Essential Nutrients

Boron (B)

Form used by plants:

H₃BO₃, B(OH)₄^- (at high pH)

H₃BO₃ is the dominant form in soils thought to be absorbed by higher plants.

Important functions:

• Regulates metabolism of carbohydrates

• Activates certain dehydrogenase enzymes

• Aids in formation of pollen tube and feeder roots

• Involved in translocation of Ca, sugars, and plant hormones

• Facilitates the synthesis of nucleic acids

• Essential for cell division and development

Ideal foliage range for apple leaves:

25-50ppm

Below normal levels <20ppm 

Excessive levels >80 ppm

Top of page

Ideal fruit range (tentative):

Whole fruit samples at harvest: deficient levels are < 10 mg/kg (ppm); excessive levels for Jonathan, McIntosh >25 ppm and for Delicious and G. Delicious >60 ppm; others are intermediate

Ideal soil range:

0.5-2.0 ppm

B is mobile in soils

Note: B can be easily leached from acid sandy soils. Boron availability is lowest between soil pH 7-9. Lime induced B deficiency is possible.

Best indicators:

Fruit and blossom content are more sensitive indices of tree B status than are soil or leaf B content.

Mobility in plant:

Not remobilized within plant

Deficiency symptoms: 

 

Vegetation symptoms:

• Younger leaves are affected first and are often small, stiff, thick, brittle leaves with smooth margins.  Main veins may be large, lacking chlorophyll, cracked, or corky (NY pub).  Chlorosis and cupping are common.  Leaves may form rosettes at nodes.

• Delayed budbreak

• "Witch's broom" often occurs as sidebuds bread and start developing.

• At B concentrations 12 ppm or less, terminal buds die and shoot dieback takes place.  Additionally, internal bark necrosis may develop in trees deficient in B.

• Ca deficiencies may appear because Ca translocation is impaired at insufficient B concentrations.

• A B deficiency decreases the rate of water absorption, root growth, and translocation of sugars in plants.

Fruit symptoms include: 

• deformed fruit, poor quality 

• cracking and russet 

• premature ripening

• internal corking (when leaf B concentration is 14-21 ppm)

• increased fruit drop

• reduced fruit set.

Top of page

Increased risk of B deficiencies on:

• Sandy soils receiving excessive amounts of irrigation water

• Calcareous soils (pH>8)

• Drip-irrigated soils

• Excess lime reduces B uptake and can induce deficiency

Toxicity symptoms:

• Fruit symptoms: reduced or no yield; increased internal breakdown after harvest, increased watercore development.  At leaf concentrations between 60-70ppm, fruit may ripen early and fall prematurely.

• Vegetation symptoms: same as deficiency symptoms.

Top of page