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CHAPTER 7. BORON NUTRITION OF CROPS
UMESH C. GUFTA
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.)
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
BORON NUTRITION OF CROPS
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 kits (contained in a
moderately soluble glass)
Solubor (partially dehydrated)
Na2B 4O7 . 10H 2O
Na,B 4 . XH 2O
UMESH C. GUFTA
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.
BORON NUTRITION OF CROPS
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.
A . BORON IN PLANTS
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-