While farmers don’t think much about applying boron, a micronutrient needed in small amounts, it is the most common micronutrient deficiency in soybeans, followed by manganese. While many growers recognize or are aware of manganese deficiency (often related to glyphosate applications), most aren’t aware of boron.
Boron is an important micronutrient that plays a role in plant development, reproduction and sugar movement. Soils typically provide sufficient amounts of boron to the plant, but in some instances it can be deficient. Because boron is needed only in very small quantities, and since an overdose is potential toxic, care should be taken in its use in order to not over apply. One of many strange features of boron deficiency is it can show up one year and not the next.
Soil type and condition greatly influence nutrient availability. Poorly drained soils, highly weathered soils, highly variable soils, or soils with a nutrient imbalance when high amounts of manure were applied are good candidates for micronutrient applications. Deficiencies can occur in areas of low rainfall when root activity is restricted, in alkaline or strongly acidic soils, sandy soils or soils low in organic matter. The boron ion (borate is like nitrate) is mobile in soil and can be leached from the root zone during heavy rainfalls, especially in lighter sandy soils. The best way to determine a micronutrient deficiency in soybeans is to use plant tissue analysis in combination with soil test data.
A boron soil and tissue test is available through soil testing laboratories. A soil test is most appropriate for sandy soils where a response to boron might be expected. A soil testing from 0 to 0.9 ppm would be classified as low. A soil testing from 1.0 to 5.0 ppm would be adequate. A soil testing more than 5.0 ppm would be excessive. For tissue, the sufficiency levels are 20 to 55 ppm. The critical level of boron in the top mature soybean leaves is about 20 ppm, but the ideal level is about 30 to 40 ppm. Greater than 55 ppm is high.
Boron-deficiency symptoms first appear at the growing points. Shortened internodes and yellowing or reddening of upper leaves are symptoms of boron deficiency in soybeans. Boron deficiency results in interveinal chlorosis with brittle leaves in the youngest trifoliates at the time of their appearance. Stunted roots with few or no flowers also may be evident. Floral buds may wither before opening, leading to abortion.
To correct a soil deficiency, 0.5 to 2 pounds per acre of boron is usually applied. If using a solid fertilizer material, boron can be surface broadcast on the soil and incorporated. Boron materials such as boric acid or Solubor® can be liquefied, then sprayed on the soil surface. Fertilizer boron should never be banded because at high concentrations it can be toxic to seeds, seedlings and plant roots.
Foliar boron is compatible with most pesticides and other nutrients. Foliar applications can be made several times during the growing season because boron is not mobile in the plant. As new growth appears, repeated spraying is required to get boron into the new growth. And a 1 lb. application rate can be divided into smaller doses and applied each time a foliar is applied on the field. One or more foliar applications of boron over the growing season may produce better results than one pound of boron broadcast and incorporated into the soil before planting.
Let us know in our forums if you have used boron in your fertility program and if you’ve seen positive results.
Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at email@example.com.