Boron

Since being identified as an essential plant nutrient over 80 years ago, hundreds of reports have documented a role for boron (B) in agricultural crops around the world. Globally B deficiency is one of the most prevalent micronutrient deficiencies. In Australia, while there are relatively few reports of B deficiency, responses to fertilization have been documented most states and low levels are seen on sandy soils on the slopes of the Great Dividing Range and in Western Australia. Lucerne and canola are the most common high demanding crops although there are a large number of fruit vegetable and field crops that should be assessed for potential B deficiency. B is also important in pine and eucalypt managed forests.

Boron in Plants
The primary role for B is in the cell walls, where it provides cross-links between polysaccharides to give structure to cell walls. Boron also plays roles in formation of sugar complexes for translocation within plants, and in the formation of proteins. Cell membrane function, nodule formation in legumes, flowering, and development of seed and fruit all depend on adequate B. Deficiency can reduce both yield and quality of crops. Flower initiation and pollen development also require adequate B (Bell 1997).

Of particular note with B is that it also has a relatively narrow range between deficiency and toxicity in plants. For example, B deficiency of canola can occur with soil B levels (Hot Water Soluble) of 0.15 to 0.5 mg B/kg while toxicity occurs at values greater than 3 mg B/kg. B toxicity typically occurs in low rainfall regions (<400 mm) where high levels of B occur naturally at depth in sodic soils. Low soil test B were seen on around 6% of samples in a survey across Australia, with lower values on Kurasols and Tenosols, and the highest values on Ferrosols and Calcarosols.

Boron in Soils
Agricultural soils range from 1 to 467 mg B/kg in total B concentration. The available forms, B(OH)₃ (boric acid) and B(OH)₄^- (borate) are usually mobile in the soil solution, but can be adsorbed to the common constituents of soil, including hydroxides of iron (Fe) and aluminum (Al), clay particles, and organic matter. Because of this, there are several factors that influence B availability in the soil:

· Organic matter is the most important source of B in the soil. In hot, dry weather, the rate of organic matter decomposition slows down especially near the soil surface. As a result, the plants can show a transient B deficiency that disappears when the rate of organic matter turnover increases. Alternatively, in cold weather, organic matter decomposition also slows, and low B release affects many brassica crops including canola and early-planted vegetables like cabbage and broccoli.
· Weather conditions: Periods of low transpiration may induce B deficiency. This may be due to either dry conditions or under water logging where stomata close or cold weather that restricts root activity and in the surface soil and can cause transient B deficiency. Fog or high humidity can also reduce transpiration and so inhibit B uptake. Symptoms may disappear as soon as the surface soil receives rainfall. Root growth resumes, but yield potential can be reduced because of the B shortage.
· Soil pH: Plant availability of B is greatest between pH_Ca 5.0 and 7.5. At higher pH values, B uptake is reduced, although many Australian alkaline soils have high initial B concentrations. Liming acid soils can lower B solubility due to sorbtion of B to Al hydroxides, but this is a temporary effect. Table 1 shows the effect of liming on B uptake by lucerne in a pot experiment. Liming to raise pH reduced B uptake but it was only the plants at the nil applied B that showed B deficiency. So, where soil B is low, liming may increase the risk of B deficiency. At subsequent harvests, the suppressive effect of the lime declined.
· Soil texture: Coarse-textured sandy soils, which are composed largely of quartz, are typically low in minerals that contain B. Plants growing on such soils commonly show B deficiencies.
· Leaching: Plant available B is mobile in the soil and is subject to leaching. Leaching of B from the root-zone is of greater concern on sandy soils and/or in areas or times of high rainfall

Table 1. The B concentration (mg/kg) in lucerne in a pot trial in response to B and raising soil pH approximately 6 weeks after treatment. HWS B was 0.28 mg/kg. Interaction LSD (p=0.05) = 9. (Sherrell 1983).

pHCa	Nil	0.23 kg B/ha	0.46 kg B/ha	0.91 kg B/ha	1.82 kg B/ha
5.1	12	33	44	57	85
5.6	13	31	38	49	60
6.2	9	26	35	39	69

Fertilizing with Boron
It is important that B fertilizers be evenly applied because of the narrow range between deficiency and toxicity. Diagnosing the need for B fertilization needs to consider the factors listed above controlling soil availability. Plant analysis and visual symptoms are often more useful as diagnostic tools than soil testing.

Boron fertilizer can be broadcast or banded into soil, or applied as a liquid foliar treatment. Broadcast application requires higher rates than banded or foliar applications. Soil application rates for the most responsive crops may be as high as 3 kg B/ha, and for medium responsive crops, 0.5 to 1.0 kg B/ha (Table 2). In-furrow applications of B if adjacent to seed can cause poor emergence. Common forms of fertilizer are shown in Table 2, with some derived directly from evaporate deposits. Ulexite is a mix of less soluble calcium and more soluble sodium borates often used in forestry applications. Most of the borate salts can be supplied either as powders or granules depending on the application method required. Soluble forms are usually preferred, except in sandy soils where newer low solubility B-phosphates can be used to reduce leaching out of the root-zone.

Foliar B can be applied when a deficiency has been diagnosed either by tissue test or visual symptoms. Application at flowering will give immediate responses, and in rapidly growing crops one or more applications may be required. Split foliar applications of 0.5 kg B/ha applied between stem elongation and flowering are effective, but care needs to be taken to keep concentrations to levels that will not injure leaves. This typically is at concentrations of 0.5% w/v. Foliar applications will have little residual activity.

Table 2. Responsiveness of crops to Boron.

Most responsive	Medium Response	Least responsive
Apple	Clovers	Bean
Brassica vegetables	Cotton	Pea
Canola	Maize	Sorghum
Celery	Lettuce	Soybean
Lucerne	Pear	Rice
Sunflower	Spinach	Winter cereals
Beet/Turnips	Tomato

Table 3. Common Boron fertilizers

Source	Formula	B %	Water solubility
Borax	Na2B4O7·10H2O	11	High
Boric acid	B(OH)3	17	High
Ulexite	NaCaB506(OH)6.5H2O	13	Slight
Colemanite	Ca2B8011.5H2O	20	Low
Sodium pentaborate	Na2B10O16·10H2O	18	High
Sodium tetraborate	Na2B4O7·5H2O	14-15	High
Sodium octaborate	Na2B8O13·4H2O	20-21	High
Boron phosphate	PBO4	10	Moderate/Very low*
Boron frits	Boric oxide glass	2-11	Very Low

*Solubility depends on conditions of manufacture (Abat et al. 2014).Boron Deficiency Symptoms
Although B is mobile in the soil, its mobility within the plant varies among species. Nutrient deficiencies tend to appear on the youngest leaves or growing points. In certain species (such as apples and almonds), B is mobile and moves throughout the plant.

The following B deficiency symptoms occur in specific crops:

· almond: new shoots do not develop. Brown and gummy nuts.
· apple: small, flattened or misshaped fruit, internal corking, cracking and russetting, dead terminal buds, brittle leaves, blossom blast.
· canola: red-brown youngest leaf, shortened cracked stems with cupped leaves and thickened margins. Interveinal necrotic spots and leaf dies from edges to vein.
· celery: crooked stem
· corn: narrow white to transparent lengthwise streaks on leaves, multiple but small and abnormal ears with very short silk, small tassels with some branches emerging dead, and small, shriveled anthers devoid of pollen.
· cotton: ringed or banded leaf petioles with dieback of terminal buds, causing rosetting effect at the top of the plant. Ruptured squares and thick, green leaves that stay green until frost and are difficult to defoliate.
· Lucerne: short internodes and stems, younger leaves turn red or yellow, death of terminal bud.
· Peanut: hollow heart

Boron toxicity symptoms
Toxic accumulation of B occurs in many low rainfall regions. Boron toxicity symptoms appear first on the edges and tip of older leaves and often appear later as the roots reach the toxic layer. Because B is mobile, extra irrigations can move B out of the root-zone, but in dryland conditions, avoidance is the only management option available. Soil mapping can assist in identifying subsoil limitations and then paddocks can be zoned, and selecting more tolerant crops or altering N management (Angus et al. 2004).

Prediction of B responses

· Soil tests can generally give reasonable prediction of B deficiency when tests are calibrated for soil groups and crop species. Both Hot Water Soluble and hot 0.01M CaCl2 extractant are used and correlate well with plant response. Depth of sampling is important and results should consider the accessibility of subsoil B to the crop when interpreting a topsoil test.
· Tissue tests are a reliable guide to plant B status, although like the soil tests, the results need to be interpreted in terms of soil moisture, root depth and anticipated future B supplies. Critical whole plant levels for canola range from 22 mg/kg (seedling) to 15 mg/kg (rosette). Critical values for cereals are around 4-5 mg/kg. Because B is relatively immobile, sampling the youngest fully expanded leaves may provide a more robust diagnosis than whole plant sampling.

Crop response to Boron
Crop species vary significantly in their responsiveness (Table 4). Canola has been identified with high B requirement, and seed concentrations are around ten times that for cereals. There is also considerable variation in response with species in terms efficiency of uptake (Stangoulis et al. 2000) and tolerance to toxicity (Kaur et al. 2006). Most legumes, as well as several fruits and vegetables, are highly responsive to B and also susceptible to B toxicity. Other vegetables show somewhat less response. Grains and grasses are generally less responsive to B. Crops vary in sensitivity to excess B, but those with high requirements do not always have high tolerance. For example, alfalfa and cabbage show only moderate tolerance to high B levels. The depth of sampling is important because B is mobile and while the topsoil maybe depleted, the lower soil layers may provide adequate B.

Table 4. Examples of crop yield response to application of Boron fertilizer.

Crop	Source	Rate	Time	Place	Yield Response	Reference
Canola	Na Borate	0.6 kg B/ha	Stem elongation	Foliar	39%	Myers et al. 1983
Canola	Boric Acid	2 kg B/ha	Sprayed at sowing	Soil	4 to 52%	Stangoulis et al. 2000.
Lucerne Seed	Na Octoborate	0.4-1.1 kg B/ha	After first cut	Foliar	+37%	Dordas, 2006
Lucerne	Na Tetraborate	3-4 kg B/ha	Annual	Soil	+46 to 62%	Haby et al. 1996.
Soybean	Na Octoborate	0.25-1.0 kg B/ha	V2 or R2	Foliar	0 to 130%	Ross et al. 2006

References
Abat et al. 2014. J. Plant Nutr. Soil Sci. DOI: 10.1002/jpln.2014.
Angus et al. 2004. Proc. 4^th International Crop Science Congress.
Bell, R.W. 1997. Plant and Soil, 149-168.
Dordas, C. 2006. Agron J 98:907-913
Haby, V.A. et al. 1998. Better Crops with Plant Food 82(1):22-23.
Kaur S. et al. 2006. Plant and Soil. 285:115-123.
Myers et al. 1983. Aust.J.Exp.Agric.Anim.Husb. 23, 172-177.
Ross, J.R. et al. 2006 Agron. J. 98:198-205.
Sherrell, C.G. 1983. New Zealand Journal of Agricultural Research, 26, 197-203.
Shorrocks, V.M. 1997. Plant and Soil, 193, 121-138.
Stangoulis J.C.R. et al. 2000. Plant and Soil 225:243-251.

Further reading
Brown, P.H. et al. 2002. Plant Biol. 4: 205-223.
Bell R.W. and Dell, B. 2008. Micronutrients for sustainable food, feed, fibre and bioenergy production. IFA, Paris. 175pp.