Bo

Boron:- Content – forms in soil - Critical limits in soils and plants. Factors affecting its availability.

The total boron content in Indian soils has been found to vary from 7 to 630 ppm. It is observed that more boron is concentrated in the surface soils than in the sub surface.

Sources of boron

1. Non metal among the micronutrient

2. Low concentration in earth crust igneous rocks (<10 ppm)

3. Tourmaline and borosilicate contains B.

Forms of boron (Boron cycle)

B exists in 4 forms in soil.

1. Rocks and minerals.

2. Adsorbed on clay surface, Fe and Al oxides combined with O.M.

3. Boric acid (H3BO3) and

4. B (OH)-4 in soil solution.

Factors affecting available boron content in Indian soils

1. Parent material: The boron content of granite is 19.3, shales and lime stone 39.2, basalt 42.5 and of alluvium 42.3 ppm.

2. Texture: Boron content of the soil depends on the coarse sand fraction than on the clay content. Arid zone soils have higher boron content.

3. Calcium carbonate: Calcareous soils are supposed to contain more total boron than non calcareous soils.

4. Effect of cultivation: Cultivated soils have more boron than their counter part virgin soils due to its accumulation through irrigation water.

5. Irrigation water: Continuous use of irrigation water containing even a small amount of boron would build up the boron content of the soil.

6. Soil pH : This element generally becomes less available to plants with increasing pH greater than 6.3 to 6.5. Adsorption of boron by iron and aluminium hydroxides is maximum at pH of 8 to 9 and 7 respectively.

7. Organic matter: It is one of the main source of boron in acid soil.

8. Soil moisture: Boron deficiency is often associated with dry weather and low soil moisture conditions.

9. Plant factors: Crops most sensitive for boron deficiency are sugar beet and celery. High boron requiring crops are apple, asparagus, broccoli, cabbage, cauliflower. Boron needs of graminae are low.

Soils where boron deficiency is common are

low in boron such as those derived from acid igneous rocks.
Highly leached acid soils like laterites and podsols.
Light textured soils.
Soils low in organic matter.
Alkaline soils with high amount of free CaCO3

Boron

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

· Maintaining a balance between sugar and starch.

· Carbohydrate Metabolism.

· Protein Synthesis.

· The translocation of sugar and carbohydrates.

· It is important in pollination and seed reproduction.

· It is necessary for normal cell division, nitrogen metabolism, and protein formation. It is essential for proper cell wall formation.

· It plays an important role in the proper function of cell membranes and the transport of K to guard cells for the proper control of internal water balance.

Cell Wall

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

Germination and Pollination

The B requirement is much higher for reproductive growth than for vegetative growth in most plant species. Boron increases flower production and retention, pollen tube elongation and germination, and seed and fruit development

A deficiency of B can cause incomplete pollination of corn or prevent maximum pod-set in soybeans.

Sugar Translocation

Photosynthesis transforms sunlight energy into plant energy compounds such as sugars. For this process to continue in plants, the sugars must be moved away from the site of their development, and stored or used to make other compounds.

Boron increases the rate of transport of sugars (which are produced by photosynthesis in mature plant leaves) to actively growing regions and also in developing fruits.] Boron is essential for providing sugars which are needed for root growth in all plants and also for normal development of root nodules in legumes such as alfalfa, soybeans and peanuts

Symptoms of boron deficiency

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

· Apple- interacting with calcium, may display as "water core", internal areas appearing frozen

· Beetroot - rough, cankered patches on roots, internal brown rot

· Cabbage - distorted leaves, hollow areas in stems.

· Cauliflower - poor development of curds, and brown patches. Stems, leafstalks and midribs roughened

· Celery - leaf stalks develop cracks on the upper surface, inner tissue is reddish brown.

· Celeriac - causes brown heart rot

· Pears - new shoots die back in spring, fruits develop hard brown flecks in the skin.

· Strawberries- Stunted growth, foliage small, yellow and puckered at tips. Fruits are small and pale.

· Swede (rutabaga) and turnip- brown or gray concentric rings develop inside the roots.

· Arecaceae (Palm Tree) - brown spots on fronds & lower productivity.references

Boron Toxicity

Excessive concentrations of B can cause reduction of crop yield and loss of quality as well. A B toxicity looks like yellowing of the leaf tips, interveinalchlorosis, and progressive scorching of the leaf margins. Boron toxicity is uncommon in soils, unless fertilizers or municipal composts high in B have been added in the past. Of the field crops grown in New York, corn and soybeans are most sensitive to B toxicity

Apple- interacting with calcium, may display as "water core", internal areas appearing frozen

Cabbage - distorted leaves, hollow areas in stems.

Reclamation of boron deficiency

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

Apply household borax at a rate 1 tablespoon borax to 12 quarts of water. This amount will treat a 100 foot row of vegetables or 10 square feet of soil. Apply two times 2-3 weeks apart.

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