Boron (B) is classified as an essential micronutrient because it is used in relatively small quantities in plants and is necessary for plants to complete their life cycle. In Minnesota almost all soils are capable of supplying adequate amounts of B for crop production. Research in Minnesota has shown using B fertilizer improves alfalfa yield and specialty crop yield/quality on a few sandy soils. Where needed, utilizing B can be profitable. However, there is a fine line between meeting B requirements and B toxicity in plants. Care should be used in the decision about using B in your fertility program.
Boron's role in plants
Boron plays a major role in the cell wall biosynthesis that primarily influences many growth factors including root elongation, tissue differentiation, pollen germination, pollen tube growth, and cell membrane functions. Boron exists mostly in dicotyledonous plants within cell wall components. Boron is also known to function in several metabolic pathways. For example, B is involved in the mechanisms for the synthesis and transport of carbohydrates and proteins. Boron is important for nodulation in legume plants because it accelerates atmospheric N fixation.
Soil organic matter (OM) is the primary source of B. Boron becomes available for plants as OM decomposes. Plant available B exists in the soil solution primarily as an un-dissociated boric acid (H3BO30). Because H3BO3 is a neutral molecule, it is not attracted to soil particles and OM. As a result, H3BO3 can be readily leached from soils with excess rainfall and irrigation. Leaching of B is greatest in coarse-textured soils with low OM. Therefore, plant response to B is most likely on sandy textured soils in Minnesota. Drought can decrease B availability in soil as lack of moisture may slow OM decomposition. In addition, B moves to the plant root by mass flow, which is restricted in dry soils. Figure 1 shows where a B deficiency is most likely in Minnesota.
Depending on the plant species, B can be mobile or immobile within the plant. Boron is mobile in some fruit and vegetable species and is associated with the production of polyols in the plant (sorbitol, mannitol). For most of the major crops grown in Minnesota (alfalfa, corn, soybean, small grains, sugar beet, potato, and sweet corn) boron is immobile and therefore deficiencies are observed on the youngest leaves first.
Deficiency symptoms in alfalfa
Boron deficiency symptoms occur as stunting on the upper part of plants. Terminal buds and youngest leaves in the shoots become discolored and may die. Internodes become shorter giving plants a bushy appearance (Figure 2). Mature leaf blades become severely deformed. Older leaves near the bottom of the plant stay green. Growing points of alfalfa become yellow and die under a severe B deficiency. Plants do not produce blossoms and winterkill easily resulting extensive yield loss. Deficiency of B in alfalfa can be confused with leafhopper burn and therefore a suspected B deficiency should be confirmed with soil and tissue tests.
Deficiency symptoms in corn and soybean
In corn, B deficiency causes short and bent cobs, barren ears and stalks, poor kernel development, elongated and watery stripes later becoming white on newly formed leaves, and eventually dead growing points. Since many other stresses can result in similar symptoms, it is important to have both soil and corn plant samples analyzed for B before confirming a deficiency.
In soybean, specific symptoms caused by B deficiency include yellowing leaves, curling of leaf tips, interveinal chlorosis, and dieback of tips. Soybean roots are stunted and flowering stops under severe deficiency conditions.
Deficiency symptoms in sugar beet and cauliflower
Tip burn and brown hollow heart can occur in the head of vegetable crops such as cauliflower and broccoli. Necrosis in the growing areas leads to heart rot in root crops.
In cauliflower, boron deficiency causes hollow stem and curds become discolored and deformed (Figure 3).
In sugar beet, the young center leaves of crown remain small and become chlorotic (Figure 4).
The petioles become fragile and crack easily and the new leaves in the growing point may turn black and rot. The root begins to die off which leads to heart rot disease.
Crops that respond to boron
Crops vary in B need. Table 1 shows the response to B that might be expected from various crops. A boron soil test, available through most soil testing laboratories, is especially appropriate for sandy soils where a response to boron might be expected. Table 3 lists current suggestions for boron use in a fertilizer program.
Table 1. Potential for a crop response to boron when applied to born deficient soils.
|Alfalfa, apple, broccoli, canola, cauliflower, celery, red beet, sugar beet, sunflower, turnip||Cabbage, carrot, clover, grape, lettuce, onion, radish, spinach, strawberry, tomato||Asparagus, barley, blueberries, field corn, cucumber, oats, pasture grasses, pea, pepper, potato, raspberry, rye, soybean, sweet corn, wheat|
Table 2. Sufficiency levels of boron for major agronomic crops, vegetables, and fruit grown in Minnesota.
|Crop||Plant part||Time||Sufficiency range - ppm|
|Alfalfa||Tops (6" of new growth)||Prior to flowering||30-80|
|Apple||Leaf from middle of current terminal shoot||July 15- August 15||30-50|
|Blueberry||Young mature leaf||First week of harvest||25-70|
|Broccoli||Young mature leaf||Heading||30-100|
|Cabbage||Half-grown young wrapper leaf||Heads||25-75|
|Carrot||Young mature leaf||Mid-growth||30-100|
|Cauliflower||Young mature leaf||Buttoning||30-100|
|Edible bean||Most recetnly matured trifoliate||Bloom stage||5-24|
|Field corn||Whole tops||Less than 12" tall||5-25|
|Base of ear||Initial silk||5-25|
|Grape||Petiole from young mature leaf||Flowering||25-50|
|Pea||Recently mature leaflet||First bloom||25-60|
|Potato||Fourth leaf from tip||40-50 days after emergence||20-40|
|Raspberry||Leaf 18" from tip||First week in August||25-300|
|Soybean||Trifoliate leaves||Early flowering||20-60|
|Spring wheat||Whole tops||As head emerges from boot||6-10|
|Strawberry||Young mature leaf||Mid-August||25-60|
|Sweet corn||Ear leaf||Tasseling to silk||8-25|
|Sugar beet||Recently matured leaves||50-80 days after planting||31-200|
Plant tissue analysis for boron
The sufficient concentration of B in diagnostic plant tissue varies by species and time of sampling. Table 2 lists sufficient concentration of B in plant tissue for major agronomic and horticultural crops in Minnesota. To determine B status, newly developed plant tissue should be sampled for majority of crops since B is not mobile in the plant. Concentration of B in excess of sufficiency level can present toxicity concerns for some crops. Boron fertilizer should not be applied to crops that contain sufficient concentration of B. Soil samples should be collected along with plant tissue to confirm whether a deficiency if present.
Soil testing and recommendation for boron in Minnesota
Boron fertilizer should be applied based on recent soil test results. A soil test for B is available through most soil testing laboratories. The hot water B test is appropriate for sandy soils or soils in high rainfall areas where a response to B might be expected. Alfalfa and some fruit and vegetable crops are the only crops in Minnesota that may respond to B application. Table 3 lists current suggestions for B use in a fertilizer program for alfalfa and selected horticultural crops.
Table 3. Suggested soil test interpretation for crops with a greater sensitivity to boron deficiency for the hot water extractable boron test.
|Soil test boron||Relative level||Boron to apply|
|Less than 1.0||Low||2-4|
|More than 5.0||High||0|
Boron field studies in Minnesota
In Minnesota, alfalfa is more responsive to B application than any other commodity crop. Boron treatments have resulted in improved alfalfa growth on demonstration plots throughout the area outlined in Figure 1.
Boron concentration will decrease when soils dry. It is difficult to determine whether decreased yield is caused by less available B or inadequate rainfall. It has often been demonstrated that B in plant tissue increases after rains occur. The rainfall moistens the soil and B is released. In this situation added B will likely not provide an increase in tonnage. Plant tests will be more reliable with normal soil moisture conditions.
Widespread reports of low B concentration in corn tissue samples collected at different growth stages have been common in recent years. Experiments where B was applied to corn in Minnesota have shown no increase in grain yield (Table 4). Boron concentration in plant tissue was not measured. Some soil test interpretations may give a recommendation for B application at some of the study sites because of low B concentration in the soil. The lack of an increase in corn grain yield demonstrates that the soil B test is unreliable for corn production and needs to be interpreted differently for crops that are less sensitive to a B deficiency. Boron fertilizer is not needed for the majority of corn grown in Minnesota.
Table 4. Corn grain yield (reported at 15.5% moisture) for plots with (+B) and without (-B) 5 lbs of a 10% granular boron fertilizer broadcast to the soil before planting
|Year||Location||St B*||Grain yield**||Grain yield**|
Research was conducted on soybean at 12 locations across Minnesota from 2013 to 2014 (Table 5) where a 2 lb rate of B per acre was applied. Application of B fertilizer almost always increased the concentration of B in soybean trifoliate samples collected at R1-R2 growth stage. Trifoliate B concentration was well above the defined sufficiency levels. This indicates B was sufficient when not applied. Soybean grain yield was not increased at any site and was decreased at two of the twelve locations. Soil test B ranged from 0.3 to 1.1 ppm and was an unreliable predictor of soybean yield response to B. Because of heightened toxicity concerns, B fertilizer should not be applied to soybean even if a recommendation is made based on soil test results.
Table 5. Soybean grain yield and R1-R2 trifoliate boron concentration from twelve Minnesota locations where 0(-B) or 2 (+B) lbs born per acre of a 10% granular boron fertilizer was broadcast to the soil surface before planting.
|Year||Site||Soil test B*||Trifoliate B Conc. **||Trifoliate B Conc. **||Grain yield||Grain Yield|
Boron fertilizers can be easily blended with other common fertilizers. Table 6 lists some common B sources along with concentrations.
Boron in sewage and manure wastes
Boron in manure is usually very low, ranging from 0.02 to 0.12 pound per ton in sewage and manure waste products. At the greatest concentration, a rate of 20 tons per acre would barely meet the plant B needs where B deficiencies are known. Sewage sludge is not considered a good B source.
Table 6. Some common born fertilizer sources and quantity needed to supply 1 lb/acre.
|Material*||Formula||Approximate B (%)||Quantity needed to supply 1 lb/acre|
|Borax||Na2B4O7 • 10H2O||11||9.1|
|Sodium pentaborate||Na2B10O16 • 10H2O||18||5.6|
|Fertilizer borate - 46||Na2B4O7 • 5H2O||14||7.1|
|Fertilizer borate - 65||Na2B4O7||14||7.1|
|Solubor||Na2B4O7 • 5H2O||20||5.0|
|Na2B10O16 • 10H2O|
Method of application
In Minnesota, B is the only micronutrient that might be needed in a fertilizer program for alfalfa.
The first signs of B deficiency in plants shows in the new growth. Therefore, foliar application of B is not sufficient for later in the growing season.
Soils that have either marginal or deficient level of B are limited to the east central and north central region in Minnesota. A soil test for B is available but this test is recommended for use only in these two areas just mentioned. The quantity of B fertilizers required to apply 1 lb of B per acre are listed in Table 6.
When needed, B fertilizers can be top dressed to an established alfalfa stand. Boron should be broadcast with phosphate and/or potash fertilizers for best results because of the low volume of fertilizer required.
For vegetable crops with a high demand for B like cauliflower or broccoli grown on sandy soils testing low in B, 2 to 4 lbs of B per acre should be broadcast and incorporated before planting.
Boron is mobile in soils and should be applied to annual crops each year when needed. This nutrient should not be applied directly to actively growing green tissue because serious plant injury could occur.
Boron fertilizers should never be applied directly in contact with the seed. Broadcast application of B is recommended over the use of an in-row treatment. Broadcast applications are safer when applied one to two weeks before seeding.
Boron applied to a perennial crop such as alfalfa will usually last for more than one year. A common practice, on known B-deficient soils, is to use a borated fertilizer mixture once every three years.
It is better to withhold B from a new seeding of alfalfa until after the first year of production, if oat is the companion crop. Oat is sensitive to rates of B needed for alfalfa.
Foliar sprays can be used on severely deficient fields. Use 0.1 to 0.3 pound of B per acre for foliar sprays. CAUTION: do not spray on hot days when the crop is under moisture stress.
Boron toxicity and tolerance
A major issue with B application is B toxicity. This toxicity can significantly affect crops because of over application of the nutrient. Crops such as soybean and edible bean are very sensitive to B toxicity. Both crops are commonly grown on sandy irrigated soils where soil test for B may indicate a potential deficiency. Application of B to sensitive crops should be avoided regardless of the soil test B concentration. Soil test B interpretations for alfalfa should not be used for sensitive crops such as soybean or edible bean.
Boron toxicity is typically exhibited in plants as a yellowing of leaves with scorching on the leaf edges (Figures 5 and 6). Toxicity symptoms can occur anywhere in the crop canopy. Since B is not mobile in most agronomic crops grown in Minnesota, toxicity symptoms will most likely occur at the time that excessive levels of B are made available to the plant. Because B is mobile in the soil, plants can in some cases recover from B toxicity with adequate rainfall or irrigation.
Soil tests and plant analyses have been developed as management tools to predict where and when B will be needed. Using B in a fertilizer program can produce substantial production increases of a very limited number of crops, resulting in improved net profit to the grower.
On sandy soils, especially if not irrigated, B is frequently needed to maximize alfalfa yields. Irrigation and natural rainfall may hasten decomposition of soil organic matter and the release of B. This will reduce the need for B fertilizer additions on some soils. The beneficial effect of B fertilizer on corn and soybean grain yield has been inconsistent. Application of B to corn and soybean is discouraged because of a greater potential for a B toxicity.
Reviewed in 2016