Boron Toxicity in Plants Causes, Symptoms, and Fixes

Boron toxicity in plants will first affect basal leaves, causing chlorosis followed by necrosis starting from leaf tips or margins and spreading inward until leaves fall in most plants where this micronutrient is immobile.

However, in plants where it is mobile, symptoms affect buds and upper, newly expanding leaves. These symptoms include cessation or reduced growth, bud dieback, cankers on the petiole, mishappen fruits, leaf axil gumming, etc.

Today, we will discuss boron toxicity in plants, including causes, symptoms in plants where it is mobile or immobile, and practical ways to fix toxicity.

Overview

Boron is one of the essential plant micronutrients for optimum growth and development. It will promote the best quality crops, higher yields (fruits, seeds, or tubers), and better resistance to diseases or pests.

Some of the key functions of boron in plants include supporting new growth sites (buds, leaves, branches, and roots tip), flowering and fruiting. It also increases sugar transport, is part of the cell wall membrane, helps regulate hormones, participates in protein and RNA synthesis, nitrogen fixation in nodules, and calcium use (absorption and transportation), among many others.

While very important, plants have a narrow boron requirement with too little, resulting in boron deficiency and too much toxicity.

When it is too little in the soil, your plant will suffer from boron deficiency symptoms which will first show on new vegetative growth points, root tips, and reproductive sites when it is too little. These signs include stunted growth, death of terminal buds, and short stubby roots. Also, leaves will curl, thicken, wrinkle, or have twisted tips, flowering will decline, and fruits will be distorted or fail to seed.

On the other hand, too much boron will result in toxicity, something we intend to look at in detail.

When does toxicity occur?

The ideal or optimum boron soil concentration level for most plants is 0.5-0.7 ppm, with 1-1.5 ppm causing toxicity in sensitive plants. However, tolerant ones can withstand up to 2 ppm without showing toxicity symptoms.

For irrigation water, toxicity will arise if the concentration is over 0.5 to 2 ppm, depending on plant sensitivity.

On the other hand, a concentration of less than 0.5 ppm of hot water-extractable boron represents a deficiency, meaning that the range between deficiency and toxicity is often narrow.

Causes of boron toxicity in plants– Why does it occur?

Some of the possible reasons for excess boron or toxicity include the following:

1. Arid and semi-arid are with low rainfall

Boron toxicity is naturally common in soils and irrigation waters in arid and semi-arid areas. In these places, this toxicity usually occurs with salinity or salt stress, especially in those with 500 mm of annual rainfall or less. The low rain prevents the leaching of soluble boron from the soil profile and rootzone hence the higher concentration.

2. Accumulation from irrigation water

Irrigation with boron-rich underground, salty, or well water from deep tube wells or using water from springs near earth-quake faults or active geothermal zones that tend to be high in boron may result in accumulation.

Also, sewerage or reclaimed irrigation water high in born may be responsible for your plants’ toxicity.

3. Coastal agricultural lands

Agricultural lands near coastal saline lands where boron-rich seawater intrudes to aquifers or because of seawater aerosol dispersion

4. Excessive boron-borate fertilizers

Soils excessively fertilized by boron or boron-rich or borate-containing fertilizer may result in toxicity. The accumulation effect of boron in these fertilizers lasts up to 3-4 years, depending on the crop, soil condition, and climate will only make things worse.  

5. Alkaline/saline soil or those with boron-rich parent material

Soils whose parent rocks are boron-rich (marine sediments, plutonic or volcanic materials like tuff or rich-fly ash.

Boron toxicity symptoms in plants

Toxicity symptoms will depend on whether boron is mobile or immobile and may also vary from one plant to another. However, some symptoms like bark cracking or becoming corking may occur on any plant with boron deficiency. Also, severe cases may lead to plant death.

Also, in some cases, there may be leaf wrinkling, deformation, thickening, or curling/cupping downward or tip curled under, most likely due to limited cell expansion or reduced tissue expansion.

1. When it is immobile

Boron toxicity symptoms will first affect the lower or basal leaves in plants where it is immobile. You will notice chlorosis starting with leaf tips, margin, and sometimes interveinal spreading inward, followed by browning or necrosis in these areas.

As it progresses, affected plants will have irregular blackened spots or areas along leaf margins or in-between veins that will go inward. Also, leaves start falling.

2. Where it is mobile 

Boron toxicity signs in mobile plants will affect buds, new or newly expanding leaves, fruits, or flowers. Symptoms include cessation or reduced shoot growth, bud and twig dieback, cankering on petioles, mishappen fruits, leaf axil gumming, etc. However, the exact symptoms will again vary from one plant to another.

3. Typical symptoms in some plants

• Celery: Although B is mobile, toxicity symptoms will be deformed, bitter-tasting, and mishappen young leaves or stems, but there will be no tip burn.
• Carrot, alfalfa, maize, and tomatoes: Emergency decline
• Conifers: conifer needles will die from tip downward, with most damage on lower, older leaves.
• Prunus, Pyrus, or Malus plant species: In these boron phloem mobile species, symptoms include young shoots dieback/twig or cessation, bud abscission, profuse leaf axil gumming, and stems or petioles corky brown lesions.
• Wheat and barley: Chlorosis and necrosis start from tips with brown lesions appearing in the margin, moving inward, and covering the whole leaf surface. The oldest leaves are first affected, spreading to the top of the plant. Severe cases will have brown lesions on the leaf sheath, spikes, awns, or stem. Also, delayed emergence, foliation, reduced stem height, less dry matter, kennel weight, spikes per plant, yield, lateral roots, and weaker roots. But leaf area, width, and length are not affected.

Tests, diagnosis, and common misdiagnosis

The various signs and symptoms of boron toxicity alone cannot help confirm it is indeed boron. Also, electrical conductivity (EC) and total dissolved solids (TDS) be okay, yet boron is a problem, just like chloride and sodium.

Instead, get a laboratory soil, water, and plant tissue test of a specific ion, i.e., boron test. Test flowers, fruits, and younger apical leaves as they will reveal true status in plants where boron is mobile and in those where it is.  

Lastly, it is possible for misdiagnosis of these symptoms with those of hose of high salinity or deficiencies in manganese and iron. Also, potassium deficiency may cause older leaves necrosis, especially with downward necrosis.

Management or how to fix/reduce boron toxicity in plants

There is no quick way to reverse boron toxicity, especially where it is immobile. You cannot remove it from leaves where it has already accumulated. However, long-term corrective measures will prevent further accumulation.

Some of the ways to manage boron toxicity include the following:

1. Move to boron-resistant plants – crops or vegetables, trees, and shrubs

Switch to plants (land-scaping or crops including vegetables) that tolerate boron if they are cold-hardy enough for your place. Here are examples of sensitive intermediate, and highly tolerant plant

• Sensitive/low tolerance: Sugarcane, rubber, peas, cucumber, pineapple, melon, beans, squash, peppers, pumpkin, strawberry, pecan, squash, sweet potatoes, soybeans, grasses, rye, wheat, barley, rice, etc.
• Intermediate or moderately tolerant: Asparagus, flax, coastal Bermuda grass, grain sorghum, carrot, strawberries, corn, peaches, pears, citrus, cucurbits, eggplant, leeks, radish, clovers, onions, tomatoes, potatoes, sweet corn, parsnip, lettuce, tea, turf, herbs, tobacco, okra, etc.
• Highly tolerant: Cotton, sunflower, apples, alfalfa, grapes, canola (oilseed rape), eucalyptus, peanut, oil palm, carnation, olive, table beets, turnip, rutabaga, carnations, chrysanthemums, bird’s foot trefoil, cauliflower, broccoli, beet, gladiolus, pine, celery, coffee, sugar beets, spinach, kale, collard, swede, swiss chard, mangold, rape, forage turnip, etc.

For other plants, including shrubs and trees, and their sensitivity or tolerance to boron, see the following:

• Toxicity problem: FAO.org
• Boron- and salt-tolerant trees and shrubs for northern Nevada: the University of Nevada Reno

2. Leach the soil

Leach boron and other salts below the plant root zone using fresh water like rain or surface water (hardly high in boron)). You must use nearly three times more water to leach than sodium chloride. Also, it would be best if you improved drainage to increase efficiency.

On the downside, it may also leach other important nutrients like sodium, potassium, sulfur, and magnesium. Also, a hardpan may prevent leaching and may not work where water tables are high, and soils with high pHs, like sodic, may not drain well or result in overwatering.

3. Irrigate plants with low boron water – municipal or rainfall

Switch to municipal, rain, or surface water if you irrigate your plants. Don’t use reclaimed irrigation water or sewerage as they are high in boron.

Ideal boron levels in irrigating water are 0.2-0.3 ppm/L (for sensitive) and up to 2 ppm/L (for highly tolerant) when not using boron fertilizers.

4. Bring high-quality topsoil

Boron-laden landscapes need quality topsoil or substrate, at least 12 inches deep, or create planting mounds that extend beyond drip lines. It will ensure roots don’t reach boron-rich soil and improve drainage and aeration.

5. Pause boron fertilizers until the next season

If you feed your plants with boron fertilizers, including foliar, pause until the following season as you try some of the above means to correct the issue.

6. Other means

There are suggestions that silicon and salicylic acid application may diminish the effects of boron toxicity. Also, altering organic matter, salinity, soil pH, liming, and using clay minerals may help.