Boron deficiency is an uncommon disorder affecting plants growing in deficient soils and is often associated with areas of high rainfall and leached soils. Boron may be present but locked up in soil

Soil

Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

s with a high pH

Soil pH

The soil pH is a measure of the acidity or basicity in soils. pH is defined as the negative logarithm of the activity of hydrogen ions in solution. It ranges from 0 to 14, with 7 being neutral. A pH below 7 is acidic and above 7 is basic. Soil pH is considered a master variable in soils as it...

, and the deficiency may be worse in wet seasons. Most of what is known about boron is from the observations of plants grown in boron deficient conditions. The primary role of boron is its involvement in the stabilization of the primary cell walls in plant cells. Boron is also involved in the carbohydrate metabolism in plants, protein synthesis, seed and cell wall formation, germination of pollen grains and growth of pollen tubes and sugar translocation.

Symptoms

Symptoms include dying growing tips and bushy stunted growth, extreme cases may prevent fruit set. Crop-specific symptoms include;

• Apple
  Apple
  The apple is the pomaceous fruit of the apple tree, species Malus domestica in the rose family . It is one of the most widely cultivated tree fruits, and the most widely known of the many members of genus Malus that are used by humans. Apple grow on small, deciduous trees that blossom in the spring...
  
  - interacting with calcium, may display as "water core", internal areas appearing frozen
• Beetroot
  Beetroot
  The beetroot, also known as the table beet, garden beet, red beet or informally simply as beet, is one of the many cultivated varieties of beets and arguably the most commonly encountered variety in North America, Central America and Britain.-Consumption:The usually deep-red roots of beetroot are...
  
  - rough, canker
  Canker
  Canker and anthracnose are general terms for a large number of different plant diseases, characterised by broadly similar symptoms including the appearance of small areas of dead tissue, which grow slowly, often over a period of years. Some are of only minor consequence, but others are ultimately...
  
  ed patches on roots, internal brown rot
  Brown rot
  Brown rot may refer to the following diseases:*Wood-decay fungus, a disease of trees and wood.*Ralstonia solanacearum, a disease of plants caused by bacteria.*Monilinia fructicola, a disease of stone fruits....
  
  .
• Cabbage
  Cabbage
  Cabbage is a popular cultivar of the species Brassica oleracea Linne of the Family Brassicaceae and is a leafy green vegetable...
  
  - distorted leaves, hollow areas in stems.
• Cauliflower
  Cauliflower
  Cauliflower is one of several vegetables in the species Brassica oleracea, in the family Brassicaceae. It is an annual plant that reproduces by seed...
  
  - poor development of curds, and brown patches. Stems, leafstalks and midribs roughened.
• Celery
  Celery
  Apium graveolens is a plant species in the family Apiaceae commonly known as celery or celeriac , depending on whether the petioles or roots are eaten: celery refers to the former and celeriac to the latter. Apium graveolens grows to 1 m tall...
  
  - leaf stalks develop cracks on the upper surface, inner tissue is reddish brown.
• Celeriac
  Celeriac
  Celeriac is also known as celery root, turnip-rooted celery or knob celery. It is a kind of celery, grown as a root vegetable primarily for its large and bulbous hypocotyl rather than for its stem and leaves. The swollen hypocotyl is typically used when it is about 10–12 cm in...
  
  - causes brown heart rot
• Pear
  Pear
  The pear is any of several tree species of genus Pyrus and also the name of the pomaceous fruit of these trees. Several species of pear are valued by humans for their edible fruit, but the fruit of other species is small, hard, and astringent....
  
  s- new shoots die back in spring
  Spring (season)
  Spring is one of the four temperate seasons, the transition period between winter and summer. Spring and "springtime" refer to the season, and broadly to ideas of rebirth, renewal and regrowth. The specific definition of the exact timing of "spring" varies according to local climate, cultures and...
  
  , fruit
  Fruit
  In broad terms, a fruit is a structure of a plant that contains its seeds.The term has different meanings dependent on context. In non-technical usage, such as food preparation, fruit normally means the fleshy seed-associated structures of certain plants that are sweet and edible in the raw state,...
  
  s develop hard brown flecks in the skin.
• Strawberries
  Strawberry
  Fragaria is a genus of flowering plants in the rose family, Rosaceae, commonly known as strawberries for their edible fruits. Although it is commonly thought that strawberries get their name from straw being used as a mulch in cultivating the plants, the etymology of the word is uncertain. There...
  
  - Stunted growth, foliage small, yellow and puckered at tips. Fruits are small and pale.
• Swede (rutabaga)
  Rutabaga
  The rutabaga, swede , turnip or yellow turnip is a root vegetable that originated as a cross between the cabbage and the turnip; see Triangle of U...
  
  and turnip
  Turnip
  The turnip or white turnip is a root vegetable commonly grown in temperate climates worldwide for its white, bulbous taproot. Small, tender varieties are grown for human consumption, while larger varieties are grown as feed for livestock...
  
  - brown or gray concentric rings develop inside the roots.
• Arecaceae
  Arecaceae
  Arecaceae or Palmae , are a family of flowering plants, the only family in the monocot order Arecales. There are roughly 202 currently known genera with around 2600 species, most of which are restricted to tropical, subtropical, and warm temperate climates...
  
  (Palm Tree) - brown spots on fronds & lower productivity.references/

Soil Conditions

Boron is present in the soil in many form, the most common being Boric Acid (H₃BO₃). An adequate amount of boron in the soil is 12 mg/kg. If the boron content of the soil drops below 0.14 mg/kg then boron deficiency is likely to be observed. Boron deficiency is also observed in basic soils with a high pH because in basic conditions boric acid exists in an undissociated form which the plant is unable to absorb. Soils with low organic matter content (<1.5%) are also susceptible to boron deficiency. Highly leached sandy soils are also characteristic of boron deficiency because the boron will not be retained in the soil. Boron toxicity is also possible if the boron content of the soil is high enough that the plant cannot cope with the excess boron. The levels at which boron is toxic to plants varies with different species of plants.

Boron Requirements

Boron is an essential micronutrient which means it is essential for plant growth and development, but is required in very small quantities. Although Boron requirements vary among crops, the optimum boron content of the leaves for most crops is 20-100 ppm. Excess boron can result in boron toxicity and the toxicity level varies between plants.

Treatment

Boric acid (16.5%boron), borax (11.3% boron) or SoluBor (20.5% boron) can be applied to soils to correct boron deficiency. Typical applications of actual boron are about 1.1 kg/hectare or 1.0 lb/acre but optimum levels of boron vary with plant type. Borax, Boric Acid or Solubor can be dissolved in water and sprayed or applied to soil as a dust. Excess boron is toxic to plants so care must be taken to ensure correct application rate and even coverage. Leaves of many plants are damaged by boron; therefore, when in doubt, only apply to soil. Application of boron may not correct boron deficiency in alkaline soils because even with the addition of boron, it may remain unavailable for plant absorption. Continued application of boron may be necessary in soils that are susceptible to leaching such as sandy soils. Flushing soils containing toxic levels of boron with water can remove the boron through leaching.

Functions

Once boron has been absorbed by the plant and incorporated into the various structures that require boron, it is unable to disassemble these structures and re-transport boron through the plant resulting in boron being a non-mobile nutrient. This results in the symptoms of boron deficiency appearing in the young leaves first.

Cell Wall

Boron is part of the dRG-II-B complex which is involved in the cross linking for pectin located in the primary cell wall and the middle lamella of plant cells. This cross linking is thought to stabilize the matrix of plant cell walls.