Boron (B)

  • Boron is one of the 13 essential elements for plant growth, and is considered a micronutrient based on the concentration required for plant growth
  • principle form of uptake:
    • boric acid (H3BO3)
    • borate (BO33-)
    • boric acid is preferred over borate
    • as a condition of increased pH (increased alkalinity), boric acid converts to borate
  • relatively immobile in the plant
    • transported primarily in the xylem via transpiration
  • boron ions primarily move through the soil via diffusion
  • soil conditions related to deficiencies in boron:
    • acid igneous soil
    • sandy soil leached by water
    • calcareous soil
    • soil low in organic matter
    • acid peat & muck soil

Boron Requirements in Plants

  • amount required for healthy plant growth: 20 ppm, in dry soil matter
  • target level for soil content IG:
    • TCEC < 10 = 1 ppm Boron
    • TCEC > 10 = 2 ppm Boron
  • minimum concentration at which no obvious deficiencies appear: 1 ppm
  • sufficient concentration: 1.5 ppm
  • excess (toxic) concentration: 2 ppm for a few crop species, but this level is comfortable for most crops (see Boron Toxicity)

Boron Deficiency

  • soil levels: less than 1 ppm will produce deficiency symptoms
  • Plants that are sensitive to boron deficiency PNSF:
    • legumes
    • brassicas (broccoli, cabbage, brussel sprouts, mustards, etc)
    • beets
    • celery
    • grapes
    • fruit trees
    • cotton
    • sugar beet
  • symptoms of boron deficiency in plants PNSF:
    • abnormal development of growing points (meristematic tissue)
    • apical growing points eventually become stunted and die
    • flowers and fruits will abort
    • for some grain & fruit crops, yield and quality are significantly reduced
    • plant stems may be brittle and easily break

Boron Toxicity or Excesses

  • first crops to show boron toxicity symptoms: GREEN BEANS
  • Sensitive to excess boron PNSF:
    • cereals
    • potato
    • tomato
    • cucumber
    • sunflower
    • mustard
  • semi-tolerant to excess boron:
    • celery
    • corn
    • lima beans
    • peppers
    • potatoes
    • pumpkin
    • radish
    • sweet potatoes
    • tomatoes
  • symptoms of boron excess PNSF:
    • leaf tips and margins turn brown and die
    • leaves eventually assume a scorched appearance and prematurely fall off

Amending Soil Boron Levels

  • Note: it’s very easy to add too much boron, so unless you have a soil test that reports your starting boron levels, it is not recommended to add any boron
  • mild excesses of boron will leach out in a few years
  • boron is easy to spread evenly, since it is soluble in water
    • dissolve in hot water then spray uniformly on soil
    • make sure to spray the boron solution directly on the soil, since leaves cannot take it up

Soil factors that affect boron uptake by plants:

  • available levels in soil
  • temperature & decomposition of organic matter
  • soil pH: high pH (basic/alkaline) soil decreases the plant uptake of boron

Plant factors that affect boron uptake by plant:

  • hybrid or variety of plant
  • stage of growth
  • interactions with other elements

Sources for Boron:

  • organic debris, plant residues, and microorganisms are the major sources of boron
  • organic matter (humus) acts as a storage facility for anions, like borate (BO33-)
  • Boron Source  Formula Composition 
    Borax Na2B4O7·10H2O  11% B
    Boric acid H3BO3  17.5% B
    Solubor Na2B8O13·4H2O  20% B

Roles in Plant Functions:

  • involved in carbohydrate reactions
  • associated with pollen germination & growth, as well as improving the stability of pollen tubes
  • part of RNA formation (uracil)
  • used in cellular activities (division, differentiation, maturation, respiration, growth, etc)
  • unique and vitally important job to do IG:
    • moisture conducting tubes in plant cells must be lined with B
    • “B comes first, and then come the rest of the plant nutrients”
    • B & silicon work together to move moisture up the vascular tubes


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