BORON – The forgotten micronutrient

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The forgotten micronutrient!

by Noel Garcia, CCA and Howard “Bubba” King

BORON (B) deficiency is the most common and widespread of all micronutrient deficiencies.


In most agricultural soils, Boron is associated with the soil organic matter. With decades of cultivation, the organic matter content of our soils has decreased substantially. Therefore, the principal source of Boron has also substantially decreased.

Most Boron enters the plant passively with the transpiration stream (mass flow – produced by water uptakethrough the roots). When transpiration is reduced, the uptake and translocation of Boron is reduced. Factors that reduce transpiration are high humidity and soil moisture that is either too low or too high, low air temperature and cloudy weather. Boron uptake is in part determined by water uptake rate. Accordingly, drought reduces Boron uptake. Also, Boron deficiency reduces root growth, thus aggravating the Boron stress.

Boron is mobile in soil, yet relatively immobile in the plant; therefore, coarse soils and high rainfall may cause temporary Boron shortages. Boron availability also decreases as soil pH increases above 6.5 pH.

Existing Boron deficiencies can be overcome with multiple foliar sprays and/or multiple injections into irrigation (drip and pivot). Foliar and Drip applications rates of Solubor (20.5% B) of 1 lb./Acre (equivalent to 0.20 lbs of actual B) are common. Applications should be repeated at 7-10 day intervals as plant analysis indicates need.

Other sources of Boron are Borax (11% B), Boric Acid (17% B), Sodium Pentaborate (18% B) and Sodium Tetraborate (21% B). CAUTION – most liquid Boron formulas contain very low amounts of actual B (2 – 4%). Be sure to use enough to be effective.

Boron deficiency symptoms may be observed as abnormal or retarded elongation of growing points and/or apical meristems. Leaves are misshapen, wrinkled, thicker and darker color. Leaves and stems are brittle due to poor cell wall formation or the accumulation of phenols. In Monocots (Grasses), the leaves will spiral or twist. In Dicots (Broad Leaf Plants) the leaf tip will not form completely or will be malformed. Dicots require three to four times more Boron than Monocots. Deficiencies can be prevented with applications of Boron in preplant and sidedress fertilizer applications.

Boron interacts with the other elements in the soil and in the plant such as Calcium (Ca), so therefore must be in balance for proper plant growth. The plant can not properly metabolize Calcium when Boron is low and viceversa. Calcium inhibition of Boron uptake is common in high pH soils.

Low levels of Boron in the plant affect Phosphorous (P) incorporation into cells and P-containing compounds. The adsorption of P may be reduced by half at the roots of Boron-deficient plants. High Nitrogen (N) and/or Potassium (K) levels can decrease the Boron level in the plants. Increasing N and/or K decreases the Calcium level and changes the Ca:B ratio in plants. Some research has indicated that high soil Calcium levels, independent of soil pH, can reduce Boron uptake. Nitrogen deficiency can decrease the vigor of plants to an extent that they may reduce their uptake of adequate amounts of many other nutrients. Boron uptake can also be affected in this manner.

Most crops require approximately two to three pounds of elemental Boron per acre. Crops that have high Boron requirements are the Brassica crops: turnip, broccoli, cauliflower, cabbage and Brussels sprouts; also, alfalfa, cotton, clover, asparagus, sugar beet, table beet, celery, apple, tomato, carrots, lettuce, radish, citrus, corn, onion, peach, peanuts, avocados, pear and conifers. Watermelons and other cucurbits are unique in that they have a high Calcium requirement, so accordingly especially demand an adequate Boron supply.

Boron toxicity can occur at 10 times required rates or above. In Citrus, on the same branch, toxicity symptoms on old leaves and deficiency symptoms on new leaves have been observed (B is relatively immobile in the plant – this makes a steady supply of B very important). In some cases of Boron toxicity, applications of a soluble form of Calcium (gypsum) have reduced the toxic effects.

Remember that while Boron is an essential micronutrient,
it is toxic to the plant at high levels.
However, as mentioned, different plants have varying tolerances and demands.


(in ppm)
(in ppm)
(in ppm)
LOW Below 0.40 Below 0.80 Below 1.40
MEDIUM 0.40 – 1.00 0.80 – 1.50 1.50 – 2.50
HIGH 1.00 – 1.80 1.50 – 2.80 2.50 – 3.50
VERY HIGH Above 1.80 Above 2.80 Above 3.50

Boron Sensitivity of Some Crops

  • Sensitive: Citrus, Avocado, Pecans (accumulates in leaf with age), Beans, Garlic, Onions, Cowpeas, Peanuts, Sweet Potato, Strawberry, Sunflower, Sesame.
  • Semi-tolerant: Broccoli, Carrots, Cucumber, Pepper, Potato, Cabbage, Celery, Corn, Squash, Cantaloupe, Cauliflower, Radish, Turnip.
  • Tolerant: Tomato, Alfalfa, Parsley, Beets, Cotton, Sorghum, Asparagus

Boron has many functions in the plant. It is directly involved in cell differentiation, maturation, division and elongation. It is required for Nitrogen and Calcium metabolism, and the formation, translocation and metabolism of sugars and other carbohydrates. Boron is required for proper pollination, pollen tube growth and seed reproduction. It is involved in cell wall lignin production and differentiation, and the formation of xylem vessels in roots and stems. Boron maintains a balance between sugar and starch. It also plays a vital rôle in the proper function of cell membranes and the transport of K to guard cells for the proper control of internal water balance.

TPSL®’s Ask the Plant® analysis has modified the Boron plant analysis standards that were published over a half-century ago which reflected the high soil Organic Matter levels of those times. TPSL®’s own research and practical hands-on evaluations over decades of plant analysis, over thousands of acres nation-wide and on hundreds of different crops / plants have determined that the original plant Boron standards are inadequate, thus relate to low yields and poor quality of fruit and plants.

Most crop Boron standards used in Ask the Plant® have more than doubled from their original standard established in the 1950s and are modified specifically by crop and specific stage of growth. This allows our clients to develop the highest quality product, promoting the earliest harvest date and producing Maximum Economic Yields based on a proactive balanced nutrient fertility program.

For additional information on Boron, see our Water Analysis Guide Sheet.

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