years selling products based on the relationship between humic acids and boron uptake and had developed a dry granulated product called humate stabilized boron. This was water soluble, could be applied via fertigation, but could also be applied via fertiliser box or broadcast. I lectured in 2004 in Atherton concerning my Biochemical Sequence and mentioned this product as a boron supplement to be applied at a rate of 25 kg/ha. A mango grower from Georgetown picked up on this relationship of boron and 25 kg/ha but failed to associate the humate bit. Not knowing that Nutri-Tech’s product was only 3% actual boron, he applied solubor at 25 kg/ha—roughly a 7 fold overdose. He was in a panic by the time his conundrum was conveyed to me. The bark on his trees was splitting and cracks were opening in his fruit. I advised him to quick, visit the (not yet in commercial production) diatomaceous earth mine in nearby Mt. Garnet and get a tipper truck of diatomaceous earth fines and spread them about 5 mm deep under his trees and irrigate as much of this into the soil as possible, and he did this immediately. To his immense relief the cracks closed up in both the bark and fruits and he saved his crop. The next year he had the best mango crop in memory.
The reasoning behind this was from observation there was a relationship between the ratio of boron to silicon in providing sap pressure and the observation of how charge affects chromatographic analysis and separation of compounds, as for example, in column chromatography where ion exchange resins affect rates of flow of various compounds. Silicon has four bonding electrons while boron has only three. Thus when boric acids intersperse with silicic acids in the lining of xylem tissues where upward transport takes place via capillary action, a higher ratio of borates increases the concentration of unpaired electrons and thus there is greater attraction for water and electrolytes. If the concentration of borates is too high this can be remedied by increasing the concentration of silicates, and thus shifting the ratio of borates to silicates back to lower values. This hypothesis is further supported by the reverse case where in calcium saturated soils with high pH there is a tendency at low boron to silicon ratios to not generate enough ionic attraction in the xylem to transport calcium laden fluids upward, resulting in swelling of the roots, which is alleviated by addition of borates rather than silicates to the soil solution.
As a scientist I’m a bit outside the fraternity as I hold no formal degrees, nor do I rely on others to do fundamental research before I consider a hypothesis such as the above. I need only make practical observations and test the hypotheses that arise in a variety of practical circumstances. To the mainstream fraternity of pedigreed science professionals this verges on anathema. They put in long years gaining accreditation and degrees to bolster the credibility of their work. I, on the other hand, find this approach as confining as a straitjacket and wasteful of my precious time. I’ve spent even longer years gaining practical experience, and have as deep or a deeper background and training in the minutiae of observation. (Dad was a keen observer and one hell of a poker player, and I spent a lot of time with him growing up.)
I believe that there has always been a lowgrade undercurrent of scientific interest in silica. Dr John (Sara, Sura, Sulu? something like that) who taught at Virginia Tech back in the mid ‘80s was keen on the study of silica and lectured on the topic at our Georgia Organic Growers’ Association convention in ’86. But I fear he was a little too much outside the square to gain tenure and lost his teaching position. I don’t know what he is doing now. The Safer’s soap rep who also delivered a talk at that ’86 convention confided in me that he always added soluble potassium silicate to his product formulations (organic insecticidal soaps) because it was the silica that made the skins of fruits and veggies strong and shiny and unappetizing for insects. Even back in the late 70’s the university of Missouri did a study of maize where those plants without corn ear worm damage analysed 6-8% higher in silicon than the field averages.
But as for boron’s role in the silica uptake of nutrients via the lining of capillary structures in the xylem, I wish you luck in finding references in the scientific literature if my credentials don’t satisfy. My investigations of boron’s role span 35+ years, so I don’t rely on the depth of scientific literature on this one. But I know and can unerringly fix a dairy paddock where the clover isn’t producing effective nodulation because boron levels show up on the soil test at 0.3 or 0.5 ppm (despite 1 or 2 ppm molybdenum). My local coop agronomist in Blairsville, Georgia had it right back in 1976 that clover or soybeans wouldn’t nodulate effectively without 1 ppm B. I asked him and he didn’t know why back then, thus setting me a puzzle I spent the next 30 years sorting out—a great mystery and a satisfying discovery.