Boron (BO3-3 )

Boron in the Soil

Total soil boron (B) content can range from around 20 lb./acre to over 200 lb./acre (10 to 100+ ppm). However, only a small fraction of this amount is available to the crop. Much of the total soil B is present as a component of Tourmaline, a highly insoluble mineral. Most of the remainder is in secondary, moderately insoluble minerals. The forms of B that are available to plants include inorganic borate complexes of Ca, Mg, and Na, plus various organic compounds formed from plant and microbe decomposition.

Functions

Boron is essential for many plant functions. Some of them are

Factors Affecting Availability

High Response Crops

While boron is an essential element for all plants, the following crops have been found to be especially responsive: alfalfa, apples, broccoli, brussel sprouts, cabbage, carrots, cauliflower, celery, citrus, clover, conifers, corn, lettuce, onions, peaches, peanuts, pears, sunflower, sweet corn, sweetpotato, sugarbeets, table beets, tomato, and turnip.

Deficiency Symptoms

Symptoms vary between crop species, but generally occur in the growing points or flower and fruiting parts of the plant. Young leaves will often show a general chlorosis, as is the case with alfalfa. The upper leaves of clover may show a reddish color. In both crops, the terminal internodes shorten, and the new growth may appear distorted. Cotton may exhibit a rosette growth due to shortened terminal growth. Corn may not show any leaf symptoms, but a B deficiency can cause the failure of the pollen tube to function properly. This results in kernel embryos not being pollinated, causing the ears to have missing kernels.

BORON DEFICIENCY SYMPTOMS

Crop

Visible Symptoms

Alfalfa

Death of terminal bud, rosetting, yellow top, little flowering and poor pod set.

Almond

Flowers fall and nuts abort or are gummy.

Apple

Pitting, skin discolored, cracking and corking.

Apricot

Twigs die back and fruit fails to set.

Beet (Table)

External spotting, cracking and canker.

Broccoli

Hollow stems, internal discoloration, brown curds.

Cabbage

Hollow stem, watery areas, heads hollow, plants stunted.

Canola

Leaves distorted. Blank or partially filled seed heads.

Carrot

Reddening of leaves and root splitting.

Cauliflower

Leaves curled, hollow stem, curds dwarfed, brown.

Celery

Stem cracked and striped brown, heart blackened.

Citrus

Thickened ring, gum pockets near axis, discolored patches.

Clover

Poor stands, growth and color. Reduced flowering and seed set. Leaves cupped and shriveled, and become brittle.

Corn (all)

Short, bent cobs, barren ears, blank stalks, poor kernel development, elongated, watery or transparent stripes later becoming white on newly formed leaves, dead growing points.

Cotton

Shedding of squares and young bolls, ruptures at base of squares, dark fluid exuding from ruptures, internal discoloration at base of boll, half-opened bolls, green leaves until frost.

Dry Bean

Interveinal chlorosis of leaves. Bushy appearance.

Grape

"Hen & Chick" symptom, dead main shoots.

Lettuce

Stunted growth, discoloration of leaves, brittle.

Peanut

Dark, hollow area in center of nut, called "hollow heart."

Pear

Blossom blast, pitting, internal corking and bark cankers.

Pistachio

Fruit set decreases, and blanks and non-split nuts increase.

Potatoes

Plants have a bushy appearance. Leaves thicken and margins curl upward.

Radish

Pale roots, brittle stems, watery flesh and flecked coloration.

Rutabaga

Roots are tough, fibrous and bitter. Upon cutting, they have soft, watery areas, often called "brown-heart."

Sorghum

Leaves are narrow and have a gray appearance with watery, transparent stripes. Seed heads are not filled.

Soybean

Yellow leaves, chlorotic between veins, downward curling of leaf tips, crinkling of leaves, dieback of tips, no flowering, roots stunted.

Strawberry

Pale chlorotic skin of fruit, cracking and dieback.

Sugar Beet

Yellowing or drying of leaves, cracking of leaf midrib, brown discoloration of internal tissue, rotting of crown.

Sunflower

Leaves appear wilted. Abnormal head fall due to weak peduncles.

Tobacco

Leaf puckering and deformed buds.

Tomato

Thickened leaves, brittle leaves, fruit fails to set.

Turnip

Hollow center or brown heart, watery areas.

Walnut

Dieback from shoot tips, leaf fall.

Wheat

Distorted heads and chlorosis of leaves.

Toxicity

The range between a correct application rate, and a toxic one is not large, so it is relatively easy to apply too much boron. Because of this, it is very important to get uniform mixing and application, especially when applying in concentrated bands or foliar. Because of the slow transport of B in the plant, symptoms generally appear on the older leaves and consist of margin or leaf tip chlorosis, browning of leaf tips, which is quickly followed by the death of the affected tissue or defoliation. The critical plant level for toxicity can range from 10 - 50 ppm in sensitive crops such as small grains and garden beans, to as high as 200 ppm in tolerant crops such as sugar beets, rutabaga, cucumbers, and conifers.

Since B is a mobile element in the soil, excess B from over-application can be corrected over time with leaching. Applications of lime and sources of soluble Ca have been shown to be effective in reducing B toxicity, and it has been reported that additional N application can be of benefit.

Using Boron in a Fertility Program

Recommended rates of B

Method

Pounds of B/A

Broadcast:

1.0 to 3.0 lb./A

In-row:

Not Recommended

Foliar:

0.10 to 0.25 lb./A

Broadcasting is the preferred application method for several reasons.

The impregnation of dry fertilizer blends with dissolved Solubor has been successfully used to assure uniform distribution.

If visible B deficiency symptoms appear, significant yield reductions have already occurred. Use plant analysis to identify and correct nutrient shortages before they cause serious yield losses. Also remember, as growers strive for those exceptional yields, the need for B increases.

Some Boron Fertilizer Sources

Product

Chemical Formula

Approximate B %

Borax

Na2B4O7 .10H2O

11%

Boric Acid

H3BO3

17%

Sodium tetraborate

Na2B4O7 .(0-10)H2O

10-20%

Solubor

Na2B10O16 .0H2O

20%