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 (BO₃^3-)
  • 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 (BO₃^3-)

• 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

Sources:

• The Intelligent Gardener
• Plant Nutrition and Soil Fertility Manual