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of the United States (Sparr, 1970), almost all provinces of Canada, and many

other countries of the world. Some of the most severe disorders caused by a

deficiency of B include brown-heart of rutabaga or turnips (Brassica napobrassica, Mill), cracked stem of celery (Apiurn graveolens L.), heart rot of beets

(Bera vulgaris L.), brown-heart of cauliflower (Brassica oleracea var. botryris L.), and internal brown-spot of sweet potatoes (Ipornoea batatus (L.)

Lam .).

Boron is unique among the essential mineral nutrients because it is the only

element that is normally present in soil solution as a non-ionized molecule over

the pH range suitable for plant growth, as Oertli and Grgurevic (1975) have

shown. According to their results, boric acid is the form of B that plant roots

absorb most efficiently. Alt and Schwarz (1973) hypothesized that B is absorbed

as the molecule and that B, at least in high supply, is passively distributed with

the transpiration stream. Vlamis and Williams (1970) found that B did not

accumulate in barley roots in response to changes in temperature or external B

concentration. They suggested that boric acid is present in the medium largely in

molecular form (not the ionized form) and does not participate in the metabolic

activity associated with the ion uptake and accumulation in roots.

Soils formed from marine sediments are likely to contain more B than those

formed from igneous rocks; soils on the average have a higher content of B than

rocks (Norrish, 1975). The original source of B in most well-drained soils is

tourmaline. Tourmaline (3-4% B) is present in soils formed from acid rocks and

metamorphosed sediments. Boron can substitute for tetrahedrally coordinated Si

in some minerals. It is likely that much of the B in rocks and soils is dispersed in

the silicate minerals in this way and would be available only after long periods of

weathering (Norrish, 1975). Most of the B in soil that is available to plants is

derived from sediments or plant material (Bowen, 1977).

Because of its non-ionic nature, once B is released from soil minerals it can be

leached from the soil fairly rapidly. This explains why soils in high rainfall areas

are often deficient in B. Results of Gupta and Cutcliffe (1978) on the podzol soils

of eastern Canada have shown that up to 62% of applied B was not recovered

from the surface 15 cm of soil 5 months after broadcast application. The amount

recovered being referred to is that fraction which was recovered by hot-water

extraction of the soil. On the other hand, the availability of B also decreases

sharply under drought conditions. This has been attributed partly to the reduced

number of microorganisms that can release B from the parent materials (Bowen,

1977). Also, moisture is not available to dissolve B from tourmaline. Lack of soil

moisture reduces the mobility of B, thus restricting its uptake by plant roots via

mass flow mechanism.

The total B content of most soils varies from 20 to 200 ppm (Berger and Pratt,

1963). Gupta (1968), working on a number of soils from eastern Canada, found

that total B ranged from 45 to 124 ppm, whereas hot-water-soluble (hws) B

ranged from 0.38 to 4.67 ppm. This indicated that only a small fraction of total B



occurred in an available form. Generally, less than 5% of total B is found in an

available form, but one report by Kick (1963) showed that hws B averaged 15%

of the total B in some Egyptian soils. Very little is known about the mineral

forms of B in soils (Lindsay, 1972). The adsorption of B on oxides of Fe and A1

is believed to be an important mechanism governing B solubility in soils and

clays (Sims and Bingham, 1968).

The purpose of this chapter is to review and report recent information on the B

nutrition of agricultural crops in the light of factors such as availability of soil B

to plants and factors affecting it, B-containing fertilizers and methods of application, symptoms and levels of B deficiency and toxicity in plants, and the

physiological role of B in plants.

11. Boron-Containing Fertilizers

Sodium salts are the most common forms of B fertilizers. Some of the

commonly used forms with their chemical formulas and percentages of B as

described by Diamond (1972), Chesnin (1972), and Morrill et al. (1977) are

given in Table I. Another recently available source of B is ulexite

(NaCaB,O,* 8H20) containing 49.6% B,03. Commercially available ulexite

contains 30% B,O, (9.43%B) and is considered to release B slowly as compared

with other sources of B. Other kinds of B-containing fertilizers include borated

gypsum, calcium carbonate, superphosphate, calcium nitrate, and various mixed

fertilizers (Berger and Pratt, 1963). Additional sources include farmyard

manure, sewage sludge, compost, and similar materials. The percentage of B in

these sources depends on the origin of the materials present in the given source.

The application rates of B vary from 0.3 kg/ha for sensitive crops such as

beans (Phaseolus spp.), to 3 kg/ha for B-tolerant crops such as rutabaga and


Percentages of Boron and Chemical Formulas of Boron Soure&

Boron source


Boric acid

Boron kits (contained in a

moderately soluble glass)

Sodium tetraborate

Borate-46, Agribor.



Sodium pentaborate

Solubor (partially dehydrated)

Chemical formula

B (%)


Na2B 4O7 . 10H 2O


Na,B 4 . XH 2O







Na2B,@,,. lOH@





20-2 1



alfalfa (Medicugo sutivu L.), and they differ according to soil types. Crops

grown on peats generally do not show toxicity at the rates that might produce

toxicity on mineral soils. For example, no B toxicity was recorded in sweet corn

grown on a peat soil even when the hws B level was as high as 10 ppm (Prasad

and Byme, 1975).

The method of application is important in determining the amounts of B to be

applied. The rates of applied B for vegetable crops as summarized by Mortvedt

(1974) are 0.5-3 kg/ha broadcast, 0.5-1 kg/ha banded, and 0.1-0.5 kgha

applied as a foliar spray. Vegetables such as rutabaga are very sensitive to a

lack of B and require more B than other vegetables to control B deficiency.

Gupta and Cutcliffe (1978) stated that this crop may require up to 4 kg B/ha

broadcast and 2 kg B/ha applied banded or as foliar spray on some podzol soils

of eastern Canada to overcome factors such as high soil pH, improper mixing,

and uneven application. In applying B-containing fertilizers, care must be taken

to apply them uniformly and to avoid excessive rates, because the range between

B deficiency and B toxicity is very narrow. Boron apparently does not react with

components of mixed fertilizers, and its availability to plants is related to its

distribution and retention in the soil (Mortvedt and Giordano, 1970). The

mobility of applied B in soils is probably greater than that of the other micronutrients .

The three most common methods of B application are broadcast, banded, or

applied as a foliar spray or dust. In the first two methods of application, an

appropriate quantity of the B source is mixed with the bulk N-P-K fertilizer and

applied to the soil. Boron has been applied coated on concentrated superphosphate and as a fine granular material mixed with individual plot row fertilizer

treatments (Mom11 et ul., 1977). In many regions, B-containing fertilizers are

sold as 0.2, 0.3, or 0.4 B, which means that the bulk NPK fertilizer contains

0.2%, 0.3%, or 0.4% actual B by weight.

For foliar sprays, Solubor and boric acid are most commonly used. The number

of sprays used varies from crop to crop and from one region to another. Solubor

is also an ideal source of B for addition to liquid fertilizers. It is a specially

developed product for speedy and economical correction of B deficiency in

fruits, vegetables, and other crops (Turner, 1975). A finely divided powder, it

may also be applied directly to plant foliage as a dust.

A detailed discussion on the effects of methods of B application on the plant

uptake of B and control of B deficiency will be presented in a subsequent section.

111. Methods of Determining Boron in Plants and Soils

Numerous color reagents are available for determining B in aqueous extracts.

Reagents such as chromotropic acid, which form intensely colored complexes

with boric acid in aqueous solution, have been developed during recent years.



However, difficulties are experienced in automating this method because both

the reagent and the borate-chromotropate complex are sensitive to light (James

and King, 1967). Chromogenic reagents, such as quinalizarin and carminic acid,

are specific and sensitive for determining B. But their use in automated procedures appears to be limited because they must be used in a concentrated H2S04

medium (Willis, 1970; Lionnel, 1970). The curcumin method has been modified

and was found to be rapid and simple (Fiala, 1973). However, reagents such as

curcumin suffer from the disadvantage that the values for B determined by such

methods vary with the amount and salt content of the material analyzed (Williams

and Vlamis, 1970). Other spectrophotometric methods are based on the conversion of sample B into fluoroborate (BF;) in an H,S04-HF system, which is then

extracted as a colored complex with Azure-C (or methylene blue) into dichloroethane and determined spectrophotometrically (Weir, 1970).

Procedures other than colorimetric include the determination of B by means of

spectrographic and atomic absorption spectrophotometric methods. However,

these methods are not as sensitive to B as the color reagents, and therefore their

use has been limited.

A new color-developing reagent, azomethine-H, originally used by Russian

workers for determining B in organic compounds, was first used by Basson el al.

(1969) for determining B in plant materials. Since that time the method has

achieved prominence for determiring B in soil, compost, and manure (Wolf,

1971); it will be dealt with in detail in this section.


Dry and wet ashing are the most common methods for extracting B from

plants. The carmine method of Hatcher and Wilcox (1950), also used with some

modifications by Gupta (1967a), and the curcumin method of Williams and

Vlamis (1970) have been the most common methods for determining B in plant

extracts during the past 10 to 20 years. However, with the advent of the

azomethine-H method in the late sixties this color reagent has become very

popular. Wolf (1971) extended its application in the determination of B from

compost and manures using a colorimetric technique. Recently, J. A. Smith and

D. A. Tel, University of Guelph, Ontario, Canada, have automated the

azomethine-H colorimetric method, using the technique developed by Basson et

al. (1969). These scientists made some modifications by adding 4 g of NaOH per

liter of EDTA reagent to give a pH 4.9 to the waste coming from the AutoAnalyzer system. The only interferences with this method are due to the presence of Al, Cu, and Fe in the plant extracts. Such interference is easily overcome

by use of a 0.25 M EDTA (disodium salt) solution (Basson et al., 1969). Sippola

and Ervio (1977) reported that the recoveries of added B ranged from 94 to 108%

in plant extracts, when determined by the azomethine-H method using spec-

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