Trace Element of the Month—Boron By Chris Eidson, Agronomist To continue the “From the Vault” series of Dr. Luke Baker’s write-ups on trace elements, we look back to “Inside Brookside” from November 2009 for comments on Boron.
Boron in soils: Boron (B), the only nonmetal trace element, has a predominant valence state (or charge) of +3 and a small radius. As a result, it always occurs in combination with oxygen in soil (as boric acid). Boron occurs in low concentrations in the earth’s crust and most igneous rocks (approximately 10 ppm). The total concentration of B in soils commonly ranges from 2 to 200 ppm, but values of 7 to 80 ppm are more likely. Interestingly, less than 5% of the total B is available for plant uptake. The most common B-containing mineral found in soils is tourmaline, which is borosilicate. It is highly insoluble and resistant to weathering. As a result, B release is exceptionally slow, which can lead to deficiency symptoms. The two most common forms that are present in the soil solution are H3BO3 (boric acid) and B(OH)4 – . At pH values from 5 to 9, boric acid is the dominant B species and B(OH)4- is present when the pH is above 9. Boron can move in soil via mass flow and diffusion. Since it moves via mass flow (like nitrate and sulfate), it tends to leach. Unlike nitrate, B can to some extent adsorb to clay minerals, aluminum/iron oxy-hydroxides and manganese oxides. However, this adsorption is pH dependent and is greatest when soil pH is between 7 and 9. Therefore, the largest potential source of plant available B in soil is organic matter. Research suggests as organic matter increases so does the potential for plant available B. For example, higher levels of plant available B in surface soil as compared to subsoil is attributed to greater amounts of organic matter. Factors affecting plant availability: Boron usually becomes less available to plants with increasing soil pH, decreasing dramatically above a pH of 6.5. In fact, research has shown that liming acid soils can on occasion temporarily induce B deficiency in susceptible plants (such as alfalfa). The reduction in B availability following liming is due to B adsorption to newly precipitated aluminum hydroxide minerals. In soils with toxic levels of B, liming can be used to help depress B availability to plants. Soil texture has a profound impact on plant available B. For instance, coarse-textured, well-drained sandy soils are low in B and crops that have a high B requirement will often respond to an application of 3 or more pounds per acre of B. However, coarsetextured soils with fine-textured subsoils generally do not react in the same way because the finely textured subsoil is able to slow the loss of B. Thus, there will be some B available in the subsoil for plant uptake. Overall, fine-textured soils reduce B loss and typically have higher soil organic matter (releases B), which decreases the potential for B deficiency. Boron can also interact with other elements in the soil. Plants seem to have a low tolerance to B when calcium supply is low. Alternatively, when calcium supply is high plants need more B. In addition, low levels of B can be further aggravated by increased levels of potassium.
Boron in plants: Plants require B for many growth processes: new cell development in meristematic tissue, proper pollination, translocation of sugars, starches, nitrogen, and phosphorus, synthesis of amino acids and proteins, nodule formation in legumes, and regulation of carbohydrate metabolism. In B deficient plants, the youngest leaves will become pale green, losing more color at the base than at the tip. The basal tissues begin to breakdown, and if growth continues, the leaves will begin to have a one-sided or twisted appearance. Flowering and fruit development will also be restricted by a shortage of B. Other symptoms include: thickened, wilted, or curled leaves, cracking or rotting of fruit, and cracked or rotted roots. In most arable soils, B toxicity is fairly uncommon unless it has been added in excess amounts through fertilization. In arid regions, however, B toxicity may occur naturally or may develop because of a high B content in irrigation waters. Boron toxicity symptoms include chlorosis of tips or margins in older leaves, necrotic spots, brownish leaf tips, and markedly depressed growth. Sources of boron: Only small quantities of B occur in animal wastes, ranging anywhere from about 0.02 to 0.1 pound per ton. Thus, with most manures, average application rates will provide sufficient boron over time.
Fertilizer sources of boron include borax, boric acid, colemanite, sodium pentaborate, sodium tetraborate, and solubor. The most common methods of application are broadcast, banded, or applied as a foliar spray or dust. Foliar applications are most commonly made in the production of fruit. Rates of B fertilization depend on plant species, soil cultural practices, rainfall, liming, organic matter, and other factors. Typically, application rates of 0.5 to 3.0 pounds per acre are recommended. The amount of B recommended depends on the method of application. For example, the B rate for vegetable crops is 0.4 to 2.7 pounds per acre broadcast, 0.4 to 0.9 pounds per acre banded, and 0.09 to 0.4 pounds per acre foliar applied. Always err on the low end when unsure.