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IV. Boron Requirement of Plants
BORON IN SOILS AND CROPS
Stiles (1946) listed the name of the first worker to call definite attention to the favorable effect of boron on the growth of the species concerned. This was given whether the worker regarded boron as essential
for the species or not. Thus, Nakamura in 1903, reported increased
growth of peas and spinach as a result of adding boron to the soil but
i t was not until 1915 that M a d claimed the essential nature of boron
for plant growth.
1, Function of Boron in Plants
Warington (1923) first showed that meristematic activity was markedly affected in the broadbean and that both roots and stem tissues were
abnormal in the absence of boron. Since this work, there have been a
great number of studies on the function of boron in plants. Johnston
and Dore (1929) found t.hat plants grown in a boron deficient nutrient
solution showed four distinct types of injury: (1) death of the terminal
growing point of the stem; (2) breakdown of the conducting tissues of
the stem; (3) a characteristic brittleness of the stem and petiole and
(4)extremely poor growth of roots which develop a brownish unhealthy
color. The total sugars and starches were more abundant in the leaves
and stems of the boron deficient plants while a greater amount of benzene-insoluble matter was found in the leaves of normal plants and in
the stems of boron deficient plants.
Haas and Klotz (1931) concluded that boron is essential to cell division in the meristematic tissues and in the cambium. In the absence
of sufficient boron the cambium and portions of the phloem were observed
to disintegrate and gum up, some of which found its way to the exterior
through a split in the cortex. When there was any xylem disintegration,
the amount was small. A normal accumulation of carbohydrates in the
leaves of boron deficient, plants was observed and ascribed to the disintegration of the phloem with consequent interference with translocation. The addition of boron to the culture solution resulted in a reduction
in the total sugar content of the leaves and a restoration of the vigor
of the plant.
Shive (1941) believed there was considerable experimental evidence
that boron is an important factor in the processes involved in organic
synt.hesis. He found that plants grown in boron deficient regions yielded
strong positive tests for pectins and negative tests for fats. Lohnis
(1940), studying the influence of boron deficiency on the anthers of
several small grains, found the primary effect of boron deficiency to appear in the cell nucleus where division was inhibited in the early stages
of the boron deficiency. Working with alfalfa, Scripture and McHargue
(1943) found that soluble nitrogenous compounds and reducing sugars
were present in greater proportions in expressed sap from boron deficient
plants than in that from normal plants. Later (1945), working with
radishes they found that direct reducing sugars, sugar hydrolyzed by
invertase, and alcohol-insoluble carbohydrates were all present in excess
amounts in the tops of boron deficient plants. The rook of the radishes
contained less direct reducing sugars but more of the other carbohydrate
fractions than did the normal ones. They concluded that boron must
function in metabolism and translocation of carbohydrates but whether
directly or indirectly through its role in nitrogen metabolism remained
Smith (1944) found that in squash leaf cells approximately 50 per
cent of the boron was immobilized in the cell wall or intracellular substance. Along with the boron, he found 70 per cent of the calcium. He
suggests that boron is of importance in the cytoplasm and in the wall
but not in the chloroplast or vacuole.
From the evidence above it can be seen that boron is very import,ant
in cell division and is apparently a necessary component of the cell wall.
Boron also plays an important role in the synthesis of proteins in the
plant as shown by the fact that in its absence, nitrogen compounds and
sugars accumulate while meristematic tissues die. Further work is necessary before more definite roles can be ascribed.
2. Interrelations with Other Ekments
The relationship of boron to calcium in plant nutrition, as well as the
relationship between boron and numerous other elements, has been the
subject of much study. There have been many conflicting reports as to
the effect of boron on the other elements, particularly on t*he base elements. There have been cases where, because boron was the limiting
factor, the content of other elements in the plant have been increased.
When boron was adequately supplied, the amounts of the other elements
in the plant declined. Under conditions of boron toxicity, the normal
physiology of the plant can be so upset as to cause either abnormal
accumulations, or lowered amounts of both cations and anions.
I n other cases if an element such as phosphorus is limitsing, the boron
content of the plant might be high due to a stunting because of a lack
of phosphorus for normal growth. I n such a case, if the phosphorus
supply is adequate the amount of boron in the plant will probably decline
due to the greater growth of the plant when supplied with phosphorus.
The same relationship could hold true of any of the other nutrient elements.
When the effects of various bases are studied, it should be remembered that on a chemical equivalent basis plants tend to maintain R
BORON IN SOILS AND CROPS
constant amount of bases. Therefore, if the supply of potassium is
increased in a plant, the amount of calcium or other bases will probably
he lower. It should also be remembered that plants require a certain
balance of one nutrient element to another for normal growth and if
the balance is greatly upset the plant will be abnormal. Thus if the
calcium supply was increased three or four-fold above normal, the result
could be magnesium deficiency even though the amount of magnesium
present would be sufficient under normal conditions. It is important,
therefore, when studying the relationship of boron to some of the bases,
to keep this fact in mind.
I n nearly all experiments, it has been found that the amount of boron
in the plant tissue increases with the amount supplied to the plant
whether the plant is growing in soil or nutrient solution. This is true
even when the amounts of boron are supplied in toxic concentrations.
This build-up in the pIant can be as much as ten-fold or more, as shown
by Parks, et al. (1944).
The following are some of the factors that. should be taken into consideration when relationship between boron and the other nutritive elements are studied.
a. Calcium-Boron Ratios. .Much of the work on calcium-boron ratios
has already been discussed under Section II-2-b. The relationship betweeen calcium and boron apparently is a very real one and, as shown
by Reeve and Shive (1944), when plants have access t o increasing
amounts of calcium they require more boron to prevent deficiency. With
high amounts of calcium, plants are able to withstand larger amounts
of boron without it becoming toxic. They found t>hata t high boron levels
there is a marked decrease in both total and soluble boron in the plant
tissue with increase in the calcium concentrations in the nutrient solut.ion.
The calcium accumulation in the tissues is largely determined by the
calcium concentration in the growing medium and appears to be independent of boron. This is in line with observations by other workers.
Not only is a certain definite amount of boron needed with calcium in
the plant to build cell walls, as well as for the other functions of boron
in the plant, but it appears that excess of boron above t,hese needs combines with calcium to form compounds no longer toxic to the plant.
Because the symptoms of boron deficiency and of calcium deficiency
which appear in the growing point are very similar, i t appears logical
that these two elements are related in their function in plant growth.
Further evidence that their function is related was given by Smith
(1944), who found 50 per cent of the boron and 70 per cent of the calcium
immobilized in the cell wall or intracellular spaces.
K . C. BERGER
b. Potassium-Boron Ratios. The main work on potassium-boron
ratio has been done by Reeve and Shive (1944). Along with their work
on calcinm-boron ratios, they grew tomato plants in water culture solutions with five different potassium levels and four different boron levels.
They found that the external symptoms of boron toxicity a t high boron
levels and the deficiency symptoms a t low boron levels were progressively
accentuated with increasing potassium concentrations in the nutrient
solution. Increasing t.he potassium concentration had the effect of increasing the boron content of the plants especially a t the higher boron
levels. They found that calcium and potassium were similar in their
capacity to accentuate the symptoms of boron deficiency with increasing
concentrations of these cations in the nutrient solution, but they found
boron toxicity at the high boron levels decreased markedly with increasing concentrations of calcium but not with potassium. This might largely
be due to the constant ion effect in plants which is now well known,
whereby, on the equivalent basis, t.here tends to remain for any given
species a rather constant total amount of cations in the plant. When one
of the cations is increased in the plant it does so a t the expense of some
other. I n this case increasing the potassium content would have the
effect of reducing the amount of calcium in the plant. and thus the effect
of high potassium would be the same as that of low calcium. The results
seem to agree with this.
Another effect of potassium in the plant might, be that of increasing
the permeability of the roots. The fact that boron concentrations in the
plant are increased with increasing amount8 of potassium would seem to
If, when the potassium concentrations was increased, greater growth
resulted, boron deficiency symptoms would be accentuated a t low boron
levels because of t,he increased growth of the plant. This probably is a
factor in some cases in the field, where potassium deficiency and potassium responses often occur.
Chapman et al. (1940) suggested a reciprocal relationship of calcium
and potassium when they found that lemon plants grown in potassium
deficient cultures supplied with boron a t the rate of 1 p.p.m. developed
symptoms of boron toxicity. Normal lemon leaves contained 20 to 40
p.p.m. of boron in the dry matter but in these plants the boron concentration was nearly 200 p.p.m.
It appears that the relationship between potassium and boron in the
plant is of much less importance than that of calcium and boron. In
many cases probably the effect of potassium on boron in plants is an
indirect one, where adding potassium to the plant decreases the uptake
of calciiim, thus upsetking the calcium-boron ratio in the plant.
BORON IN SOILS AND CROPS
c. Nitrogen-Boron Relatiomhip. Because the region of the meristem
is one of the first to be affected when boron deficiency exists, it seems
likely that boron not only is an important factor in cell wall formation
but also that it might be tied up with protein formation in the protoplasm. There is considerable evidence in the literature to show that
when boron is deficient, there is an accumulation of nitrogen compounds
and simple carbohydrate compounds in the plant. This would indicate
an inability of the plant to complete protein synthesis in the absence
Schropp and Arene (1942) in a study of the nitrogen content and
metabolism in plants found more nitrogen and a smaller proportion of
protein-nit,rogen in boron deficient plants than in normal plants. Baumeister (1941) working with Phaseolus vulgaris in water cultures, with
and without boron, found that when the plants were grown with only a
small amount of calcium sulfate present, the nitrogen being supplied by
the cotyledons only, the plants accumulated excess nitrogen in shoots,
leaves, and roots, in comparison with the parallel series in which boron
was also supplied. When nitrogen was supplied to the nutrient solution,
disturbances in the nitrogen content and metabolism of boron deficient
plants were seen earlier in the stems and roots than in the leaves.
Scripture and McHargue (1943-1945), working wit,h alfalfa, radishes,
and spinach, found that soluble nitrogenous compounds and reducing
sugars were present in greater proportions in the expressed sap from
boron deficient alfalfa and radishes than in that from normal plants.
The radish roots from deficient plants contained less direct reducing
sugars but more of the other oarbohydrate fractions than did those from
normal plants. Further support for the theory t.hat boron is used in
protein formation is found in their work on, spinach. The proportion of
protein-nitrogen to alcohol-soluble-nitrogen in spinach leaves increased
with increasing boron supplies.
Working with nasturtiums, Briggs (1943) found a progressive decrease
in nitrate absorption in boron deficient plants as compared with that of
normal plants. Ammonia-nitrogen, soluble organic nitrogen, and carbohydrates accumulated in such plants. In plants showing boron toxicity,
the amount of protein nitrogen was increased and the amount of soluble
d . Other Elements. Parks et al. (1944) studied the relationship of
boron to a number of other elements. Their data show that differences
between the content of various elements as affected by the boron supply
could not be correlated with the type of ion (cation or anion), the valence
of the ions, or the total growth of the plant. This is probably due to the
K. C. BERQER
fact that most of their treatments contained toxic quantities of boron.
I n their paper, they list conflicting reports as to the effect of the boron
supply on the magnesium, phosphorus, and iron contents of various
plants. As has been previoiisly stated, many of these results have been
obtained due to abnormalities in tlie physiological functions of the plant
due to exheme deficiencies or excesses of boron in the plant, or have been
clue to the fact that some one of the nutrient elements has been limiting
plant growth, thus causing accumulation of other elements.
I n the normal physiological functions of the plant it appears to be
established that there is a direct relationship between calcium and boron
and between nitrogen and boron. Although other elements have been
considered to have an effect on boron in the plant it seems t,hat under
normal conditions their effect is mostly indirect by influencing the uptake
of either calcium or nitrogen.
3. Symptoms of Boron Deficiency
Visual symptoms of boron deficiency have been the subject of much
investigation and have been summarized recently in useful form by Dennis and O’Brien (1937), Dennis and Dennis (1943) , Dennis (1948) , and
McMurtrey (1948). One striking thing about boron deficiency is that
invariably it affects the terminal growth which indicates that boron is
not translocated in the plant., but is fixed in insoluble compounds, and
that it is needed in cell division. McMurtrey (1948) lists the visual
symptoms of a number of crops and in nearly all of them, the main
visual symptom of boron deficiency is that terminal growth ceases, internodes become shortened, and the plant in many cases acquires a rosetted
appearance. These symptoms are nearly alike in such widely divergent
species as apple, beet, and alfalfa. Of course, because the terminal
growth dies, flowers are often blasted and fruit and seeds frequently fail
Calcium deficiency symptoms in most plants are much like those of
boron deficiency in that the terminal growth is commonly affected first,
and yet are usually differentiated by different types of chlorosis of the
It is essential to remember that with boron, as with phosphorus and
other plant nutrient elements, deficiency may be present long before
visual deficiency symptoms occur, and increases in yield may be obtained
t.hrough the application of borax. This was shown to be true by Berger
and Truog (194413). They worked with red beets and sugar beets and
obtained statistically significant increases in yield in 5 out of 6 trials in
which boron deficiency symptoms were absent. I n only 1 of the 6 trials
were boron deficiency symptoms present when an increase in yield was
BORON I N SOILS A N D CROPS
obtained. Significant increases in yield were only obtained when tlie soil
had less than 1 p.p.m. of available boron.
As is true with other deficiency symptoms in any given plant, a number of different visual symptoms of boron deficiency occur, depending
upon the severity of deficiency. The first of these symptoms is internal,
as shown by Walker (1944), who found that when boron deficiency of
garden beet and cabbage is brought about gradually in sand cultures the
progressive effects on the histology of the plant can be studied in successive tertiary rings and that long before any external symptoms develop,
various profound histological changes occur.
Probably the next stage of boron deficiency in most species is a
necrosis of terminal gr0wt.h and shortening of the apical internodes. As
the deficiency becomes more severe chlorosis often appears, flowers blast
and fruit fails to form. Finally the entire plant dies, Thus there are
a whole series of boron deficiency symptoms with the mildest manifested
only by reduced growth and certain chemical changes, the second st.age
by cellular changes, and next a series of microscopic changes leading to
premature death of the entire plant.
It is interesting to know that some boron deficiency symptoms were
thought to be diseases and were named and described as such long before
their causes were understood. Thus, in 1924, Foster and Weber described a nonparasitic disease of celery which they named “cracked stem”
and ascribed as its probable cause a combination of climatic factors, use
of an unbalanced fertilizer, and excessive use of lime. It was not until
1935 that Purvis and Ruprecht t,raced the cause of these symptoms to
a deficiency of boron. It is interesting, however, that Foster and Weber
(1924) exactly described the conditions which caused the boron deficiency. In some cases a great many years elapsed between the recognition of the disease and the final discovery that it is caused by boron
deficiency. This is what Atwater (1941) refers to as the “ancient history
of boron deficiency symptoms.’) A considerable number of these socalled nonparasitic diseases associated with boron deficiehcy have been
reported and described. Among them are top sickness of tobacco, heart
rot of beet, cork disease of apple, brown rot of cauliflower, cracked stem
of celery, “raan” of swedes, and many others.
Along with boron deficiency symptoms, a great deal of work has been
done on boron toxicity. This was summarized recently by Eaton (1944)
who has grouped plants into t.hree classes : sensitive, semi-tolerant, and
tolerant. It is interesting to note from tables in this publication that
sensitive plants, in general, contain high amounts of boron when grown
in a solution containing 5 p.p.m. of boron. Semi-tolerant plants, grown
in the same solution, contain an intermediate amount of boron, while
K . C. BERQER
the lowest amount of boron is found, in general, in the tolerant plants.
For the most part tolerant plants not only have a high boron requirement, but also a high calcium requirement.
4. Boron Requirements of Plants
Although there has been a tremendous amount of work done on boron
fertilization of crops in the field, it is still difficult t o classify many orops
according to their boron requirement. I n order to do this properly, it is
necessary to have knowledge of the boron content of the crop when
grown in a normal soil, the response obtained with the crop on boron
deficient soils, and the extent of the deficiency found in various parts
of the country. When the boron content of the growing medium is high,
there is no relationship between boron needs and boron content of the
plant. When the boron content of the medium is similar to that in which
the plants normally grow however, the boron content of plants grown in
such a medium is an indication of t.he boron need of the crop. Crops
with a low boron content will have a low requirement for boron, and in
general, the crops with a high content of boron will have a relatively
Amounts of Boron Found in Topa of Plants Grown in Two Soils
of dry matter
Data of Bertrand and De Weals (1936).
Data of Bertrand and Silherstsin (1937)
of dry matter
BORON IN SOILS AND CROPS
high requirement for boron. In Table I are given the boron content of
plants grown in one soil by Bertrand and De Waals (1936) and in another Foil by Bertrand and Silberstein (1937). As can be seen, the boron
content vrtried from 2.3 p.p.m. in barley to 94.7 p.p.m. in poppy. The
results given in this table are representative of boron contents found in
plants grown in humid region soils in various other parts of the world.
In Table I1 an attempt is made to arrange plants according t o their
boron requirement. The information from which this listing was made
Boron Requirement of Some Common Field and Vegetable Crop Plants
Probable available boron content of soils (p.p.m.1 required for
Plants with high
> 0.5 p.p.m.
Plants with medium
0.1 to 0.5 p.p.m.
Plants with low
was obtained not only from the boron contents of the plants but also
from the field experience of a great number of workers in various parts
of the world. It is impossible in a paper of this kind to list all the
sources. There will probably be changes in this classification, not only
from one category to another, but also by addition and deletion as more
information is obtained. An attempt is also made to give an approxi-
K. C. BEBGEB
mation of the available boron content of soils required for the optimum
growth of the various crops. It is realized of course that this will vary
considerably from area to area, will change with soil and weather conditions, and that it is only ti rough approximation. It should be remembered that in a soil producing half of a normal crop, because of a lack
of water or plant food elements, the amount of boron needed for normal
growth will also be only about one-half of the normal. Furthermore, to
obtain maximum growth of the plants, it will be necessary to have more
boron in a soil high in available calcium than in one low in calcium.
There is also a difference in ability of various plants to feed on boron
which is strikingly shown up in the difference between red beets and
sugar beets. When grown in the same soil both of them have about the
same content of boron in the leaves although it is necessary to have more
available boron in the soil for red beets to prevent boron deficiency than
it is for sugar beets simply because of the smaller root system of the
When plants become deficient in boron, they contain a certain amount
of the element. Death of the plant will occur before this amount will be
lowered. There has probably been more work done on alfalfa than on
any other crop to determine the amounts of boron needed for normal
growth and methods for detecting boron deficiency. Rogers (1947) has
Critical value for the boron content of alfalfa plants as reported
by various investigators '
Amount of boron reported in deficient
plants or plants with need for boron
McLarty, Wilcox, and Woodhridgr
Rprger and Truog
Haddock and Vandrcavryr
Dregne and Powers
Jordan and Powers
Dunklee and Midgley
Brown, Munsell, and King
Whetstone, Robinson, and Byws
Dawson and Gustafson
Munsell and Brown
7.0 to 11.5 (normal plants 12.0
to 22.5 p.p.rn. B)
17.0 (Also 17.0 with no response
13.0 to 17.0 response to boron
12 to 19
23 in leaves, yellows
BORON IN SOILS AND CROPS
listed the amount of boron found by various investigators in deficient
plants or in plants with a need for boron. These are given in Table I11
and it can be seen that there is considerable discrepancy. Rogers states
that, if alfalfa, crimson clover, or burr clover contain less than 10 p.p.m.,
boron response to additions of borax is indicated on the coarse textured
red and yellow podzolic soils of Alabama. The same limiting amount
has been found to be true with alfalfa in Washington, Oregon and Wisconsin.
There undoubtedly is a certain critical minimum boron content in
plants as there is a certain maximum boron content above which toxicity
symptoms appear. As stated previously, these symptoms vary with
species and various growing conditions. With the exception of alfalfa,
little work has been done on this problem.
Although borax has been in use for about 4 centuries, it was only
about 90 years ago when it was first discovered in plants and only a
little over 30 years ago when it was first claimed to be essential for
plant growth. The largest share of the work on boron has been accomplished in the last 15 years. Very satisfactory chemical methods are now
available for the determinat.ion of boron in plant tissues and for the
determination of total and available boron in soils.
The amount of available boron in soils can also be determined by
growing plants in soils and comparing the growth with those grown in
nutrient cultures with known amounts of boron. This method is fairly
accurate but is slow, requiring two to eight weeks for completion of a test.
Boron availability and fixation have been the subject of much recent
work. Because available boron is lost from soil most rapidly by leaching, problems of boron deficiency are largely found in soils of the humid
regions. I n arid regions, soils not leached are usually high in available
boron and irrigation waters often contain considerable quantities of this
The available soil boron is apparently in two foriris, organic and
inorganic, ant1 these are in equilibriiim with the fixed forins of boron.
Boron is supplied to the soil largely from tourmaline but the rate of
decomposition is slow. In heavily cropped soils, boron is often depleted
through crop removal.
Free calcium in soils tends to hold boron from leaching and in alkaline soils, in the presence of free calcium, boron is fixed in a temporarily
unavailable form, partially by organic matter : m l partially by soil
minerals, the activity of which predominates in the clay fraction.