Borax in the garden

Contents

Borax is used for various house chores but did you know this naturally occurring mineral can be used in the garden too? Check out!

What is Borax?

Borax (Na2B4O7·10H2O) is a natural mineral. An important boron compound. It is an essential ingredient in many laundry and cleaning products you buy from grocery stores. Besides this, it has many uses on its own.

Is Borax Safe?

Read this article for help, and this one too.

Where Can You Find Borax?

You can find borax in the cleaning supplies section of most grocery stores.

Borax Uses

1. Kill Weeds

Borax can be used to kill weeds. Add 10 ounces of powdered borax to 2.5 gallons of water, mix thoroughly, and use a sprayer to coat the leaves of unwanted weeds in your yard. Keep overspray off of any plants you want to keep, avoid saturating the soil with the solution, and avoid contact with bare skin. Find out a few more natural herbicide recipes on Treehugger!

2. Remove Rust from Tools

Mix borax and lemon juice together to make a paste. Apply this paste to your rusty tools and allow it to set for at least 30 minutes, and then scour with a scrub brush. Repeat these steps if necessary, and always rinse clean with water when finished.

3. Ant Killer

If you want to get rid of ants in your garden, try this borax bait recipe. For this, you’ll need borax, honey or maple syrup in equal amounts. Mix well and place this in the affected spot. To learn more about this recipe, .

4. Disinfect and Clean your Garden Tools and Surface

Mix half a cup of borax in a gallon of warm water. Make use of this solution to soak and scrub pots and tools. Once clean, rinse thoroughly and leave the items in the open air and sunshine to dry. You can also clean gloves and other gardening objects.

5. As a Fertilizer

Boron (B) is the second most widespread micronutrient deficiency problem worldwide after zinc. It improves plant’s health and growth. Its deficiency commonly results in empty pollen grains, poor pollen vitality and a reduced number of flowers per plant. The lack of boron also causes stunted root growth.

The common symptoms are dying growing tips, bushy stunted growth, low productivity but these symptoms are very common. So the way to determine Boron deficiency is to get your soil tested. Soil that is acidic, sandy and low in organic matter often has a deficiency of Boron. Also, cabbage family crops and a few other plants like celery, strawberry, apple, etc. require Boron in a higher amount than other plants and they get help from the application of borax.

Application

If the soil test for Boron is less than 1 ppm, apply household or agricultural grade borax (11 percent B) at the rate of 1 tablespoon per 100 square feet where Boron requiring plants will be grown. Apply the borax evenly and mix thoroughly with the soil. It may be easier to dissolve 1 tablespoon of borax in 1 gallon of water and apply the solution evenly with a sprinkling can. Apply 1 fluid ounce of solution per plant.

6. Kill Green Fly on Roses

Dissolve 25 gm borax in a little hot water and make up to 600 ml by adding cold water. It will kill greenfly on roses and other plants.When applied to the stems of fruit and other trees, it destroys all insects in and about the bark. Read here in detail!

Boron is a mineral salt with acidic properties. It is naturally occurring, and it is the only ingredient in Borax. This product has been around for ages, but borax uses go beyond its excellent reputation as an all natural cleaner.

Borax (sodium tetraborate decahydrate), also known as sodium borate, disodium tetraborate, or sodium tetraborate, makes a good laundry booster, and it has powerful, natural disinfectant properties. Reacting this with a mineral acid like hydrochloric acid creates what we call boric acid. Many personal care products use Borax as a preservative. In this article, we will share some smart Borax powder uses in your garden. Read on to learn more about Borax in your garden.

Augment Your Soil With Borax

Synthetic fertilizers and other products coupled with mismanagement of soil can lead to soil deficient in boron. This is particularly the case of very sandy soil. Soil lacking boron grows produce lacking in boron. To get the most value and nutritional benefit from the veggies you grow, it’s easy to see why creating rich soil is key.

How Can I Tell If My Plants Need Boron?

Plants show signs of boron deficiency in a variety of ways. Some exhibit dead leaf tips, others may present darkened fruit and/or leaves. Roots may also have dead areas, and root veggies may have black centers or black spots. Cruciferous vegetables, such as superfood broccoli and cauliflower may grow cracked, hollow stems.

Fertilize With Boron

Here’s how to add boron to soil.

There are a number of ways to use boron as a fertilizer. Adding Borax (boron) to your soil in the right amounts will deliver healthier plants and healthier food.

You do need to be careful not to overuse the boron fertilizer product as it can be toxic. Keep in mind that it can kill weeds, and it can also kill desirable plants.

The product can easily be used as a dry fertilizer applied directly to the soil. It only takes a very small amount of borax to have a positive effect. For example, if you have a very large garden, you only need to use about 6 tablespoons of the borax powder broadcast evenly and tilled into the soil before planting to realize significant plant growth benefits.

You can create a good liquid fertilizer mixture for general watering by mixing one part boron with ten parts of water. Try a little bit on just a few plants to see how it works. If it seems too strong, dilute it a bit and keep sampling.

For a still useful very weak solution, mix a tablespoon of boron into three gallons of water. If in doubt with boron supplementation, start out with the weaker solution and monitor the results. If you get the improvement you want, you have done enough!

Related Reading:

  • Is Epsom Salt Good For Plants
  • How To Use Epsom Salt For Sweeter Tastier Tomatoes
  • How To Use Epsom Salt For Brighter, More Beautiful Roses

In very diluted solution, it is possible to use boron as a foliar fertilizer. To do so, you would add two ounces of borax to five gallons of water. Add just a tiny amount (several drops) of dish soap to help the solution disperse evenly. Spray very lightly and evenly and do not drench your plants with this solution.

Use your boron mixture sparingly because this mineral builds up in the soil. A single application can last for three years, so even a light annual application could eventually build up to become toxic.

Use Borax To Kill Weeds

Borax mixed with water at a strength of half a cup of powder to one gallon of water makes a powerful herbicide that you can apply carefully at the base of weeds to hasten their demise.

This solution can also be applied directly to the leaves of the unwanted plants as a spray, but you must be careful to avoid having over-spray come in contact with your desired plants. It is best to use a pump sprayer or a brush to apply the solution directly to the leaves of individual weeds you wish to kill.

Borax can also be used in its powder form to kill weeds. You can sprinkle it directly onto the plants for quick results. Be sure not to apply it when you are expecting rain because this could result in runoff and damage to your veggies.

Smith Brothers Borax ad roughly 1870-1900 from Boston Library

A Natural Ant Killer Borax Pest Control

Here’s a good borax insecticide recipe:

To prevent ants from eating up your produce, create borax and confectioner sugar ant traps. Mix the dry ingredients with a bit of water to create a paste. You can spread this borax solution paste on flat surfaces (such as the lids of yogurt or margarine tubs) and place your borax ant killer traps strategically around your garden.

Placing it away from your crops can help lure the ants away. They will gather the mixture as food and carry it into their colony. Borax ant problems away!

Clean & Disinfect With Borax A Natural Cleanser

Create a natural disinfectant for your tools and hard surfaces by mixing half a cup of Borax with a gallon of hot water. This solution is also useful for fighting mildew and deodorizing.

Use this warm solution to soak and scrub pots and tools. Once clean, rinse thoroughly and leave the items in the open air and sunshine to dry.

Naturally, you can use Borax as a laundry booster to wash your gardening gloves, towels, aprons and other garments and washable gardening items.

Handle Borax With Care

Borax is a natural product, and it is generally safe to use, but don’t make the mistake of handling it carelessly. It can be toxic if used or consumed in large amounts.

Always remember, some of Borax for plants most effective uses involve killing unwanted plants and insects and combating fungus. While these powers make it an excellent support tool for gardeners, always remember to wear safety gear and handle with care!

Image: flickr

Borax is a white powder that easily dissolves in water. Made of boron and sodium, this naturally occurring compound is a very practical substance with many uses in and around the garden.

Ant Treatment

Gardens overrun with ants can benefit from a borax ant trap. The Ecology Center recommends using a borax-based ant trap as a non-toxic ant eradication method.

Directions

  1. Mix a cup of sugar with a cup of borax.
  2. Distribute the mixture around anthills.

Since the borax is white, like sugar, the ants will mistakenly carry it back to their colony. There the borax will kill the ants.

Soil Fertilizer for Sunflowers

Borax is especially useful as a fertilizer for the sunflower plant. Borax contains the element boron which is essential for plant growth. BC Living notes that sunflowers respond favorably to a borax fertilizer. The fertilizer gives the plant strength and durability.

  1. Mix 1/2 teaspoon of borax with 1 gallon water.
  2. Water the sunflower early in the morning before the sun gets too hot. The optimal time to apply the fertilizer is before the blooms open and while they are only halfway mature.
  3. Repeat in one month.

Treatment for Boron Deficiency in Vegetables

The Royal Horticultural Society notes that boron is essential for healthy plant development. Some plants suffer from boron deficiency. Broccoli displays this deficiency with hollow stalks and beets with black spots. Other crops may also develop a boron deficiency including cabbage, strawberries, cauliflower and strawberries. To confirm that plants may have boron deficiency, conduct a soil test to be sure.

  1. Add 2.5 ounces of borax into 5 gallons of water and a few drops of dish soap.
  2. Spray the solution evenly on plant foliage. Do not concentrate the solution in one area.

Weed Control

Creeping Charley is a perennial weed that can easily intrude a vegetable garden. According to the University of Illinois Extension, borax is an effective herbicide that stops this annoying weed before it takes over the whole garden.

  1. Dissolve ten ounces of borax in four ounces of warm water.
  2. Put the solution in 2.5 gallons of water.
  3. Spray only on the lawn when warm. Do not spray if rain is expected.

Flower Preservation

The University of West Virginia Extension Service recommends using borax to preserve flowers. Flowers that are preserved using borax hold their shape and seem to experience minimal shrinkage.

  1. Mix 2 parts borax to 1 part cornmeal.
  2. Sprinkle a layer of the mixture in the bottom of a box.
  3. Lay the flowers down, not touching, in the bottom of the box.
  4. Cover the flowers with more of the mixture and put the lid on the box.
  5. Let the box sit somewhere for 14 days.

Borax Is a Gardener’s Friend

Borax can be used in its natural form as it is found in lake deposits. The commercial borax available is synthetically produced. It is the same as the natural form but without water. Half the commercial borax produced is mined from lake deposits in California. Borax is a cost effective substance that can be used in and around the garden to help solve a number of common garden problems and is suited for use in organic gardens. It’s truly a natural wonder that has many uses in the garden and around the home.

Boron Nutrition in Deciduous Tree Fruit Orchards

Introduction

Boron (B) is a microelement that is essential only in vascular plants and diatoms.

The essentiality of B resulted from evolution of xylem, passive transport of B in the transpiration stream, and accumulation of B to levels of metabolic significance in the shoot apices. Acquisition of an essential role for B in the normal functioning of the apical meristem served as a means of preventing B toxicity.

Roots originated from stems and have a similar requirement for B. Roots are more sensitive to B deficiency because they do not receive B through the transpirational stream and must absorb B from the surrounding medium.

The primary effect of B deficiency appears to be disruption of the normal functioning of the apical meristem (evidence suggests a fundamental role in pyrimidine metabolism), with changes in auxin metabolism, lignification, phenol accumulation, and sucrose transport being secondary effects.

Excessive accumulation of B can cause B phytotoxicity.

Boron Chemistry in Water and Soil

Adsorbed B and soil solution B are in equilibrium in soil, with most of the B occurring being adsorbed.

Most well-drained soils are low in B. High soil B occurs primarily in semi-arid and arid areas because of high water tables coupled with little or no drainage or because of additions of B in irrigation water.

Boron dissolved in water occurs primarily as monomers of orthoboric acid, B(OH)3. As pH increases, orthoboric acid spontaneously reacts with an additional water molecule to create a hydroxyborate anion, B(OH)4-. Boron species other than B(OH)3 and B(OH)4- can be ignored for most practical purposes in soils.

Boron is specifically adsorbed to clay minerals, hydrous metal oxides, and organic matter in soils. Boron can also be coprecipitated with calcium carbonate.

In low B soils, a large portion of total B is associated with the organic matter. Boron is released to the soil solution by mineralization of the organic matter and becomes available to plant roots.

Soil Boron Availability to Plant Roots

Plants respond mainly to the concentration of B in soil solution. Because B moves passively in the transpirational stream, plant uptake of B usually increases with increasing temperature.

Soil B availability decreases with:

  1. Decreasing total soil B;
  2. Increasing clay mineral, hydrous metal oxide, organic matter, and lime contents in soil;
  3. Increasing pH above pH 6.5;
  4. Very wet or very dry soils;
  5. Increased leaching;
  6. Cold soil temperature.

Foliar Boron Availability

Foliar application of B solutions is an efficient way to increase the B content of fruit trees.

The efficiency of B uptake by fruit surfaces is lower than that of leaves and does not appear to be influenced by the presence of lenticels. Boron uptake through bark is negligible.

Foliar B uptake increases the longer the applied B solution remains as a fine film on the leaf or fruit surface. Leaf and fruit injury can result from salt scorching on hot clear days when evaporation rates are high. Leaf injury can also result on cool humid days when evaporation rates are low because of B toxicity resulting from excessive uptake of B from foliar sprays.

Some of the B applied in foliar sprays appears to be washed from the tree to the soil surface by rainfall or sprinkler irrigation water where it can then be absorbed by tree roots.

Boron Deficiency Symptoms

Boron deficiency was very common in deciduous tree fruit orchards until the 1920s, when B was identified as an essential element and corrective fertilizer programs were developed and implemented.

Apples

Fruit symptoms:

  1. Small, flattened or misshapen fruit;
  2. Drought spot;
  3. Internal cork;
  4. Cracking and russet;
  5. Premature ripening;
  6. Increased fruit drop;
  7. Seed count may be low.

Vegetation symptoms

  1. Internal bark necrosis (bark measles);
  2. Dead terminal buds and shoot dieback, sometimes with witches-
    broom effect as sidebuds break and start developing
  3. Shortened internodes;
  4. Dwarfed, stiff, thick, brittle leaves with smooth margins.

Pears

Blossom symptoms

  1. blossom blast.

Fruit symptoms

  1. Reduced fruit set;
  2. External and internal cork;
  3. Cracking.
  1. Internal bark necrosis (bark measles);
  2. Dead terminal buds and shoot dieback, sometimes with witches-
    broom effect as sidebuds break and start developing;
  3. Shortened internodes;
  4. Dwarfed, stiff, thick, brittle leaves with smooth margins.

Apricots, Peaches, Cherries

  1. Cracking, shriveling, deformation;
  2. Internal and external browning;
  3. Cork formation around pit and in flesh;
  4. Differential ripening within a single fruit;
  5. Increased fruit drop.

Vegetative symptoms

  1. Usually appears after growth has started in the spring;
  2. Buds fail to break or break and fail to develop normally;
  3. Blossom blast;
  4. Death of terminal buds and twig dieback; retarded shoot growth;
  5. Dwarfed, narrow leaves with upturned edges, often with thickened midribs; may blacken and fall off.

Boron Toxicity Symptoms

Boron toxicity was recognized in the 1860s but did not become a problem in deciduous tree fruit orchards in the PNW until B fertilizer programs were initiated.

  1. Reduced or no yield;
  2. Increased internal breakdown after harvest;
  3. Increased watercore development after harvest;
  4. Premature ripening.
  1. Dead terminal buds and shoot dieback;
  2. Marginal leaf chlorosis and necrosis; defoliation.
  1. Reduced or no yield.
  1. Dead terminal buds and shoot dieback;
  2. Late developing leaves;
  3. Small, curled, and elongated or cup-shaped leaves;
  4. Marginal leaf chlorosis and necrosis; defoliation.
  1. Reduced or no yields;
  2. Malformation;
  3. Poor pit development;
  4. Earlier maturation;
  5. Poor flavor.
  1. Tips of new shoots wither and die-back;
  2. Cankers and gummosis develop along stems;
  3. Brittle, partially deformed leaves; may have small necrotic
    spots along midrib that may drop out, creating a shot-holed
    effect, and small cankers on the underside of midribs and
    petioles;
  4. Enlarged nodes at base of buds may be present.

Diagnosis of Boron Nutritional Status

Fruit and blossoms are more likely to exhibit symptoms associated with B deficiency or excess than are the vegetative portions of the trees. Fruit and blossom B content are more sensitive indices of tree B status than are soil or leaf B content. Boron fertilizer treatments usually result in relatively greater increases in fruit B concentrations than in leaf B concentrations. Differences in fruit B concentrations usually persist throughout the growing season, while differences in leaf B concentrations often disappear later in the growing season. Midsummer leaf and at-harvest fruit B concentrations are often poorly correlated. Tree B content or horticultural responses are usually poorly correlated with soil B analyses.

Diagnostic Guidelines for Soil Boron

Most studies have found poor relationships between soil B and fruit tree B content or B malnutrition symptoms; however, soil B test guidelines for orchard soils based on the hot water extraction procedure have been published (N.B., 1 mg/kg = 1 ppm).

Tree B Level Soil B Content
Deficient <0.5 mg/kg
Optimal 0.5 to 1.0 mg/kg
High 1.0 to 2.0 mg/kg
Excessive >2.0 mg/kg

To my knowledge, these guidelines have never been critically evaluated.

Soils should be sampled by depth increments of 0-6, 6-18, and 18-30 inches (most of the soil B is likely to be in topsoil; however, toxic levels can occur in subsoil and will be missed by sampling only the surface).

Tentative Diagnostic Guidelines for Boron in Apple and Pear Tissues

All analyses are dry matter basis (N.B., 1 mg/kg = 1 ppm).

Apple fruit (whole fruit samples at harvest)
Deficient <10 mg/kg
Excessive
Jonathan, McIntosh >25 mg/kga
Red Delicious, Golden Delicious >60 mg/kga
Others intermediatea
Pear fruit (whole fruit samples at harvest)
Deficient <20 mg/kgb
Excessive
D’Anjou >45 mg/kgc
Bartlett >55 mg/kgc
Pear flower (full bloom)
Deficient <15 mg/kg
Excessive
D’Anjou >90 mg/kgc
Bartlett >115 mg/kgc
Apple and Pear leaves (midsummer)
Deficient <20 mg/kgd
Excessive >80 mg/kge
a Apple fruit quality impaired.
b Criteria for blossom blast control. Maintaining fruit at this level should eliminate B-related cork.
c Associated with vegetative phytotoxicity symptoms; fruit quality was not impaired.
d Deficiency symptoms can be present at higher concentrations.
e Temporarily high leaf levels may occur without ill effect after foliar B sprays.

Tentative Diagnostic Guidelines for Boron in Apricot, Peach, and Sweet Cherry Tissues

All analyses are dry matter basis (N.B., 1 mg/kg = 1 ppm).

Fruits (at harvest)
Deficient
Apricot <20 mg/kga
Excessive
Apricot >200 mg/kga
Peach >60 mg/kga
Leaves (midsummer)
Deficient <20 mg/kgb
Excessive >50 mg/kgb
a Associated with impaired fruit quality.
b Associated with tree phytotoxicity and impaired fruit quality.

Guidelines for Fertilizer Boron Application

The commercial guideline for soil application of B in Washington is a surface-broadcast application of 3 lb actual B per acre, made once every three years. An argument has been made that this infrequent application of a high B rate creates large and potentially deleterious fluctuations in soil and tree B; hence, an annual spray application at a low B rate would better benefit fruit production by providing a more constant supply at lower concentration. This hypothesis has not been tested except for pears grown in non-irrigated areas in Washington, where it was found to be valid. The suggested foliar spray rate is 1.0 lb B/acre for B-deficient orchards. Soil application is suggested if the soil test level is below 0.5 mg/kg or if B deficiency symptoms are present. Suitable forms of B fertilizer for soil application include sodium borates, borax, and boric acid.

Aircraft applications of B should be made during the dormant season to enhance likelihood of application uniformity.

If the soil test level is 0.5 to 1.0 mg/kg and B deficiency symptoms are lacking, a single annual maintenance foliar spray of 0.5 lb B/acre is suggested for the irrigated apple, pear and stone fruit production areas. Boron fertilizer specifically formulated for foliar sprays are available. Preferred application timing is early fall after harvest but the spray can also be applied with possible lesser effect in the early spring at the prepink-to-pink blossom stage. Most foliar B fertilizers are reported to be compatible with most pesticides used in cover sprays (consult the label). The timing of maintenance B applications does not appear to be critical for apple trees; applying the B in the first cover spray or split between the first two cover sprays appears to be acceptable. Applying B in concentrate sprays does not appear to reduce the efficiency of B uptake; however, slight marginal scorching or leaves has been reported for sprays containing 0.4 lb B/100 gal, and severe leaf injury at 0.8 or more lb B/100 gal. There is some anecdotal evidence suggesting that a single annual spray is inadequate for trees grown on sandy soils; multiple applications at low B rates may be preferred.

Pears have a higher B requirement than other tree fruit species in the early spring. A slightly different annual B maintenance program is suggested for non-irrigated pear orchards, where B deficiency is more likely to occur. Sprays applied for blossom blast control tend to have little effect on fruit B. The WSU spray guide therefore suggests increasing the maintenance rate to 1.0 lb B/acre; however, a possibly more effective alternative program is to apply one postharvest or first-white to full-white spray of 0.5 lb B/acre, followed by a second application at the same rate in the first or second cover-spray.

No B fertilizer applications are suggested if soil B exceeds 1.0 mg/kg or if B toxicity symptoms are observed. If B toxicity is confirmed, soil reclamation may be necessary.

Boron phytotoxicity in Washington orchards is rare. An informal but critical review of grower anecdotes suggests that B phytotoxicity is usually associated with two preventable management errors: (1) mistaking the bag of B fertilizer for the nitrogen fertilizer, and (2) overlap of B applications flown on by airplane.

Application of B through the irrigation system is not currently recommended because of the likelihood of nonuniform application and consequent risk of B phytotoxicity.

Boron In Soil: The Affects Of Boron On Plants

For the conscientious home gardener, boron deficiency in plants should not be a problem and care should be taken with the use of boron on plants, but once in awhile, a boron deficiency in plants can become a problem. When boron in soil is too high or too low, plants will not grow correctly.

Effects and Use of Boron on Plants

Boron is a micronutrient necessary for plant growth. Without adequate boron in the soil, plants may appear healthy but will not flower or fruit. Water, organic matter and soil texture are all factors that affect boron in soil. The balance of too little or too much between plants and boron is a delicate one. Heavy boron soil concentration can be toxic to plants.

Boron helps control the transport of sugars in plants. It is important to cell division and seed development. As a micronutrient, the amount of boron in soil is minute, but among micronutrients, boron deficiency in plants is the most common.

Deep watering will relieve heavy boron soil concentrations by leaching the nutrient away from the roots. In good soil, this leaching won’t cause boron deficiency in plants. The organic material used to enrich and fortify the earth will release the micronutrient back into the soil. On the other hand, lightly water the plants and boron levels can rise and damage roots. Too much lime, a common garden additive, around your plants and boron will be depleted.

The first signs of boron deficiency in plants shows in the new growth. Leaves will yellow and growing tips will wither. Fruit, particularly noticeable in strawberries, will be lumpy and deformed. Crop yield will suffer.

If you suspect a boron deficiency problem with your plants, using a small amount of boric acid (1/2 tsp. per gallon of water) as a foliar spray will do the job. Be careful as you use boron on plants. Again, heavy boron soil concentrations are toxic.

Turnips, broccoli, cauliflower, cabbage, and Brussels sprouts are all heavy boron users and will benefit from a light yearly spray. Apples, pears and grapes will also benefit.

BORON IN PLANTS

BORON UPTAKE BY PLANTS

Boron uptake by plants is controlled by the boron level in soil solution rather than the total boron content in soil. Boron uptake is a passive (non-metabolic) process. It moves with water in plant’s tissues and accumulates in the leaves; therefore, Boron uptake and accumulation are directly dependent on the rate of transpiration.

Boron mobility in the phloem is now known to be plant-specie dependent.

BORON TOXICITIES AND DEFICIENCIES

Boron deficiency symptoms:
Limited budding, bud break, distorted shoot growth, short internodes, increased branching, flower buds falling and inhibition of fruit and seeds development

Symptoms of boron deficiency in pepper

Boron toxicity symptoms include:

Chlorotic leaf tips, leaf necrosis, and later leaves falling and even plant death.

BORON IN SOIL

Boron is soils can be categorized into 3 groups:
Boron as a mineral component
Boron adsorbed to soil particles, such as clay minerals, iron or aluminum oxides and organic matter. Adsorption of Boron to clay particles is reversible so the adsorbed boron can be released to the soil solution. It also has high affinity to iron and aluminum oxides.
Boron in soil solution as boric acid (H3BO3) and borate ions (BO3-). The boron in soil solution is available to plants (mainly as H3BO3).

The ratio between the Boron concentration in the soil solution and the Boron adsorbed to soil particles is affected by the components of the soil (clay minerals, free oxides and organic matter) and also by other factors such as type and concentration of salts in the soil, pH and temperature.

Actually, most of the boron in soil is adsorbed to organic matter, acting as a pool of boron from which the boron can be readily released into the soil solution.

PREVENTING BORON ACCUMULATION IN THE ROOT ZONE

Since toxic levels of Boron are only slightly higher than deficiency levels, it is important to keep a non-toxic level of Boron in soil solution. In order to achieve that, root zone should be flushed either periodically or continuously.

The water amount and the irrigation intervals should be determined in the same way done for treating salinity buildup in soil.

COMMON BORON FERTILIZERS

Boron Source Formula Composition
Borax Na2B4O7·10H2O 11% B
Boric acid H3BO3 17.5% B
Solubor Na2B8O13·4H2O 20% B

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Restoring Soil Nutrients

The P of the N-P-K formulation, phosphorus, proves critical to photosynthesis, plant maturity, healthy roots, and energy transfers within plants, but it’s difficult to apply effectively in a soluble form. Elements with charges opposite that of phosphorus, such as calcium and iron, capture the nutrient and render it unavailable to plants. Remedy the problem by spreading phosphate rock on the soil. It’s barely soluble, remaining stable until soil organisms and plant roots release it in a form that plants use.

Rock phosphate should also be supplied to the compost pile. Sprinkle it lightly whenever adding raw materials. Compost receives phosphorus from those items, too. Wood ashes, bone meal, citrus wastes, cottonseed meal, manures, fish wastes, and dried blood all supply phosphorus.

Watch for purple leaves, veins, and stems. They strongly indicate a phosphorus-deficiency. Leaves of corn and white pea beans turn yellow at blossoming time when they don’t receive enough phosphorus. The deficiency also causes the undersides of tomato leaves to turn purple. Radish leaves develop a reddish color on the undersides, and they yellow progressively during maturity. That condition may appear to be caused by a lack of nitrogen, but if kernels don’t fill the rows or ear tips and leaves yellow, read the problem as a lack of phosphorus.

Potassium is the familiar K of the N-P-K trio. It remains in solution after being absorbed and flows through plants completing several important functions necessary for good health: (1) It helps the manufacture of sugars and their movement within plants, which has a direct bearing on a plant’s ability to resist diseases. (2) It adjusts the openings of leaf pores (stomata) to make them open widely when moisture is available and close tightly during drought.

Potash (potassium compound) availability (3) increases photosynthesis by increasing the amount of chlorophyll in leaves. That results in plants that better utilize available light. Your plantings in partial and full shade are more apt to thrive in a potassium-rich environment.

Plant leaves provide some of the most reliable indications of potash deficiencies. Their edges will yellow, turn brown, and curl. Corn leaves show yellow streaks between veins and brown edges and tips develop. The spaces between leaf nodes will also grow unusually small, giving the leaves an appearance of being tightly packed. Curled carrot leaves indicate a potassium deficiency, especially when a below-ground inspection reveals stunted, misshapen carrots that have an “off” flavor. The carrots may be long but of a small diameter.

Excellent natural sources of potash for a summer quick fix include a foliar feeding of fish emulsion and liquid seaweed and root feeding with wood ashes, well-rotted cow manure, cottonseed meal, aged-poultry manure, and compost enriched with corn stalks. Granite dust and glauconite (greensand) provide potash over the long haul.

Calcium, Magnesium, & Sulfur

Calcium, magnesium, and sulfur are often thought of as secondary elements. Plants and soil may not need large supplies of these nutrients, but the roles they play prove as essential to growth as those of nitrogen, phosphorus, and potassium.

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Calcium serves several critical functions and concentrates primarily in leaves. It builds the cell walls of plant tissues and neutralizes acids produced by plants as toxic by-products of metabolism. It regulates the availability of other nutrients, builds plant proteins, and prevents magnesium toxicity.

Your tomato plant’s upper leaves yellow from a calcium deficiency. That symptom differs from a lack of nitrogen, which causes yellowing of lower leaves. Stems of tomato plants that lack calcium feel soft to the touch, and fruits exhibit blossom-end rot. The roots grow short and display a brown rather than a fleshy color.

Red patches appear on pea leaves when calcium levels are inadequate. The patches appear near the centers of the leaves and eventually spread to the outer margins. The plants lack vigor and appear dwarfed.

Limestone is the most widely used calcium-rich material. When purchasing limestone, shop for dolomitic limestone because it contains magnesium as well as calcium. Bone meal, oyster shells, wood ashes, and compost are other natural sources, the latter two materials supplying it the fastest.

The next secondary element that you should familiarize yourself with is magnesium. Photosynthesis would be impossible without magnesium because chlorophyll depends on its presence. Magnesium aids in a plant’s use of other nutrients, including nitrogen, phosphorus, and sulfur. What may appear as a phosphorus deficiency, for example, may be a magnesium deficiency that results in a plant’s inability to absorb phosphorus.

Lack of magnesium is characterized by discoloration of leaf tissue between veins, which remain green. The leaf tissues gradually turn yellow, often beginning at leaf margins, which gives them a streaked appearance. The leaves of some plants turn a reddish or purple color while the veins remain green.

Squash leaves, cucumbers, carrots, and lima bean leaves mottle and brown when the plant’s supply of magnesium proves inadequate. Corn leaves show yellow and/or white stripes; turnip leaves form brown margins and yellow mottling on inner surfaces; tomato leaves develop a yellow color that darkens away from veins, and the leaves become brittle and turn upward. Once a magnesium deficiency has been determined, it’s easily rectified by applying dolomitic limestone. The magnesium in the limestone’s dolomitic component is all that’s needed to put sufficient amounts of the element back into the soil.

Sulfur is the third secondary element needed for healthy plant growth. It helps plants produce proteins and enzymes, but there’s an inherent problem with retaining sulfur, especially in sandy soil. Unlike many other elements, sulfur isn’t readily held by soil particles. Instead it leaches past the root zone where, in time, it becomes unavailable to plants. The high porosity and low organic content of sandy soil amplify the problem.

Chlorosis is the primary result of sulfur deficiencies. The yellowing caused by a lack of sulfur differs from that of a nitrogen deficiency. The entire leaf doesn’t dry and become brittle as when lacking nitrogen. The soil’s need for sulfur strikes your legumes first because of those plants’ great demand for it. The typical yellow leaves develop, increase in numbers, and growth becomes noticeably interrupted. Yellowing cabbage leaves is another good indicator of insufficient sulfur, cabbage being another crop heavily dependent upon adequate supplies.

Don’t be quick to suspect a sulfur deficiency in the soil, although it’s never out of the question. The fact is that sulfur finds its way into the soil by other means. Acid rain sometimes contains enough sulfur to negate any deficiencies in the soil. Water supplies, too, often contain sulfur in quantities sufficient to supply the small amounts needed by plants when watering.

If a soil test indicates a sulfur deficiency, purchase sulfate of potash-magnesia. Containing approximately 27% sulfur, the product is derived from ancient marine deposits.

Trace Elements

Zinc

Nine nutritional elements exist as trace elements, and plants need them only in minute amounts. Oxygen, carbon, and hydrogen come from air. That leaves the necessary six: zinc, boron, manganese, iron, copper, and molybdenum.

Zinc is a catalyst that aids in several chemical reactions in plants, including the creation of amino acids. Zinc deficiency results in tomato leaves that appear smaller than usual. They turn yellow and mottled, with scattered dead spots, and leaf internodes grow shorter than normal. On corn plants, older leaves lose their color prematurely.

Zinc shortages can occur in most soils, acidic and alkaline. Manures and compost easily remedy zinc deficiencies, although ground, raw, phosphate rock contains traces of zinc, too, as does seaweed.

Boron

Boron is a trace element responsible for at least sixteen plant functions. It influences cell development and division, fruiting, flowering, stem growth, and a host of other plant activities.

Lack of boron causes a variety of slow, irregular growth habits such as dwarfing and bushiness because the growing plant tips die, which encourages lower leaves to sprout. Tomato stems blacken at the tips, and the young leaves on main stems yellow, then die quickly. Top growth halts and side growth replaces it, giving the plant a dwarfed, bushy appearance. Boron deficiency leads to blackheart disease in many root crops and, although phosphorus or potassium can cause it, the disease’s presence in beets and turnips indicates a lack of boron.

Correct a boron deficiency with compost that received phosphate rock, seaweed, or small amounts of sawdust, plenty of oak leaves, peat moss, or other acidic organic material. A liquid seaweed foliar feeding provides immediate results.

Manganese

A manganese shortage causes plant disorders with symptoms such as chlorosis that may appear to be caused by shortages of other nutrients. Manganese differs from most trace elements in that heavy applications of organic matter into the soil does not assure its availability. Alkaline soil can halt the nutrient’s availability, making increased soil acidity a goal when manganese deficiency is suspected. Organic matter (especially seaweed) supplies all of the manganese required by plants, but a correct soil pH ensures that plants will use it.

When the lush green color of spinach changes to gold with white, dead areas, suspect a shortage of manganese. Tomato leaves turn light green with yellow showing farthest from main veins. The yellow areas develop white dead spots similar to those on spinach leaves. The entire plant dwarfs and lacks blossoms and fruits.

The leaves of snap beans turn yellow when soil lacks manganese and brown spots often appear between leaf veins. The stems of cucumbers, cabbage, broccoli, cauliflower, peppers, and eggplant grow small and spindly from a manganese deficiency. The leaves gradually turn yellow, then white, but veins remain green (an easily recognized symptom).

Carrot tops are excellent suppliers of manganese. Add them to the compost pile after the harvest. Leaf mold is another quality supplier of manganese.

Iron

Next up is iron. The trace element plays major roles in the production of chlorophyll and in the nitrogen-fixation process. It also helps reduce nitrates to ammonia for the synthesis of plant proteins.

Chlorosis is a primary symptom of an iron shortage, but it’s not a reliable indicator without a soil test because yellowing indicates other nutrient deficiencies as well. Additional symptoms include new shoots that stop growing, and in severe cases, top growth dies. Fruit-tree leaves turn yellow and develop brown areas, and fruits generally
lack flavor. Excessive lime and/or phosphate may inhibit a plant’s intake of copper, so apply small amounts when copper or other trace-element deficiencies exist.

Chelation is a process that makes iron available to plants. In short, iron in an insoluble form is attracted to organic chelates that plants readily absorb. When plants absorb chelates, they also take in the captured iron.

The best way to put iron and chelates into the soil is by the addition of organic matter. Manures (especially chicken manure), garden and kitchen wastes, greensand, dried blood, and/or seaweed added to the compost pile provide all of the iron that plants need.

Copper

Copper, like several other nutrients, tends to act as a catalyst. The metal activates plant respiration and iron usage. Plants lacking copper develop a condition known in the Northeast as withertip. A look at the leaves on stem tips reveals wilting that watering doesn’t revive. Fruit trees suffering from withertip are especially reliable indicators of copper deficiency.

Tomato shoots exhibit abnormal growth and less-than-vigorous root systems develop when a lack of copper exists. Blossoms don’t form and leaves curl upward with bluish-green coloring. The edges of lettuce leaves whiten when copper is unavailable, and the bleached appearance eventually encompasses the entire plant.

Applications of compost or well-rotted manure correct a copper shortage in summer soil. Liquid seaweed sprayed on leaves, especially on growing shoots, helps the plants along until they can absorb it in chelated form from compost or manure.

Molybdenum

Molybdenum is responsible for nitrogen fixation by Rhizobium bacteria on legume roots. Nonlegumes use molybdenum to reduce complex soil nitrates to ammonia, which provides plants with a simple form of nitrogen that they use readily. Molybdenum becomes scarce when it becomes chemically bound to other elements in acidic soils. Suspect your potato patch and other acidic areas as candidates for molybdenum shortages.

The early stages of a molybdenum deficiency often disguise themselves as a nitrogen deficiency because molybdenum helps plants absorb nitrogen. The yellowing and mottling of older leaves beginning near the bases of plants and plants that appear weak and spindly signal a nitrogen deficiency possibly caused by a lack of molybdenum. Particularly susceptible to molybdenum shortages are many of the brassicas (broccoli, cauliflower, brussels sprouts, kale, etc.) and lettuce. The leaf material between veins doesn’t develop properly and a leafs midrib overdevelops instead. That gives the leaf a whiplike appearance, a condition sometimes referred to as whiptail.

Remedy a molybdenum shortage by planting crops such as vetches that store the micronutrient. Turn the plants under after they mature. The stored molybdenum is released to the soil as the crops decompose. For instant relief, side dress plants with several inches of compost and spray leaves with liquid seaweed.

Boron ( B ) – Powder   –   Material Information

Discovered in 1808 by L.J. Lussac and L.J. Thenard (in Paris) and Sir Humphrey Davy (in London).

Boron is a non-metallic element which occurs in several allotropes. It is rarely found in nature, normally occurring as borates or orthoboric acid (the abundance of boron in the earth’s crust is 10 ppm, the principal ore being borax, Na2B4O7.xH2O). Amorphous boron is the more common allotrope and exists as a dark powder which is unreactive towards water, oxygen, acids and alkalis. Boron finds importance within nuclear reactors due to its neutron absorbing capabilities, boron steel being used as control rod material. Boron compounds are used for a number of applications including the manufacture of certain grades of glass and detergents.

Boron will react directly with most metals to produce metal borides which are hard, inert binary compounds of various formulae and arrangements of the boron atoms. For example, as single atoms (M2B), pairs (M3B2), single and double chains (MB, M3B4), sheets (MB2), B6 octahedra (MB6) and B12 clusters (MB12). Boron also forms the binary compound, boron nitride, which is of interest as it is isoelectronic with carbon and occurs in two structural modifications; one is a layer structure similar to graphite which is soft and lubricating, whilst the other (formed under high pressure) has a very hard, stable, tetrahedral structure as found in diamond.

Why boron is an essential element for vascular plants

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The Significance of Boron in Plant Nutrition and Environment-A Review

INTRODUCTION

More than eighty years ago it was known that boron is one of the essential elements for plant growth. The importance of boron as a plant nutrient was first demonstrated by Warington (1923) with characteristic deficiency symptoms of die prematurely broad beans and later on by Brandenburg (1931) found heart and dry rot in sugar beet and mangolds, respectively. On the other hand, boron is also regarded as a poisonous element (Buchel and Bergmann, 1964), because of its high potency, even small quantities gave damage to plants e.g, germination inhibition, root growth inhibition, shoot chlorosis and necrosis (Bergmann, 1984). According to Russell (1973) boron is probably the trace element which most commonly limits crops yield and consequently is most widely used in agriculture, horticulture and in forestry. Boron deficiency has been reported to be most pronounced on leguminous crops such as lucerne, red clover and alfalfa and cruciferous crops such as cabbage, cauliflower, rutabagas, turnips and radish (Murphy and Walsh, 1972), because of their relatively large boron demand. It is well documented in the literature that monocotyledons require less boron than dicotyledons, because the roots of monocotyledons had a lower capacity to adsorb boron than roots of dicotyledons (Tanaka, 1967). In connection with the significance of boron in plant nutrition and the occurrence of boron disorder symptoms, Bergmann (1984) stated that boron prevents the dropping of grapes and frost resistance of fruit trees presumably due to the beneficial effect on carbohydrate and protein metabolism. He further stated that the scab disease of potatoes should also be reduced by applying boron, through improvement of skin resistance. Ju et al. (1982) reported that at low boron supply boron deficiency was apparent as brown heart in turnip, although there were no external symptoms, but that growth was inhibited at toxic boron levels. Recently, these findings were also confirmed by Tariq and Mott, (2006a and b) on radishes, using sand culture technique. Kotur (1991) reported that higher boron rate increased the cauliflower yield 90% over control and reduced curd rot 3% over control. Moreover, he found the curd rot came down from 52 to 7% at normal boron supply due to correction of boron deficiency and reduction of incidence of black rot (Kotur and Kumar, 1989). Similarly, Mishra (1972) obtained a 210% increase over control in curd yield, when boron was applied at normal concentration. The assessment of boron nutrition and its requirement by various vegetable crops have been extensively studied and reviewed by Gupta (1983), Bergmann (1984), Shorrocks (1984), Francois (1986) and many other researchers. So, it has been found that boron is necessary for plant growth, especially for cruciferous and root crops. However, it is clear from the literature that boron plays a significant role in plant nutrition though with a very narrow range between deficiency and toxicity (Hesse, 1971; Tariq, 1997).

The chemistry of boron in soil: Boron is a member of the 3rd periodic group. It is a non metal among the micronutrients. Boron has an extremely complex chemistry and is capable of unusual bond types, especially in combination with hydrogen. However, in aqueous solution, the element shows a charge of 3+ and has an approximate ionic radius of 0.023 nm and with an electronegativity of 2.0 on the Pauling scale and around 50% ionic character of bond with oxygen. Boron occurs in aqueous solution as boric acid, B(OH)3 and hydrolyses reversibly to the borate ion according to the reaction below (Baes and Mesmer, 1976):

B(OH)3 + H2O = B(OH)4 – + H+ ↔ pKa = 9.2

When B occurs in solution at concentration above 0.1 M, poly borate species are formed by the addition of one OH¯ ion per borate ion present. The most common species of boron are B(OH)4¯, B2O(OH)5¯, B3O3(OH)4¯ and B4O5(OH)4-2, with the trimeric anion being the poly borate formed under most conditions (Baes and Mesmer, 1976). Boron always occurs in combinations with oxygen, usually 3-fold coordination and occasionally with 4-fold coordination (Krauskopf, 1972). The chemistry of B in soils has been extensively reviewed by Evans and Sparks (1983) and Keren and Bingham (1985) and this material is not repeated in this review. In terms of the present study, which is concerned with the significance of boron in plant nutrition and environment, only the following points, pertinent to this theme, are noted: The principle B species expected to be found in soil are H3BO3 and, in part, B(OH)¯4. The neutral species H3BO3 is the predominant species expected in soil solution. It is only above pH 9.2, that the species H2BO3¯ become predominant. Apparently, the higher polymers of B are unstable unless B concentrations exceed 10-4M, which is a level seldom encountered in soils (Lindsay, 1972). Lindsay further indicates that most B fertilizers are in the form of B4O7-2 and are expected to hydrolyse to H3BO3. In soils, B is considered to be the most mobile and often deficient element compared to other trace elements. According to Sillanpaa (1982) based on an FAO global study of micronutrient status of soils, boron deficiency is most widespread and the known boron deficiency areas worldwide is at least eight million hectares (Bussler, 1979). On the other hand soils over fertilized with boron, irrigated by sewage sludge, or saline water, may contain toxic amounts of boron.

Concentration: Boron in soil is found in a chemical pool and concentrations can be roughly categorized by climatic zone. The water soluble B concentration in most soils varies between 2 and 200 μg g-1 but a more frequent range is from 7 to 80 μg g-1 (Krauskopf, 1972). Temperate and boreal regions contain low concentrations of B ranging from 1-2 μg g-1 in sand and podzol soils (Evans and Sparks, 1983). Tropical humid regions also contain low concentrations of B, in the range of 1-2 μg g-1. However, arid and semiarid region soils contain high boron concentrations from 10-40 μg g-1 or more (Aubert and Pinta, 1977). Soil B falls into three categories i.e., total, acid soluble and water soluble. Generally, less than 5% of total boron is available to plants (Berger and Truog, 1945). However, Fleming (1980) categorized water soluble B values in soils to give a general guide for B supplying power to plants:

The availability of soil B depends on soil texture, pH and liming, organic matter content, soil moisture and relationships with certain cations and anions in soils (Tisdale et al., 1985). In fact it is not feasible to mention the detail of all these factors affecting the availability of boron in soils, however, keeping in view the present study, it would be useful to review the chemistry of B availability in relation to soil environment. The readers refers to detail study of the factors affecting the availability of boron (Evans and Sparks, 1983; Tisdale et al. 1985; Keren and Bingham, 1985).

Adsorption: There are several descriptions in the literature of B reactions with other components in soil. However, the mechanisms of these reactions in soil are still not well understood. These reactions are highly pH dependent and always occur at pH above 7.0 (Kabata-Pendias and Pendias, 1984). As Overstreet and Dean (1951) and Bingham and Page (1971) reported, the mechanism of B retention by soils does not parallel that of other soil anions (Cl¯, SO4-2, NO3¯ and PO4-3). Boron retention is lowest in acid soils, but increases rapidly in the range pH 6 to 10, indicating that B availability is crucially dependent on soil pH. Heavy applications of Ca(H2PO4)2 result in a lower availability of B in acid soils, because soil acidity produced by phosphate induced a greater fixation of B (Bingham and Garber, 1960). It is well understand that due to the liming of acid soils, Ca ions combine with soluble B to form the highly insoluble Ca-metaborate and thus reduce the availability of B. But the addition of K should increase the availability of B in the soil due to the formation of K-tetraborate of high solubility (Donald, 1964). However, it depends on the degree of K saturation of the soil colloids, as Hadas and Hagin (1972) found that K-saturated soils fixed more B than untreated soils. Bishop and Cook (1958) noted that both CaCO3 and MgCO3 were equally effective in decreasing water soluble boron in soil, but CaSO4 was found ineffective. Similarly, NaOH increased the water soluble boron by increasing the soil pH. Similarly, Gupta and Macleod (1981) showed that at equivalent rates of Ca in acid soils, CaCO3 reduced the B concentration in plants more than CaSO4, indicating a pH rather than a Ca effect. Su et al. (1994) found that boron adsorption increases with pH increase as a result of increasing amounts of CaCO3 addition to soil. The effect of pH on boron adsorption by oxides and clay minerals is well established with the maximum amount of boron adsorption occurring in the pH range 7 to 9 (Goldberg and Glaubig, 1986). However, the reasons for the increase in boron adsorption with increasing pH are complex, involving an increase in B(OH¯)4 concentration, an increase in OH¯ concentration, a change in the concentration of Al and Si in solution and an increase in net negative charge and of surface hydroxyl ion dissolution (Hingston, 1964).

Precipitation: In sodic soils B toxicity is ameliorated by the addition of gypsum, which converts readily soluble Na-metaborate to sparingly soluble Ca-metaborate (Bhumbla and Chhabra, 1982). Singh and Singh (1984) also observed that the increased absorption of Na and B by plants at high soil pH was due to the formation of Na-borate, which is the most soluble salt of B in soil. Singh and Randhawa (1978) studied the relative effect of various amendments (MgCl2, MgSO4, CaCl2, CaSO4, AlSO4 and FeSO4) on the solubility of boron in saline-alkali soils. They observed that all the amendments reduced the concentration of water soluble boron, but Mg salts were shown to be relatively superior to the others. Unfortunately, they did not speculate on possible mechanisms for the effect of Mg salts on reducing the solubility of boron. However, Rhoades et al. (1970) stated that boron is actually incorporated into the crystal lattice of MgOH (on a precipitated surface), which either distorts its lattice or else forms a new compound. Little is known about the relationships between B with metallic micronutrients such as Zn, Cu, Fe and Mn in soils.

Solubility: It is reasonably clear from the literature that the differences in the solubilities of K, Ca and Na-metaborates contribute to absorption and availability of boron. There are various boron compounds which are used as commercial fertilizers and the availability of boron to crops also depends on its solubility in soil. Boric acid and borax are the most common fertilizers. Borax is generally used for soil application, while boric acid is used for both soil and foliar application. Colemanite and Ulexite are slow in releasing boron as compared with other sources. Others kinds of boron-containing fertilizers and materials include: farm yard manure, sewage sludge, compost, borated gypsum, calcium nitrate and various mixed fertilizers (Gupta, 1979).

Table 1: Boron solubility in (g L-1) of cold water
Source: Souchelli (1969)

However, the solubility of various boron compounds are given in Table 1.

It is gathered from the overall literature review of boron chemistry that the solubility and retention of boron in soil is depend on the various soil components and ions, specifically cations (K, Ca, Mg and Na).

Biochemical and physiological functions of boron in plants: As an essential micronutrient the mode of operation of boron differs considerably from that of other micronutrients. Metallic micronutrients such as Zn, Cu, Fe, Mn and Mo are effective as components or as activators or inhibitors of enzymes in the plant, but a similar function for boron has not been established (Sutcliffe and Baker, 1981; Bergmann, 1984 ), although its effect can be detected experimentally in various metabolic processes. In spite of the essentiality of boron for plants, the biochemical and physiological function of this element is still not well understood (Price et al., 1972; Bergmann 1984; Kabata-Pendias and Pendias, 1992). However, the role of boron in plants has formed the subject of many investigations and several functions have been assigned to boron of which the following is summarized below:

Carbohydrate metabolism and transport of sugars: Boron has been considered to be functional in the transport of carbohydrates and translocation of sugar is thought to be enhanced by the formation of borate-sugar complexes (Gauch and Dugger, 1954; Price et al., 1972; Marcus-Wyner and Rains, 1982; Katyal and Randhawa, 1983). In sugar beet the sucrose content of the storage roots tended to decrease in the same treatment at which limiting boron resulted lower yield (Valmis and Ulrich, 1971; Tariq et al., 1993).

Phenol and auxin metabolism: The accumulation of auxins and phenols may be associated with leaf necrosis when boron is deficient (Bohnsack and Albert, 1977; Mengel and Kirkby, 1982; Marcus-Wyner and Rains, 1982). The boron is presumably responsible for the metabolic changes and cell damage in boron deficient tissue (Marschner, 1986) and it is thought that boron complexes the phenolic compounds in plant cells, reducing their potential toxicity (Lee and Arnoff, 1967).

Water relations: Boron is concerned with the water relations in cells and regulates the intake of water in to the cell (Wallace, 1961). In this connection Briggs (1943) in early work reported that boron deficient (N. aquaticum L.) plants showed decreased moisture percentage, less succulence, less metabolic activity and a lower growth rate, in comparison to boron-sufficient plants. This was confirmed many years later by Sharma and Ramchandra (1990) who also reported that boron deficient plants had low water potential, stomatal pore opening and transpiration.

Tissue development, differentiation and formation of cell walls: Boron is required for proper development and differentiation of tissues (Katyal and Randhawa, 1983). Boron may affect the deposition of cell wall material by altering membrane properties (Goldbach and Amberger, 1986) and the deficiency of boron causes the breakdown of the walls of parenchyma cells (Shorrocks, 1984). The responsive effect of boron deficiency on cell division causes slow down in root extension and followed by a degeneration of meristematic tissue in plants (Jackson and Chapman, 1975). Cohen and Lepper (1977) concluded that a continuous supply of boron is not essential for cell elongation of intact squash plants but is required for maintenance of meristematic activity. The evidence from the various literature review showed that boron is important in cell walls (Jackson and Chapman, 1975; Cohen and Lepper, 1977; Gupta et al., 1985). Both boron deficiency and toxicity cause lower chlorophyll levels and net photosynthesis (Petracek and Sams, 1987). Boron may induce cell wall synthesis by an influence on the activity of the plasmalemma (Sutcliffe and Baker, 1981). Whittington (1957) also reported that boron is essential for the maintenance of meristem in plants.

Reproduction: Boron is involved in the reproduction of plants and the germination of pollen (Wallace, 1961). Tompson and Batjar (1950) and Montgomery (1951) concluded that the boron nutrition of the pollen grain was of particular importance. Birnbaum et al. (1977) found that cotton ovules callus when boron is lacking in the growth medium. The role of boron in promoting pollen tube growth is well established but the mechanism of its action is still unknown (Sutcliffe and Baker, 1981). Thus a significant positive correlation could be found between boron in the plant and number of flowers, the proportion of flowers not aborted and the weight of fruit (Bergmann, 1984; Oyewole and Aduayi, 1992). In the case of rape, clover, alfalfa and beet, boron hinders abortion of the ovaries. Moreover, it reduces the proportion of sterile seeds in cotton, soybean, alfalfa, maize and sunflower (Bergmann, 1984).

Disease resistance: There have been several reports of increased disease resistance with the application of boron, such as potato scab disease (Bergmann, 1984) and ergot on barley and damping off fungi on tomato and cabbage (Shorrocks, 1984). Although, Kabata-Pendias and Pendias (1992) reported that unlike other micronutrients, boron is not essential to the life of fungi and some algae (Shoklnik, 1974). But mycorrhizal plants appear to have a greater need for boron supply than do non-mycorrhizal plants (Lambert et al., 1980).

Boron in soil-plant relations: According to the observations of Rayans et al. (1977) and Bingham et al. (1981) plants respond mainly to the concentration of boron in soil solution. The chemical species in soil solution is primarily uncharged boric acid H3BO3. It was suggested that boron is absorbed as molecular boric acid in a physical process regulated by the boron concentration gradient (Oertli and Grgurevic, 1975; Bingham et al., 1970). Boron uptake has been correlated with the concentration of H3BO3 in solution, because leaf boron generally increased in a linear fashion as the concentration of the nutrient solution or soil solution increased (Gomez-Rodriguez et al., 1981; Salinas et al., 1986; Szabo, 1988; Taylor and Macfie, 1994; Tariq et al., 2005; Tariq and Mott, 2006a and b). There is still controversy in the literature about whether boron uptake is either a passive or active process. Generally, boron is taken up as undissociated boric acid or in borate form, presumably through transpiration in the xylem stream of plants and its uptake also varies with the stage of plant growth. Bowen (1972), Bowen and Nissan (1977) and Reisenauer et al. (1973) indicate that boron is actively absorbed by plants in ionic form particularly when boron concentration is low in soil and in plants boron translocates readily through xylem in the transpiration stream. Oertli and Richardson (1970) have also emphasized that transpiration, xylem stream and leaf venation are factors primarily involved in the accumulation of boron in leaves. Gopal (1970) stated that most of the boron absorbed by roots is insoluble in water and is carried by a passive stream from the roots to the upper parts of the plant and accumulated in higher quantities in leaves as they are the chief organs and also end points of transpiration. Generally, boron has a tendency to accumulate in the margins of plant leaves (Jones, 1972) and once accumulated it cannot be redistributed under any conditions (Gomez-Rodriguez et al., 1981). Thus boron concentration often increases from the lower to the upper portion of the plant (Shuman, 1994). Rashid et al. (1994) and Jones (1991) observed that boron concentration in leaves of rape seed and mustard was greater than in the whole shoots, as its accumulation occurred in leaves as boron carried in the transpiration stream and deposited at the leaf margins when water is transpired. Since boron is immobile in plants, once transpired a large accumulation occurs in the tips and margins of older leaves (Szabo, 1988). Therefore, its concentration also varies among plant parts (Tariq, 1997). Miller and Smith (1977) reported that the alfalfa (Medicago sativa L.) lower leaves contained 98, upper leaves 75, tips 47, upper stem 27 and lower stem 22 μg g-1of boron. Gupta (1971) also reported that the seeds of some cereals contain less boron than the rest of the plant. Gupta et al. (1985) stated that boron is readily translocated from old leaves to young plant parts, the first deficiency symptoms will be in the growing points i.e., the stem tips, root tips, new leaves and flower buds. In contrast, toxicity symptoms typically show first on older leaf tips and edges as either a yellowing, spotting or drying of leaf tissues. It is also evident from the previous work of Valmis and Ulrich (1971) that the supply of boron affects the distribution of boron in various plant parts. They found in sugar beet plants the blades had a higher boron content than the petioles where the boron supply was adequate, but this relation was reversed in the boron deficient plants. However, in monocotyledons plants like corn boron accumulation was found greater in the marginal section of leaves than in the midrib section (Touchton and Boswell, 1975).

Deficiency, sufficiency and toxicity
Soil: Boron, from the stand point of plant nutrition, is unique among the trace elements in that very small quantities are necessary for normal crop production, but slightly higher concentrations cause injury. As is frequently reported in the literature the range between deficiency and toxicity of boron is very small. Soils were classified by Kalmet (1963) according to the water soluble content into deficient (< 0.1 μg g-1 ), inadequate (0.1-0.2 μg g-1), moderate (0.4-0.6 μg g-1) and rich (>0.6 μg g-1). Sillanpaa (1982) also classified the water soluble boron concentration in soils based on FAO, global study into deficient (<0.3-0.5 μg g- 1) and excess (>3-5 μg g-1). Later on, Shorrocks (1993) reviewed the categories of water soluble boron in soils and his categories are:- very low (< 0.25 μg g-1), low (0.25-0.5 μg g-1), medium (0.51-1.0 μg g-1), high (1.1-2.0 μg g-1) and very high (> 2.0 μg g-1). It is evident from the above classification of water soluble boron in soil that in general, > 0.5 μg g-1 of boron in soil is sufficient for the normal growth of most crop plants (Cox and Kamprath, 1972). However, the classification of water soluble concentration depends on soil type, plant species, source of irrigation and environmental conditions. The recommendation for the boron fertilization is based on these water soluble boron categories, nature of soils, environmental conditions and the requirements of crop species.

Plant: Gupta (1979) reported that the deficient and toxic levels of boron are associated with plant disorders and/or reductions in the yield of crops. The deficient, sufficient and toxic concentrations of boron in the cruciferous and root crops are reviewed as reported by several investigators, because these crops are dicotyledons and the requirement for boron is more than monocotyledons. Gupta (1983) listed the deficient, sufficient and toxic levels of boron for radish (Raphanus sativus L.) cv. Cherry belle, tops when roots began to swell as < 29, 96-217 and > 217 μg B g-1 DM, respectively. Shelp et al. (1987) found good growth of radish (Raphanus sativus L.) cv. Cherry belle, at boron levels of 150-170 μg g-1 DM in the leaves and 28-60 μg g-1 DM in the roots. Toxic levels were attained when leaves and roots contained 260 and 40 μg g-1 DM, respectively. Similarly, Tariq (1997) observed good growth of radishes cv. French breakfast when boron concentration was 74-159 μg g-1 DM in the leaves and 23-24 μg g-1 DM in the roots. Toxic concentrations were possibly attained, when leaves and roots contained 256-586 and 48-51 μg g-1 DM, respectively. The differences found in the findings of Gupta (1983), Shelp et al. (1987) and Tariq (1997) for the sufficiency levels of boron, seems to be due to different crop variety and growth media used. In the case of rutabaga (Brassica napobrassica L.) the concentration of boron in leaf tissue at harvest were classified 20-38 μg g-1 deficient, 38-140 μg g-1 sufficient and >250 μg g-1 toxic (Gupta and Munro, 1969). Similarly, Neubert et al. (1970) categorised the boron concentration <20 μg g-1 deficient, 31-200 μg g-1 sufficient and >800 μg g-1 toxic for sugar beet (Beta vulgaris L.) at fully developed leaves stage. Generally, it can be concluded from the critical boron deficiency and toxicity levels that for most cruciferous and root crops the concentration of boron in plants <15 μg g-1 to be deficient, 25-100 μg g-1 adequate and >200 μg g-1 toxic for growth and production. It is clear that both boron deficiency and toxicity will result in reduction of crop yield and quality.

Boron in relation to environment: Boron is released to the environment from natural sources such as oceans, volcanoes and geothermal steam. Argust (1998) identified and quantified the orders of magnitude for major reservoirs and flows of boron in the environment and reported that the largest flows of boron in the environment arise from the movement of boron into the atmosphere from oceans, at between 1.3×10(9) kg and 4.5×10(9) kg B annum-1, drainage from soil systems into ground waters and surface waters accounts for between 4.3×10(8) kg and 1.3×10(9) kg B annum-1, while boron mining and volcanic eruptions represent the next most significant boron flows, accounting for approximately 4×10(8) kg and 3×10 kg B, respectively. It may also release from the industries that use it, e.g., manufacturing glass, combusting coal, melting of metals and through the addition of agricultural fertilizers. Cosmetics and laundry products also containing boron. For example, use of domestic and industrial detergent is the single most significant source of borates in the environment (Wells et al., 1998). Boron is readily leached from coal ash at ambient environmental conditions and represents a potential threat to water and soil environments in proximity to coal-ash wastes dumps and landfill with large quantities of ash (Wood and Nicholson, 1998). Similarly, reduced growth of lettuce after the use of fly ash was considered to be due to excessive salinity and boron (Page et al., 1979). Boron accumulates in plants and is found in foods, especially fruits and vegetables. Richardson (1980) reported that boron contents were consistently higher in vegetable crops on treated land and than in adjacent fields which received no sewage sludge. Hermann (1994) examined boron pollution within 10 km radius emission from coal-fired power station. He observed that both in soils and plants the boron concentration significantly depended on the distance to emitter, most frequent wind directions and depth of sampling. Periodic increase in boron concentration may be expected even up to the toxic levels to cultivated plants. In other study, fly ash-amended compost used as manure for certain crops (Menon et al., 1993) showed the string beans, bell pepper and egg plants assimilated high levels of boron and this may be the reason for their poor growth. Prausse (1991) also reported that amounts of boron recorded from a highly contaminated arable area (3000 ha) in Germany, that has received air emissions, fly ash and coal dust over the past 50 years. He noted that boron was the dominant pollutant in soil and showed negative correlation with yield from crops harvest. Francois (1991) also found direct correlation between Bsw. and boron concentration in the leaves and bulbs of garlic and onion plants. Mehrotra et al. (1989) indicated an antagonistic relationship between soil boron and SAR of irrigation waters. Ayars et al. (1993) concluded that accumulation of boron poses a major threat to the sustainability of agriculture if drainage volumes are to be reduced by using drainage water for irrigation. The overall literature review regarding the impact of boron on environment indicate that boron is especially toxic to human and animals and their entry in to the food chain and the environment must be kept with in acceptable limits.

CONCLUSIONS

Boron chemistry in soil is depend on the various soil components and ions and in plants the mode of operation of boron differs considerably from that of other micronutrients. Boron plays a significant role in plant nutrition and in environment. From the foregoing literature, it is obvious that an extreme deficient or toxic levels of boron may be responsible for secondary effects on account of the reduction in plant growth and resulting in a change of physiology and biochemistry of plants. There is a small range for boron between deficiency and toxicity in soil, plant and water systems.

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