Form taken up by plants: H3BO3
Soil Mobility: Yes
Plant Mobility: Intermediate mobility in the phloem
Deficiency symptoms: Necrosis of leaves and terminal buds, this often leads to a bushy appearance. Thick, brittle, pale green leaves. Irregular shedding of flowers or fruit. Fruit develops corky lesions and splits irregularly ("Black-heart" or heart-rot in vegetables).
Soil O. M. interactions: Boron is complexed by O.M. and can be a major source of B to plants. Mineralization of O.M. releases B to soil solution. Tourmaline is the mineral source of B in soils and is a very insoluble borosilicate mineral.
Effect of pH on availability: Boron availability decreases with increasing pH. Overliming acid soils can cause temporary B deficiency due to B adsorption on freshly precipitated Al(OH)3 and interaction with Ca2+.
Role of soil characteristics: Boron is generally less available on sandy soils in humid regions due to increased leaching. Especially true on acid soils with low O.M. B availability increases with increasing O.M. Most alkaline and calcareous soils contain sufficient B since primary B minerals have not been highly weathered or leached out as in humid region soils.
Roles of B: In plants: Root elongation and nucleic acid metabolism, cell wall synthesis, phenol metabolism, IAA metabolism, tissue differentation, aids in membrane function and integrity, and aids in pollen tube growth.
In humans: Ca and Mg metabolism, B is needed for proper absorption of Ca for healthy bones and joints, necessary for proper cell function. Optimum level 3 mg/day. Sources: Almonds, beer, soybean, vegetables, wine.
Concentrations in soil: Total B in soil is small ( 20-200 ppm)
Deficiency levels in plants: Monocots: 5-10 mg/kg Dicots: 20-70 mg/kg Gum-bearing plants (poppy): 80-100 mg/kg
Toxic levels in plants: Corn: >100 mg/kg Cucumber: >300 mg/kg Wheat: >10 mg/kg Alfalfa: >100 mg/kg
Toxic levels in soil and water: Boron can be toxic on some alkaline soils when soil test or extractable B exceeds 5 ppm. Irrigation water that contains >1 ppm B can also produce toxicity. B availability index: Hot water soluble B test Soil solution: 0.2 ppm deficient >1 ppm toxic Soil weight basis: 1-5 ppm marginal >5 ppm toxic
B fertilizers: Borax: (Na4B4O7 10 H2O) 10-11% B Boric acid: (H3BO3) 17% B Colemanite: Ca2B6O11 5H2O 10-16% B Sodium pentaborate: Na2B10O16 10 H2O 18% B Sodium tetraborate: Na2B4O7 5 H2O 14-15% B Solubor: Na2B4O7 5 H2O + Na2B10O16 10 H2O 20-21% B
Other sources of B: Animal wastes: 0.01 to 0.09 lb/ton of waste at 72-85% moisture. Sewage effluent: 0.3 to 2.5 ppm B. Average is 1.0 ppm. Coal: Up to 400 ppm. Average 50-60 ppm. Coal combustion produces a residue called fly ash has up to 600 ppm. Averages 50-60 ppm.
References: Marschner, H. 1997. Mineral Nutrition of Higher Plants. 2nd ed. Academic Press Inc. San Diego, CA. Mortvedt, J. J. 1972. Micronutrients in Agriculture. Soil Science Society of America. Madison, WI. Phillipson, Tore. 1953. Boron in plant and soil with special regard to Swedish agriculture. Acta Agriculturae Scandinavica. III:2. Raun, W. R., G. V. Johnson, and S. L. Taylor. 1996. Soil-Plant Relationships, Oklahoma State University, Agronomy 5813 class notes. Taiz, L. and E. Zieger. 1991. Plant Physiology. Tisdale, S. L., W. L. Nelson, J. D. Beaton, and J. L. Havlin. 1993. Soil Fertility and Fertilizers. 5th ed. MacMillan Publishing Co. New York, NY.