FOCUS
J. Hortl. Sci.
Vol. 4 (1): 1-27, 2009
INTRODUCTION
In the recent past, there has been massive
investment in horticulture both in public and private sectors
with the expectation that it would increase profitability of
farmers, besides enhancing employment opportunities for
the rural poor, while simultaneously providing consumers
with good quality products. But, the above expectations
remain largely unfulfilled due to several research gaps.
Effective use of micronutrients in horticulture is one such
research gap. Micronutrients can tremendously boost
horticultural crop yield and improve quality and post-harvest
life of horticultural produce. The purpose of this article is to
highlight areas where the potential of micronutrients has
not been fully realized.
According to Stout (1962), “If plants are considered
as biological machines, their bodies are constructed from
macro-elements, their working parts consist of proteins and
enzymes revolving about N atoms and the
‘MICRONUTRIENTS’ provide the special lubricants
required for a variety of energy transfer mechanisms within
the plants”. This statement from a scientist who was
Importance of micronutrients in the changing horticultural scenario in India
M. Edward Raja
Division of Soil Science and Agricultural Chemistry
Indian Institute of Horticultural Research
Bangalore -560 089, India
E-mail: medward@iihr.ernet.in
ABSTRACT
Sustenance and well-being of humankind are linked to the stocks of essential nutrients in the bio-geosphere and
the capacity for cycling and manipulation. Micronutrients play a major role in crop production due to their essentiality
in plant metabolism and adverse effects that manifest due to their deficiency. Besides affecting plant growth,
micronutrients also play a major role in disease resistance in cultivated crop species. A hitherto lesser-understood
phenomenon is their role in determining quality and the post harvest life of harvested produce. In the Indian context,
this situation has become alarming due to the widespread occurrence of micronutrient imbalance throughout the
country. Though soil application of soluble forms of micronutrients has been widely practiced in the past, it calls for
introspection, considering the nature of occurrence of micronutrient related maladies. Novel approaches include
application of crop-specific foliar formulations of micronutrients, application of chelated forms of micronutrients
and the genetic biofortification of crops. In view of the importance of micronutrients in human diet, it is felt that
biofortification of horticultural crops will play a definite and major role in addressing nutritional security of the
nation in the coming years.
Keywords: Micronutrient, horticultural crops, deficiency, foliar nutrition, organic farming
involved in identification of Mo as essential micronutrient,
succinctly portrays the importance of micronutrients in plant
metabolism.
Micronutrients assume significance in horticultural
crop production due to their ability to:
● Improve quality, size, colour, taste and earliness, thereby
enhancing their market appeal
● Improve input use efficiency of NPK fertilizers and
water
● Provide disease resistance, thereby reducing
dependence on plant protection chemicals
● Increase post-harvest/shelf life of horticultural produce
thereby avoiding wastage
● Prevent physiological disorders and increase marketable
yield
● Enhance nutritional security by biofortification
In the 20th Century, revolution in crop yield increase
began with the discovery of micronutrients starting with iron
(Fe) in 1868, and ending with molybdenum (Mo) in1938.
This led to a paradigm shift from “scientific discovery” to
2
“scientific management”, which included three scientific
components to increase productivity, viz.,
◆ Genetic components (improvement in heterosis, disease
and pest resistance)
◆ Physiological components (better photosynthetic
efficiency, decreasing photorespiration, etc.)
◆ Management components (precision in fertility, avoiding
nutrient deficiency or toxicity, improvement in organic
matter status, etc.) and appropriate use of information
on climate, soil, water and specific characteristics of
cultivars, etc.
In the crop production system, there are about 16
non-controllable, limiting factors (light intensity, day-night
temperatures, etc.) and around 40 controllable, limiting
factors (soil-available NPK, micronutrients, soil pH, organic
matter, etc). Limiting factors translate to inputs. Some inputs
have a cost component like, nutrients and compost while
some do not, like, timeliness of operation, crop rotation to
avoid allelopathy-related problems, etc. Judicious
management of controllable and non-controllable factors is
necessary for successful crop production. Controllable
stresses are of two types: Liebigs type and Mitchserlich
type. In the former, unless a limiting factor is corrected, no
response to other inputs will be seen (eg., soil acidity, soil
salinity and nitrogen deficiency). But, in the Mitchserlich
type, limiting factors do not hinder correction of other
factors.
Micronutrients in crop production
In the early stages, micronutrient disorders were
described as diseases (Stiles, 1946). Subsequently, their
essentiality as nutrients was confirmed and great strides
were made in horticultural crop production by the use of
micronutrients. Heart rot of root vegetables in Europe was
cured by B application; “Pecan rosette” of Pecan trees was
cured by Zn in Florida, and, Mottle leaf of Citrus by Zn, and
Exanthema of Citrus in California and Australia by Cu. In
Australian soils, Anderson (1956) proved the essentiality of
Mo for N
2
fixation and increased clover yield from 1 to 5 t/
ha by addition of 30 g of the micronutrient per hectare. This
effect equalled the effect of a thousand kilograms of lime
application, since, lime releases soil Mo. Stout (1962)
wondered at the power of a tiny amount of Mo. By comparing
it with uranium, he observed that “a gram of Mo may harness
more energy by greater conversion of sunlight into plant
materials than can be obtained from a gram of uranium’.
We, in India, are unable to replicate the dramatic response
to micronutrients observed in Australia. The first important
reason is the soil wealth of India. A majority of soils do not
exhibit extremes in important physical and chemical
properties like pH, texture, water-holding capacity, organic
matter, NPK fertility and micronutrients. Another reason is
that we do not have vast expanses of B deficient soils similar
to those found in Eastern and Southern China, nor do we
have tracts of Fe deficient soils as occur in Australia, Spain
and Italy. Micronutrient deficiencies in India, by themselves,
do not restrict yield drastically but do so by acting additively
with other stresses, reducing yield substantially.
Micronutrient scenario in India
About 40-55% of Indian soils are moderately
deficient in Zn, while 25-30% are deficient in B. Deficiency
of other micronutrients occurs under 15% of soils (Takkar
and Kaur, 1984). These deficiencies/limitations by
themselves do reduce yield significantly but, combined with
2 or 3 of the other 40 controllable yield-limiting factors/
stresses, these act additively and reduce yield substantially.
In the Indian scenario, micronutrient deficiencies are of the
Mitscherlich type. Almost all micronutrient deficiencies or
toxicities in India fall in the mild to moderate category, with
exception of B deficiency in mango and cauliflower in
Konkan and Chota Nagpur regions, respectively. Since
skilled manpower and infrastructure to identify the
micronutrient disorders/toxicities especially at hidden hunger
stage itself by leaf/soil analysis are limiting in India, the
damage done to Indian horticulture is enormous.
Unfortunately, this is not fully recognized by decision makers
and scientists. As 80-90% of Indian soils are deficient in
nitrogen and phosphorus, their deficiencies are visible in
terms of leaf colour, size, growth-habit, flowering and yield.
Correction of these disorders is therefore more visibly
convincing. But, 70-80% of micronutrient disorders in
horticultural crops occur as hidden hunger. Leaf and soil
analysis alone can detect it at the right stage. In a country
of around 2 to 3 million farm-holdings with horticulture as
the main enterprise, it is next to impossible to carry out leaf
or soil analysis of micronutrients to detect hidden hunger.
This is another reason why we do not take advantage of
micronutrient correction.
The changing horticultural scenario
In 1860, the air and water systems were so pure
in the world that it was necessary to add chlorine in the
form of sodium chloride for healthy growth of plants.
Whereas, by 1954, purification of air and water became a
Herculean task, to prove Chloride as an essential
micronutrient by T.C. Broyer. At present, chloride content
J. Hortl. Sci.
Vol. 4 (1): 1-27, 2009
Edward Raja
3
has reached “toxic” levels from being “deficient”. There is
a tremendous change in yield-potential of crops, and soils
health and its nutrient supplying potential. Hence, farmers
need to be made aware of this changed scenario. Increasing
the density of banana plants from 2500 to 4400 plants/ha,
and mango from 100 plants/ha to 250 plants/ha (with use of
dwarfing rootstocks and hormone sprays for regular-bearing)
has resulted in severe depletion of soil nutrients. In India,
traditional tomato varieties with yield potential of 30t/ha and
F1 hybrids with a potential of 150t/ha, are being grown in
the some soil type. With the help of fertigation, cropping
intensity has increased from 100% to 300% in several parts
of the country. The quantity of nutrients removed and the
rate, at which these are removed, are vastly different.
Physical, chemical and biological health of soil was not a
major problem prior to “Green Revolution” of 1960’s,
whereas, in the present horticultural scenario of heavy NPK
fertilizer use, fertigation and precision-farming, soil health
and balanced nutrition has become a casualty. There is 30
to 40% decline in organic matter, with adverse effect on
micronutrient availability. Decline in availability of organic
manures due to greater use of inorganic fertilizer, has made
micronutrient supply precarious. Replacing micronutrients
that have been removed, or, increasing organic matter to
make native nutrients available, has not received sufficient
attention. Need-based input management of fertilizers,
pesticides and water is more of an option than a necessary
practice by farmers of the country owing to the poor
dissemination of information generated in research. The
widespread micronutrient disorders are believed to be a
reason for stagnation in agricultural productivity.
How to get “Macro” effect out of “Micronutrients” ?
a. Identifying and eliminating Liebig stresses : Liebig
Law of Minimum states that only an increase in the factor
most-limiting will result in an increase in yield. Otherwise,
the inputs are wasted. Moisture stress, salinity, soil acidity,
extreme deficiencies of NPK, if left uncorrected, cannot
result in a response to micronutrients. Overcoming soil-
salinity in grape by using salt-resistant rootstock ‘Dogridge’
paved the way for response to other inputs in horticultural
practices in Maharashtra.
b. Enhancing response to micronutrients by the Law
of Maximum : Since micronutrient disorders in India are
predominately of the Mitschertich type, correcting another
stress is not a pre-requisite for obtaining response from
micronutrient application. This law states that the largest
net response to an input comes when there are no other
limiting factors. The magnitude of response (to
micronutrients) will increase as more and more limiting
factors (abiotic and biotic stress) are corrected. The
corollary to this law is that the attained yield is greater than
the sum of individual parts because various parts interact to
multiply the value of others.
c. Inter-disciplinary approach, a must : Only by following
an interdisciplinary approach, we can maximize returns from
micronutrient application. Identification and simultaneous
correction of other stresses, along with micronutrient stress,
can give a highly significant, profitable and visible response.
Hence, the present practice of evaluating micronutrient
response by applying it alone will limit magnitude of the
response. A blueprint approach of identifying and correcting
all possible limiting factors including micronutrients has to
be done. In India, micronutrients have been so far used for
increasing only crop yield, while, other quality parameters
like colour, size, and firmness are seldom taken into
consideration. Another important area where micronutrients
can play an important role is disease resistance, since they
function as enzyme activators and play an important role in
lignin biosynthesis and other diseases resistance mechanisms.
Predominant micronutrient disorders and their
management in horticultural crops
Though deficiencies of micronutrients were initially
referred to as “diseases” in fruit crops, that lead has been
lost. A non-exhaustive list of common micronutrient
disorders that are observed in horticultural crops is furnished
in Table1. Apart from handling sporadically-visible
deficiencies, a systematic research in this area is only a
recent development. This paper highlights the intricacies of
micronutrients like B, Fe and Zn, which have a great potential
in all areas of horticultural crop production mentioned earlier.
BORON (B)
B nutrition in horticulture crops
B deficiency and response to it have been recorded
in 132 crops in more than 80 countries over the last 60 years.
It is estimated that over 15 million ha worldwide are annually
fertilized with B. It is through field bean, a vegetable-cum-
pulse (Vicia faba) that essentiality of B was proved. The
fact that B is needed for successful fertilization is of critical
importance. Though monocots need less B than dicots, they
also suffer from B deficiency due to low B at seed set.
Since B is the only micronutrient that affects all components
of horticulture (yield, quality, post-harvest life, disease
resistance and use-efficiency of other inputs), it is to be
Micronutrients in horticultural crops
J. Hortl. Sci.
Vol. 4 (1): 1-27, 2009
4
given highest importance, to derive maximum benefit. A
number of soil and environmental factors affect boron uptake
horticultural crops. Knowledge of these will improve
assessment of B deficiency and toxicity under various
conditions.
Chemistry of boron availability
B is mobile in soil and immobile in plants. It is the
only micronutrient lost to leaching. When B is released from
soil minerals, or is mineralized from organic matter or added
to soils through irrigation water / foliar application, part of it
remains in the soil solution, while, part is adsorbed by soil
particles. Minerals that contain B are either very insoluble
(tourmaline) or very soluble (hydrated boron minerals).
These do not usually determine solubility of B in the soil
solution, which is controlled mainly by boron adsorption
reactions. Equilibrium exists between soil solution and the
adsorbed B (Russell, 1973). Since plants, including papaya,
obtain B from the soil solution and the adsorbed pool of B
acts as a buffer against sudden changes in level of B in the
soil solution, it is important to know how boron is distributed
between the solid and liquid phases of soil. Factors affecting
the amount of B adsorbed by soils, and, availability of boron
in soils include: pH, soil texture, soil moisture, temperature
and management practices such as liming.
Parent material
In general, soils derived from igneous rocks and
soils in tropical and temperate regions of the world, have
much lower B content than soils derived from sedimentary
rocks, or those in arid or semi-arid regions. Soils of marine
or marine shale origin are usually high in B. Low B content
can be expected in soils derived from acid granite and other
igneous rock, fresh-water sedimentary deposits and in
coarse-textured soils low in organic matter (Liu et al,1983).
Plant availability of B is also reduced in soils derived from
volcanic ash and soils rich in aluminium oxides (Lebeder,
1968).
Soil reaction (pH)
Soil reaction is one of the most important factors
affecting availability of B in soils and its uptake. When the
soil solution has high pH, B becomes less available to plants.
Therefore, applying lime to acid soils can sometimes result
in B deficiency symptoms in plants. The level of soluble B
in soils has close correlation with pH of the soil solution
(Berger and Troug, 1945). B uptake by plants growing in
soil with the same water-soluble B content was greater when
pH of the soil solution was lower (Wear and Patterson, 1962).
Boron adsorption from soils increased when pH rose to the
range of 3-9.
Soil texture and clay minerals
Coarse-textured soils often contain less available
B than fine-textured soils. For this reason, B deficiency often
occurs in sandy soil (Fleming, 1980; Gupta, 1983). The level
of native B is closely related to clay content of the soil
(Elrashidi & O’ Connor, 1982). At the same water-soluble
B content, B uptake was highest in plants growing in the
soil with the coarsest texture (Wear & Patterson, 1962). It
increased as the clay content increased. Of the clay types
commonly found in soil, illite adsorbed more B than either
kaolinite or montmorillonite. Kaolinite in acid red soils
absorbed the least. It was found that B adsorption was
greater for Fe and Al coated kaolinite or montmorillonite
than for uncoated clays. They concluded that hydroxy Fe
Table 1. Relative sensitivity of selected horticultural crops to micronutrient deficiencies
Crop Sensitivity to micronutrient deficiency
B Cu Fe Mn M o Zn
Bean Low Low Medium Medium Medium Low
Broccoli Medium Medium High Medium High —-
Cabbage Medium Medium Medium Medium Medium —-
Carrot Medium Medium ——- Medium Low Low
Cauliflower High Medium High Medium High —-
Celery High Medium —— Medium Low —-
Cucumber Low Medium —— Medium —— —
Lettuce Medium High —— High High Medium
Onion Low High —— High High Medium
Pea Low Low —— High Medium Low
Radish Medium Medium —— High Medium —-
Spinach Medium High High High High High
Table beet High High High High High Medium
Tomato Medium Medium High Medium Medium Medium
Turnip High Medium —— Medium Medium ——
Source: Lucas and Knezek (1991
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Vol. 4 (1): 1-27, 2009
Edward Raja
5
and Al compounds present as silicates or as impurities were
dominant over clay mineral species per se in determining B
adsorption characteristics.
Soil moisture
Boron availability generally decreases as soils
become dry, so that boron deficiency is more likely in plants
suffering from water deficit. This may be because plants
encounter less available B when they extract moisture from
soil at a lower depth during dry conditions. Wetting and
drying cycles increased the amount of B fixed. Flood
irrigation resulted in leaching of B.
Temperature
Boron adsorption rises with higher soil temperatures
and reduces availability. However, this may reflect on
interaction between soil temperature and soil moisture, since
B deficiency is often associated with dry summer conditions.
High sunlight and low temperature also aggravate B
deficiency.
Organic matter
Many researchers have suggested that levels of
soil organic matter influence availability of B to plants. The
strongest evidence that organic matter affects availability
of soil B is derived from studies that show positive correlation
between levels of soil organic matter and amount of
available B and uptake by plants (Gupta, 1983, Chang et al
1993). The association between B and soil organic matter
is caused by assimilation of B by soil microbes. Although B
present in soil organic matter is not immediately available to
plants, it seems to be a major source of available boron
when released through mineralization.
Irrigation water
Water used for irrigation also has B content and
water from semi-arid regions or saline soils has boron content
of 0.001 ppm to 0.01ppm
Low boron concentration and its impact
What makes B unique among all other
micronutrients in horticultural crops is its effect on
reproductive physiology. Low B affects the plant right from
seed-set to fruit-set and formation (Fig 1). This is because
of its role in cell wall development, cell elongation and
membrane stability.
Higher B content needed for reproductive parts
Sexual reproduction is more sensitive to low B than
vegetative growth, and a marked reduction in fruit-set can
occur without expression of B deficiency symptoms in
vegetative parts. The most intricate aspect of B nutrition is
highlighted by the fact that reproductive parts in both
monocots and dicots require 2-4 times more B. The vast
difference in B in low supply and adequate supply needs to
be kept in mind while supplying B for optimum yield.
Maintaining high B levels in reproductive parts is a vital
component of efficient B management for yield in
horticultural crops.
Boron mobility in horticultural crops: Horticultural crops
vary widely in their boron mobility in phloem; hence, B
deficiency is more widespread than any other micronutrient
deficiency (Gupta, 1983). Occurrence of brown heart in
turnip, radish and storage roots of rootabaga and hollow
stem in cauliflower and broccoli are due to B deficiency
(Shelp and Shattuck, 1987a; Shelp et al, 1987, 1992a). Poor
fruit and seed set in nut crops, even when there is no
Source: Dell and Huang (1997)
Fig 1. Life-cycle of an angiosperm emphasizing stages when
inadequate boron supply may directly or indirectly impact
reproductive development. Consequences of B deficiency shown
at (a)–(f) are: (a) impaired inflorescence/flower formation; (b)
infertile or aborted pollen; (c) reduced recognition of pollen by
the stigmatic surface; (d) impaired pollen germination; (e)
impaired pollen tube growth in astylar tissue leading to reduced
seed and fruit set; (f) impaired seed development, eg, hollow heart,
shrivelled seed; (g) abnormal seedlings, reduced seedling vigour
Micronutrients in horticultural crops
J. Hortl. Sci.
Vol. 4 (1): 1-27, 2009
6
symptom on leaves, indicates that B deficiency is
physiological in nature (Nyomora et al, 1997). For tissue
analysis, growing tissues are sampled in B immobile plants;
whereas, in plants where B is mobile, even fruits and mature
leaves are sampled. For B management in anticipated B
deficiency, foliar spray is adequate in B mobile plants (apple),
whereas, in B immobile (mango) plants; correction is difficult.
Both soil and foliar spray, especially at flowering, are
essential in B immobile plants.
Prognosis and diagnosis of B deficiency
Prognosis by B analysis is done for ascertaining B
deficiency for preventive management, whereas, diagnosis
is done for curative management. Critical B concentration
for different crops varies between 3-7 mgkg-1 (for wheat)
to 50-75 mgkg-1 (for mango), indicating the vast difference
in crop requirement for B and the need for a sensitive
prognosis programme for optimum fruit and vegetable
production. Young, Fully Expanded Leaf (YFEL) seems to
be ideal for forecasting the response to B application.
Source: Brown and Shelp, 1997Table 2. Boron distribution (mg B kg
-1) in shoots of field-grown
apple and walnut
Leaf age Crop
Apple Walnut
(Malus domestica) (Juglans regia)
Old 50 304
Mature 57 225
Young, expanded 56 127
Expanding 73 62
Meristematic 70 48
Table 3. Critical boron concentration (mg B kg-1) or concentration range in leaves of plants for prognosis for B deficiency
Species Leaf and plant-age or growth-stage Critical Country and Source
B concentration or
range (mg/kg)
Bean YFEL – 37 daysafter sowing 20–24 Columbia; Howeler et al (1978)
(Phaseolus vulgaris)
YFEL – 75 days after sowing 16–18
Broccoli YFEL blade when 5% heads formed 9–13 Canada; Gupta and Cutcliffe (1973, 1975)
(Brassica oleracea)
Brussel’s sprout YFEL blade when5% heads formed 7–10
(B. oleracea)
Cauliflower YFEL blade when 5% heads formed 8–9
(B. olreacea)
Potato YFEL – 7 weeks after sowing 24 Australia; Pregno andArmour (1992)
(Solanum tuberosum)
Rutabaga Youngest mature leaf blade at 5–6 leaves 37–44 Canada Gupta and Cutcliffe (1972)
(Brassica napobrassica)
Wheat YEB – booting 3–7 Thailand; Rerkasem and Loneragan (1994)
(Triticum aestivum)
Mango Young leaves 50-75 India; Agarwala (1988)
(Mangifera indica)
Tomato F1 Hybrid Young leaves 35-40 India; Iyengar and Edward Raja (1988)
(Lycopersicon escule1
J. Hortl. Sci.
Vol. 4 (1): 1-27, 2009
Fig 2. Leaf-B concentration (mg kg-1 dry wt) in field-grown apple
and walnut. Leaves were collected at the end of the growing season
in 1995 in the pomology orchard, Davis, California, USA. The two
species were grown in close proximity and received the same
irrigation. B distribution in leaves also highlights B mobility and
its effect. In a B mobile plant (apple), the meristem has more B
than do old leaves, but, is low in meristem in immobile plant
(walnut)
Apple Walnut (terminal leaflet)
Edward Raja
7
Table 4. Boron concentration in plant parts exhibiting B deficiency symptoms
Species Plant organ showing B in affected plant Reference
deficiency symptom part(mg-1kg)
Wheat (Triticum aestivum) Youngest emerged leaves < 1 Huang et al (1996)
Ear at booting 3–7 Rerkasem and Loneragan (1994)
Carpels at booting <6 Rerkasem and Lordkaew (1996)
Anthers at anthesis <9
Rutabaga (Brassica napobrassica) Youngest mature leaves 2–7 Gupta and Cutcliffe (1972)
Mango (Mangifera indica) Fruit <20 Ram et al (1989)
Effect of B on yield
Metabolic requirement for B varies with plant and
plant species. The data (Table 4) highlight that vegetative
parts exhibit B deficiency symptoms at low B levels, while,
the reproductive parts show symptoms at higher B levels.
Monocots like wheat are known to exhibit symptoms at lower
B concentrations than dicots like sunflower, which need 10
times more B. The fastest response to any nutrient
deficiency is observed in the case of B. Within 3 hours of
withholding B, root growth stops, and, deficiency symptoms
are visible even when adequate B is present in the soil but
is unavailable, due to low soil-mixture or poor transpiration
(Dell and Huang, 1997).
Plant factors and prognosis for B deficiency
Plant species differ in their capacity to take up B
even when grown in the same soil. These differences
generally reflect different boron requirements for growth.
In most dicotyledonous species such as papaya, the
requirement is 80-100 mg. Difference in B demand of
graminaceous and dicotyledonous species is probably related
to difference in their cell wall composition. Interestingly,
these two plant groups also differ in their capacity for silicon
uptake, which is usually inversely related to B and Ca
requirement (Loomis and Durst, 1992). All three elements
are located mainly in the cell wall. Reports on Ca/B
interaction are thus far inconclusive (Gupta, 1979).
However, these interactions are likely to have a physiological
basis. Both elements are likely to have similar structural
functions in the cell- wall and at cell-wall plasma membrane
interface, and, similar interactions in uptake & shoot
transport, and in IAA transport. These common features
also explain certain similarities in symptoms of calcium and
boron deficiency in peanut seeds and lettuce (Crisp and Reid,
1964).
Revolution in mango yield in India by B nutrition using
the Brazilian experience
In India, mango is grown in about 1.6 milion ha,
with productivity of 6-7t/ha, compared to 20-25 t/ha in
Mexico/Brazil and 25-30 t/ha in South Africa. Poor
micronutrient nutrition, especially B, is one of the causes
for such a huge yield gap (Edward Raja et al, 2005).
Deficiency of B results in poor and non-uniform flowering,
low fruit-set, increased fruit drop and poor quality produce.
Mango is a B loving crop and the critical level ranges
between 75-100 ppm (Agarwala, 1988). Rossetto et al
(2000) recorded tremendous response to B by application
of 300g borax/tree as soil application. This response varied
from 200% for cv.Tommy Atkins to 500% for cv. Haden
2H and Vandyke, but one cultivar Winter did not respond to
B(Table 5). This highlights the tremendous potential of B
for increasing yield in mango. But another point to note here
is that Brazilian soils have low pH and hence availability of
applied B is high. Edward Raja et al (2005) observed a
significant yield response to B in cultivar Alphonso in
Konkan, which has climate and soil similar to that in Brazil.
Why is widespread B deficiency seen in mango in
Konkan region (India)?
1. Since B is the only micronutrient lost to leaching,
heavy rainfall (2200mm/yr) in the region results in
low soil B status (<0-3 ppm)
Fig 3. Young fruits of mango cv. Van Dyke. Fruit with leathery-
color typical of low boron (left) and normal green fruit (right)
Source: Rossetto et al (2000)
Micronutrients in horticultural crops
J. Hortl. Sci.
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8
2. Since B uptake by xylem occurs through passive
uptake, high humidity (60-80%) in the region also
reduces B uptake by mango trees
3. Probable mismatch between need and availability;
Boron is needed in Nov/Dec when flowering and fruit-
set occurs (as, it is important in pollination). Since
90% of mango is grown as rain-fed crop, the soil
becomes dry in December when available is B low
and B demand is highest. This mismatch between
availability and need is probably another major reason
for hidden hunger and visible deficiency of B in India,
and in Konkan in particular
Occurrence of boron deficiency and response in papaya
Among fruit crops, papaya is extremely susceptible
to boron deficiency common in latisols and old slate alluvial
soils in upland areas of Taiwan (Wang and Ko, 1975; Chang,
1993). This is more likely when papaya trees are planted in
sandy soils during dry season. One of the earliest signs of
boron deficiency is mild chlorosis in mature leaves which
become brittle, and tend to curl downwards. A white “latex”
exudate may flow from cracks in the upper part of the trunk,
from leaf stalk, and from the underside of main veins and
petiole. Death of the growing points is followed by
regeneration of side-shoots that ultimately die. In fruiting
plants, the earliest indication is flower-shedding. When fruits
develop, they are likely to secrete white latex. Later, the
fruit becomes deformed and lumpy. The deformation is
probably a result of incomplete fertilization, as most of the
seeds in the seed-cavity are either abortive, poorly developed
or absent. If symptoms begin when the fruit is very small, it
does not grow to full size. Papaya fruits having a rugged
surface and secreting latex are typical symptoms of boron
deficiency. In studies on boron deficiency in papaya in
Taiwan, samples were taken from the 10th leaf blade
(without petiole), counted from the 1st leaf (the most-
recently-matured leaf, with a leaf blade that has only just
fully-developed, and which has a brownish colored petiole).
Standard sampling of this kind can effectively reflect
variations in boron content in different orchards. Boron
content of the tenth blade of papaya trees with deformed
fruits was always found to be lower than 20 ppm, while that
of leaves from normal trees was generally 25-155 ppm.
Curative management of boron deficiency
For tree crops application of B as Borax at planting
is suggested for example, Borax @ 10g/banana plant, 50-
100g/mango plant, 20-25g/papaya plant should be applied,
supplemented with foliar spray at 25% flowering. At
flowering, Solubor (20% B) is an ideal source of B for foliar
spray, followed by boric acid (17% B). Boron is a phyto-
toxic element and care should be taken to avoid toxicity.
Older leaves show toxic symptoms of necrosis of margins.
Slow-release B source in soil, with foliar spray of Solubor,
is an ideal approach to avoid toxicity and deficiency in high-
rainfall areas.
ZINC (Zn)
Zinc nutrition in horticultural crops
Among micronutrients, Zn occupies an important
place due to its ability to positively influence plant growth
and development. Zinc enhances seed-viability, seedling-
vigour and imparts resistance to biotic and abiotic stresses
(Cakmak, 2008). Zinc is highly immobile in soil and its
deficiency is common in mango, banana, guava, litchi, apple,
grape and pomegranate. Little-leaf and rosette symptoms
are the most common visual indicators of Zn deficiency.
Table 5. Average yield of four mango cultivars, expressed in kilograms per hectare, over a six year period (1993-1998), showing the
effect of soil boron application in half the blocks in the last three years, plus, the medium leaf-content of boron; Each figure is the
mean of 27 observations (3 rootstocks, 3 blocks, 3 years) in Votuporanga, SP, Brazil
Boron Cultivar kg ha -1 Leaf boron mg kg-1 kg ha -1 Leaf boron mg kg -1 Yield
1993-94-95 July 95 1996-97-98 Dec.98 Increment
Blocks without boron Winter 8,379 a* 8.2 19,489 a* 7.7 2.3
TommyAtkins 6,816 a 9.0 9,807 b 7.6 1.4
Van Dyke 6,608 b 8.4 2,697 c 8.2 0.7
Haden 2H 1,951 b 8.7 3,375 c 8.1 1.7
Mean 5,188 8.5 8,842 8.1
‘Without boron’ effect ‘With boron’ effect Yield increment
Block showing boron Winter 6,426 a* 8.2 17,114 a* 26.2 2.6
effect, from 1996 Tommy Atkins 4,288 a 9.1 16,272 a 29.9 3.8
Van Dyke 1,288 b 7.6 16,874 a 23.9 13.1
Haden 2H 1,406 b 10.0 14,820 a 29.6 10.5
Mean 3,352 8.7 16,270 27.4
Mean, followed by the same letter, does not differ by Tukey test at 5%
Source: Rossetto et al (2000)
J. Hortl. Sci.
Vol. 4 (1): 1-27, 2009
Edward Raja
9
Chemistry of Zn availability in horticultural crops
a. Soil reaction (pH)
Among soil chemical factors, soil pH plays the most
important role in Zn solubility in soil solution. In pH range
between 5.5 and 7.0, Zn concentration in soil solution
decreases 30 to 45-fold for each unit increase in soil pH,
thus increasing the risk of Zn deficiency in plants
(Marschner, 1993). Increasing soil pH stimulates absorption
of Zn to soil constituents (eg. metal oxides, clay minerals)
and reduces adsorption of the adsorbed Zn. Lindsay (1991)
reported that at pH 5.0, concentration of Zn2+ in soil solution
is sufficiently high, about 10-4 M (6.5 mg/ kg). When soil pH
increases from 5 to 8, concentration of soil solution Zn2+
reduces by nearly 1000 times and becomes nearly 10-10 M
(approx. 0.007 mg kg–1). Consequently, increase in soil pH
is associated with very sharp decrease in concentrations of
Zn in plant tissues (Marschner, 1995).
b. Moisture
Transport of Zn to root-surface in soils occurs
predominantly by diffusion, and this process is highly
sensitive to soil pH and moisture (Wilkinson et al, 1968).
Soil moisture is a key physical factor providing suitable
medium for adequate Zn diffusion into plant roots. The role
of soil moisture is very critical in soils with low Zn availability
(Marschner, 1993). Zinc nutrition in plants is, therefore,
adversely affected under water stress conditions, particularly
in regions where topsoils are usually dry during later stages
of crop growth. Occurrence of Zn deficiency stress and
consequent decrease in crop yield were found to be more
severe under rainfed (compared to irrigated) conditions
(Bagchi et al, 2007).
c. Organic matter
Soil organic matter plays a critical role in solubility
and transport of Zn to plant roots (Marscher, 1993). In a
study with 18 different soils, there was a strong inverse
relationship between content of soil organic matter and
soluble Zn concentration in the rhizosphere (Catlett et al,
2002). These results indicate that the pool of readily available
Zn to plant roots may be extremely low in soils with high
pH, and, reduced levels of organic matter and soil moisture
(Takkar, 1999; Cakmak et al, 1999). Removal of
micronutrients by different crops indicates that removal of
micronutrients is not substantial compared to soil reserves
of both available and total micronutrients (Graham, 2002).
Available Zn levels in mango orchards of peninsular India
indicate adequate soil reserves of Zn, but leaf Zn status
indicates deficiency in a majority of the soils, due to a
combination of low moisture, low organic matter and high
pH (Agarwala, 1988).
Zinc deficiency correction in tree crops
Confusion prevails in the minds of growers and
scientists regarding the right choice for Zn amendment
[ZnSO
4
or
ZnEDTA (chelate) and its mode of application].
All studies have indicated that 0.5% ZnSO
4
as foliar spray
is better than other treatments for correction of Zn
deficiency, and this method is more efficient in economic
and environmental terms. But, 2 – 4 foliar sprays are essential
for consistent correction and to obtain leaf zinc concentration
of 25 – 75 ppm, required for optimum yield. Apart form
foliar spray, applying composted manure is essential for
making soil Zn too available to the plant. Hence, no exclusive,
single method is advisable. Use of chelated Zn sources for
soil or as foliar spray is not needed, since these are par with
ZnSO
4.
Zn-solublizing bacteria can mitigate the widespread
zinc deficiency in fruit grapes. Subramaniam et al (2006)
observed that a strain of Pseudomonas fluorescens
solubilized soil-Zn. Along with Zn-solubiling bacteria, foliar
spray of B, Mn and Fe resulted in increased yield and quality
in grapes.
IRON (Fe)
Iron nutrition in horticultural crops
Iron deficiency is easy to identify but difficult to
correct. Iron is chemically unavailable in the soil, and
physiologically unavailable in the plant. The paradox is that
soil has about 10000-100000 ppm total iron, but the plant
needs only 30-50 ppm. It is not the quantity of Iron that is
important but the quality. Description of the thirsty sailor
crying in the midst of the seawater, “water, water
everywhere, not a drop to drink” aptly describes availability
of soil-iron to the plant. Another paradox in iron nutrition is
lime-induced iron chlorosis in plants, wherein, deficient
leaves have more Fe than healthy leaves, making leaf
analysis for Fe unreliable for judging iron-nutrition.
Diagnosis of iron chlorosis in tree crops
Prognosis of Fe deficiency is a challenging task,
since iron deficiency (iron-chlorosis) is an important
nutritional disorder in horticultural crops, in general, and tree
crops, in particular. It does not occur due to low level of Fe
in the soil but from impaired acquisition and use by plants.
The most prevalent cause of iron chlorosis is bicarbonate
levels in soils (Pestana et al, 2003) or the bicarbonate present
in irrigation water (Tagliarani and Rombola, 2001). Prognosis
Micronutrients in horticultural crops
J. Hortl. Sci.
Vol. 4 (1): 1-27, 2009
10
of iron deficiency is important, since correction is a costly
and tedious process.
Soil tests
For annual crops, soil tests are useful but, for tree
crops, it is of limited value since the roots are deep and
unevenly distributed. Soil tests for lime-induced chlorosis
need to focus on
a. Use of extractants capable of chelating the metal
b. Determination of active lime-content
c. Lime in silt-clay and fractions of soil
Plant analysis
a. Visual scoring: This is a fast and economical method
(Samz and Montanes, 1997). The score ranges from 0
(without symptom) to 5 (trees with dead branches and pale
young leaves). It can be quantified by SPAD apparatus
that measures leaf transmittance at two wavelengths, 650
and 950 nm, and is a measure of chlorophyll. But the limitation
is that by the time chlorosis appears, correction is no longer
possible.
b. Plant analysis: Leaf analysis is still the common method
and is based on growth rate of plants and their nutrient
content. But, it has several limitations in lime-induced
chlorosis, viz.,
i. Chlorosis Parad: This is the phenomenon of absence
of correlation between leaf Fe concentration and
degree of chlorosis. Iron concentrations on dry weight
basis are frequently more in chlorotic leaves than in
green leaves, which is due to inactivation of Fe.
ii. Analysis of active iron : Analysis of active iron
[(Fe (II)] is carried out using extractants like acetic,
nitric and hydrochloric acids, O-phenanthrolone
(Rashid et al, 1990). But these methods also have
limitations as they remove Fe from phytoferrin, which
is part of the pigment and make Fe not available for
other metabolic role. More over by the time the active
Fe is estimated, it may be too late for correction.
iii. Flower analysis : Flower analysis is the currently
more acceptable method for deciduous fruit trees and
citrus, since correction is possible before fruit set.
The Fe content in flowers is well correlated with leaf
chlorophyll status in deciduous trees with the
exception of sweet orange where it is negatively
correlated (Pestana et al, 2003).
iv. Nutrient ratios in flowers : Since Fe analysis of
flowers is not acceptable for both deciduous and
evergreen tree crops, some phenomena like increase
in K in flowers due to iron chlorosis is used for
prognosis. The K: Zn ratio in the flowers is fairly
consistent and a value above 450 indicates the
potential for preventive correction of iron chlorosis.
v. Enzyme assay : Inspite of all the refinements in
mineral analysis, the difficulty in differentiating
metabolic / active Fe from non-active Fe is still
difficult. The assay for enzyme chlorophyllase
activity is another useful option for assaying for
chlorosis for its prevention especially hidden hunger.
This form of iron chlorosis is looming large over tree
crops like grapes, mango, citrus and banana grown in
winter months in calcareous soils.
Correction of Fe chlorosis
Though Fe is one of the most abundant elements in
soil, its deficiency in plant tissues is a major challenge. Short-
term correction by organic manures (produced in India) is
not suitable due to inadequate quality standards (CN ratio,
humification, pH, exchange capacity). A wide CN ratio
induces Fe deficiency rather than correct Fe deficiency, by
producing bicarbonates from the CO
2
released from
undecomposed carbon. Citrus, mango, grape and apple are
known for their susceptibility to Fe deficiency, but prognosis
is at its infancy. Presently, applying organic manure and
use of multi-nutrient sprays are the only feeble attempts in
iron deficiency management. The estimate of loss by hidden
hunger for Fe has not been done, since, it is not recognized.
Iron deficiency prevention by use of ideal rootstocks
About 1/3rd of Indian soils are calcareous, and iron
deficiency is a major nutritional problem in such soils (Ray
Chaudry and Govindarajan, 1969). They are highly buffered,
with pH 7.5 to 8.2, and have bicarbonate affecting soil and
plant Fe availability. Chlorophyll content decreases and
carotenoid pigment increases in such soils. The iron
deficiency occurs as visible and hidden hunger. This lime-
induced chlorosis delays fruit-ripening, resulting in impaired
quality in peach and orange (Pestna et al, 2001). Mandarins,
limes and lemons are moderately tolerant to Fe deficiency.
Work of Pestana et al (2000) indicated that Troyer citrange
rootstocks are very tolerant. Nikolic et al (2000) observed
differential foliar tolerance to iron chlorosis in grape.
Kadman and Gazit (1984) identified mango rootstocks
tolerant to Fe deficiency. Edward Raja (2009) identified
mango cultivars tolerant to Fe deficiency. Correction of Fe
deficiency is the most difficult if suitable rootstock is not
available.
J. Hortl. Sci.
Vol. 4 (1): 1-27, 2009
Edward Raja
11
Iron chlorosis in ornamental crops
Ornamental crops like rose, gerbera, gladiolus, and
chrysanthemum are susceptible to Fe deficiency. Crops like
jasmine (Jasminum auriculatum and J. sambac and
Crossandra suffer Fe deficiency. Free calcium carbonate
and high pH are the reasons for the incidence of iron
chlorosis. Jasminum grandiflorum is tolerant to iron
chlorosis (Kannan and Ramani, 1988). Remedial measures
like soil or foliar application are only temporary.
Identification of high yielding, efficient cultivars of crop
plants should be the goal to manage iron chlorosis on a long-
term basis. Edward Raja (1990) screened 22 chrysanthemum
cultivars for tolerance to iron and observed that only four
cultivars were tolerant to chlorosis. Since chrysanthemum
is highly susceptible to Fe deficiency, crossing these tolerant
cultivars with susceptible ones may help manage chlorosis
by transferring the tolerance. In polyhouse grown rose, Fe
deficiency is a serious problem, resulting in 20 – 30% of
roses getting rejected as unmarketable. Of the ten cultivars
screened for tolerance, cv. Kanfetti and First Red were
found to be moderately tolerant. Chen and Barak (1983)
observed that foliar spray of FeSO
4
with a non-ionic
surfactant L-77, and, application of Fe-1 EDDHA, were
effective in correcting iron chlorosis in soil at pH 7.7.
Chemical degradation in soil / irrigation water and iron
deficiency
Due to increasing use of chemical fertilizers, the
quality of irrigation water is deteriorating. Monitoring
irrigation-water quality in grape orchards around Bangalore
indicated increasing level of bicarbonate (HCO
3
) 1.1 to 4.6
meq/l and NO
3
-N from traces to 0.8 meq/l over a period of
10 years from 1998 (Table 6). NO
3
-N enhances rhizosphere
pH by physiological alkalinity and HCO
3
makes Fe inactive
in the leaf and results in chlorosis- paradox. This is danger
in all horticultural crops, especially grape, since hidden hunger
for Fe deficiency will only increase in future.
A study on build-up of heavy metals like Zn and Cu
in grape orchards of Rural Bangalore revealed that heavy
metal content increased by 60 to 120% over a period of 10
years. These heavy metals, when present in excess induce
iron chlorosis. Hence, balanced nutrition with adequate
humified organic manures, alone can reduce the dangers of
widespread iron chlorosis. Therefore, foliar correction of
micronutrients, especially Zn, is recommended. Else, Zn
toxicity induced Fe deficiency, coupled with poor quality
irrigation, will further aggravate Fe-deficiency in horticultural
crops.
Table 6. Change in quality of irrigation water over a period of 10
years in grape orchards in Bangalore district
Parameter Unit Value
In 1998 In 2008
p H — 6.50 7.12
EC dSm-1 0.44 0.909
Cl meq/l 1.88 2.572
NO
3
meq/l Traces 0.820
SO
4
meq/l 0.160 0.180
CO
3
meq/l 0 0
HCO
3
meq/l 1.102 4.590
Ca meq/l 2.200 2.699
meq/l 1.822 1.922
Na meq/l 1.89 2.464
RSC meq/l Traces Traces
SAR meq/l 0.660 1.666
MANGANESE (Mn)
Occurrence of Mn deficiency in acid lime in
orchards of southern India have been reported by Edward
Raja (1992). Mn deficiency in older trees (25-30) of mango
has also been recorded (Edward Raja, unpublished data).
COBALT (Co)
The beneficial effect of cobalt in nodulation is well-
known and hence it is imperative that adequate cobalt supply
is made to lignin vegetables like French bean, garden pea,
pea and other vegetable beans, and take advantage of their
capacity for symbiotic N
2
fixation. Economy in N also
assures better soil-health due to reduced NO
3
pollution and
better organic matter status even in marginal soils.
NICKLE (Ni)
Ni as a micronutrient
It is becoming apparent that Ni is likely a far more
limiting factor in agriculture than previously supposed (Bai
et al, 2006; Wood and Reilly,2006). Thus potential sources
of Ni fertilizers are likely to be increasingly required,
depending on usage situations. Discovery of field-level nickel
deficiency in agriculture provides an opportunity to correct
micronutrient deficiencies using biomass of hyper-
accumulating plants. Ni is increasingly recognized as an
essential mineral nutrient element for higher plants. Ni
deficiency was discovered to be the cause of a mysterious
malady of pecan, termed “mouse-ear”, and of an increasingly
common replant malady in old or second generation, pecan
orchards. This has established a need for commercial Ni
fertilizers (Wood et al, 2004). Deficiency can also have a
major impact on primary and secondary metabolism (Bai et
al, 2006) and can also potentially influence plant resistance
to certain diseases (Wood and Reilly, 2006). Walker et al
Micronutrients in horticultural crops
J. Hortl. Sci.
Vol. 4 (1): 1-27, 2009
12
(1985) observed that Ni was needed in cowpea in
reproductive stages. According to Brown et al (1984), Ni
has a wide range of functions in plant growth, plant
senescence and N metabolism.
DIAGNOSIS OF MICRONUTRIENT
DISORDERS
Micronutrient management at the field level involves
prognosis and diagnosis, followed by correction of the
disorder. For diagnosis of hidden hunger, leaf analysis is
being practiced. According to Loneragan (1997), expertise
in diagnosis of micronutrients is the most challenging aspect
of micronutrient management, since, it poses more difficulties
than macronutrients. Most of the difficulties arise from
experimental material (seeds, fertilizers and sampling /
analysis of farmers’ crops) and experimental trials. The
concept of ‘critical level’ by Cate and Nelson (1962) and
optimum leaf nutrient norms by Beaufils (1967) have been
used by many research workers to develop leaf nutrient
norms to diagnose leaf tissue to check whether it is deficient
or healthy. Though these efforts are an improvement over
the earlier diagnostic methods, we need to exercise
circumspection in using these data. A perusal of Table 7
indicates leaf nutrient norms developed for mango for
commercial cultivars of India and South Africa. The former
is for a yield of 7-10/ha and the latter for a yield level of 30
t/ha. Productivity of mango in India is the lowest (6.8 t/ha)
in the world. Therefore, development of leaf nutrient norms
for such low yields is a point worth considering and hence
the question. Is it relevant to analyze the leaf for low and
unprofitable yield ? One more caution to be exercised is
checking variability in leaf nutrient norms. The optimum value
of manganese for Alphonso (Table 7) ranges between 13-
408 ppm. Can a plant be healthy at this vast range ? Also,
the Fe value for Alphonso is vastly different from Fe value
for Totapuri. In this context, only diagnosis based on some
metabolic function like photosynthesis or, any enzyme in
which the nutrient is structurally associated, is relevant.
Valenzuele and Romero(1988) recommended the use of
biochemical indicators like penexidare, catalane, chlorophyll,
carotenoid and anthocynain to analyze Fe deficiency.
Success in diagnosis is fundamental to success in correction
and profitable yield/quality.
DRIS and micronutrient diagnosis
DRIS is one of the important methods for diagnosing
the limiting nutrient and its strength lies in diagnosis by ratio
norms. Though research work on DRIS as a tool for nutrient
diagnosis, was initiated as early as in 1988 (Bhargava and
Chadha, 1988) no leaf analysis lab in the country uses
nutrient ratio norms for providing service to farmers. Leaf
nutrient standards for low, optimum and high ranges in high
yielding orchards (using standard deviation from the mean)
also need to be validated in the field before being used in
leaf analysis service. Identifying limiting nutrients without
analyzing B content, the most important micronutrient for
horticultural crops is also is of limited use. If the leaf is not
analyzed for B, even if another nutrient is identified as most
limiting may not help in the response to that nutrient, since
B figures in the Liebig stress category.
Soil test values and tree micronutrient status
Since leaf analysis is more useful than soil test for
making nutrient management decisions in perennial crops,
not much work has been done on this aspect. But, work
conducted earlier indicated poor relationship between soil
test and plant micronutrient status. Conventional soil tests
are generally done to predict soil capability for supplying
micronutrients during growth. The fundamental requirement
of a soil test is that it should extract all or proportionate
amount of plant-available nutrient which should correlate
with or predict crop-response to application of the nutrient
tested. Viets (1952) opined suggested that micronutrients
are found in five chemical pools :
1. Water soluble
2. Exchangeable
3. Absorbed, chelated or complexed
4. Secondary clay minerals and metal oxides
5. Primary minerals
Edward Raja and Iyengar (1986) fractionated
alfisoils (red soil), vertisoils (black soil) and high altitude
soils for native and applied Zn, and found that only the first
3 fractions (watersoluble, exchangeable and complexed Zn
fractions) contributed to uptake by tomato, but more than
80-90% of applied Zn accumulated in the last two fractions.
The incubation study on fate of applied Zn in seven different
soil types established that bulk of the applied Zn became
unavailable within 48 h of application to soil (Iyengar and
Edward Raja, 1988). In calcearous and clayey soil, about
70-80% applied Zn became unavailable (DTPA extractable),
whereas, in the acidic and high organic-matter rich coffee
soils of Kodagu, 30-40% of the applied Zn was still available.
Hence, soil application to correct Zn deficiency is
recommended mainly in acid soils rich in organic matter.
Since use-efficiency of applied micronutrients (Zn, Mn, Fe
and Cu) is very low (3-5%), it is better to resort to foliar
spray than soil application to correct micronutrient disorders
in perennial fruits (Alloway, 2008).
J. Hortl. Sci.
Vol. 4 (1): 1-27, 2009
Edward Raja
13
Photosynthesis and micronutrient deficiency
Micronutrient deficiencies affect carbohydrate pools
and photosynthesis. Reduction in chlorophyll content leads
to chlorosis in leaves, ultimately affecting the chloroplast
system and photosynthesis (Balakrishnan et al, 2000). Fe,
Mn and Zn levels affect the chlorophyll content.
Micronutrient disorders exercise their influence by affecting
photosynthesis and carbohydrate accumulation/translocation
and there is a need for determining adverse effects of
micronutrient disorders by effect on photosynthetic apparatus
and photosynthesis. This approach would give a better idea
of adverse effects of deficiencies.
Micronutrients and quality of horticultural crops
Horticultural crops differ in quality, which can affect
profitability for the same level of productivity. Micronutrients
have the capacity to improve quality, size, colour, taste, and
earliness of horticultural produce. Sufficient data are
available to show the positive effect of boron on juice purity
in sugar beet. This occurs due to decrease in excess nitrogen
content in roots. It has been suggested that B might be
involved in regulation and uptake of nitrate ions. Part of the
effects of deficiency are due to toxic accumulation of nitrates
in the plant. It has been amply demonstrated that increased
doses of nitrogen in boron deficient condition (which may
happen in intensive orchards among different European
countries) may reduce B uptake and suppress yield. With
an aim of studying the role of boron on sugar transport in
grapes, and its effect on the coloration of red wines, Borax
carried out field trials in L Rioja in 1999. Application of B
resulted in better coloured red wine.
As mentioned earlier, mango is a B-loving crop and,
therefore, continuous adequate quantity of B is essential
for yield, quantity and post-harvest life. Soil application in
July at 100-150g Borax/plant followed by foliar spray in Dec.
Jan. and March is essential for high yield, quality and post-
harvest life. Trials conducted in Konkan and Maharashtra
indicated that adequate B resulted in reduction in spongy
tissue from 35% to 10%. Work conducted in farmer-fields
indicated that 1% Solubor resulted in higher yield (8t/ha)
and enhanced post- harvest life from 4 days to 14 days in
B-sprayed plants (Edward Raja, 2009). Flowering was early
by 3 weeks and, therefore, harvest was advanced by 3
weeks, enabling farmers to market their produce early to
get a better price. The quality of horticultural produce in
terms of colour, size, TSS and nutrients/nutraceuticals are
important factors in deciding consumer acceptance and
marketability, ultimately deciding the profitability to the
producer. Micronutrients have a definite and significant
effect on quality. Fruit-cracking due to cuticle damage is a
serious problem in tomato grown under protected cultivation
in South Africa. The study by Jobin et al (2002) indicated
that B+Ca spray on the fruit resulted in reduced fruit-
cracking. Application of micronutrients (Zn, Fe and B) by
spray on Kinnow mandarin increased yield, juice and
ascorbic acid content besides reducing acidity and improving
TSS (Mishra et al, 2006). Boron is needed for cell-wall
synthesis and reduction in cracking in tree fruits. Studies by
Singh et al (2003) indicated that B application by spray @
0.1 to 0.4% along with GA
3
(10 to 100 ppm) resulted, in
increase in yield and reduced cracking of fruits in
pomegranate. The marketable yield increased by 10-15%
and profit by 20%. A study conducted in Mexico on the
effect of Ca, B and Zn on quality and storage of peach
indicated that pulp firmness, TSS, titrable acidity and storage
life improved with B applied as pre-harvest spray. In
greenhouse production systems, yield and quality of tomato
was a problem. In a study on the effect of B on tomato,
Smith and Comtrink (2004) observed that B application
increased Ca, Mg & Zn content, besides improving firmness,
colour, total solids, and shelf-life in tomato.
Micronutrients and input-use efficiency
All studies on response to micronutrients involve
application of macronutrients, and, any yield increase due
to application of micronutrient necessarily indicates that the
efficiency of applied NPK fertilizers is enhanced. Indian
Micronutrients in horticultural crops
J. Hortl. Sci.
Vol. 4 (1): 1-27, 2009
Table. 7. Optimum leaf nutrient norms for important mango
cultivars
Parameter Alphonso Totapuri All varieties of
(India) (India) South Africa
N % (Range) 0.78 - 1.65 0.84 - 1.53 1.0 - 1.2
P % (Range) 0.02 - 033 0.064 - 0.147 0.08 - 0.1
K % (Range) 0.77 - 1.73 0.52 - 1.10 0.8 - 1.1
Ca % (Range) 0.76 - 1.63 1.67 - 3.20 2.0 - 3.3
Mg % (Range) 0.40 - 0.65 0.40 - 0.65 0.2 - 0.3
S % (Range) 0.035 - 0.131 0.0147 - 0.215 0.1 - 0.2
Fe mg/kg 657 - 963 48 - 86 190 - 310
(Range)
Mn mg/kg 13 - 408 57 - 174 170 - 150
(Range)
Zn mg/kg 7.8 - 18.3 25 - 33 30 - 75
(Range)
Cu mg/kg 14.3 - 17.8 3.10 - 8.00 9 - 18
(Range)
Boron mg/kg 40-80 (3-0.6 Mo)
(Range)
Yield (t/ha) 6 10 30
Source: South Africa mango growers Year Book 2003 / IIHR, Folder
No-45-0 2007
14
farmers use about 2 million tons of fertilizers, valued at
Rs.4,00,000 crore and a subsidy of Rs.48,000 crore is borne
by the government, for fertilizers. If micronutrient application
can save even 20% on fertilizers, the benefit is substantial,
besides the added benefits to environment. More important
is the fact that in B-deficient soils, water stress can damage
crops more fiercely than in B-sufficient soils. In some studies
on oilseed rape (Brassica napus) in B-deficient soils
(without added boron), water stress treatment significantly
increased root dry weight, but decreased shoot dry weight,
resulting in decreased shoot/root ratio. Applied boron may
improve the translocation of N compounds (Miley et al,
1969).
French bean is a poor nodulating legume, hence,
inorganic N fertilizer is applied (due to inadequate
symbiotically-fixed N). Low availability of Fe and Zn in the
soil were identified as one of the causes for poor nodulation.
Supplying Fe and Zn resulted in better nodulation and N
fixation, according to Hemantaranjan and Garg (1986). It is
well-established that Fe is an integral part of the nitrogen-
fixing complex i.e, nitrogen-fixing enzyme (nitrogenase), leg
hemoglobin and terridoxin (Evans and Russell, 1971).
Subsequent study by Hemantaranjan (1988) indicated that
application of chelated iron as Fe-EDTA and Fe-EDDHA
at 5-10 ppm increased functional nodules and N-fixed
symbiotically and total dry matter in French bean. Dry matter
yield increased from 25 g to 46.3 g with Fe-EDDHA, nitrogen
content from 360 mg N/pot to 673 mg N/pot, an increase of
above 60% N fixed. Rai et al ( 1984 ) also recorded
increased N fixation in lentil due to Fe application. But, excess
Fe decreased N fixation, indicating a need for the correct
dose of micronutrients and a possibility of toxicity. Application
of 5 to 10 ppm Zn and Fe individually, and together, resulted
in better ‘functional nodulation’ and seed in Varanasi soils
pH 7.5. These findings encourage us to focus on
micronutrient nutrition of legumes in general, and French
bean in particular, for reducing inorganic N use in vegetable
cultivation.
Molybdenum is also involved in functional nodulation
and N fixation in legumes along with Fe and, hence, adequate
Mo supply is also needed to encourage symbiotic N fixation
and reduce dependence on fertilizer nitrogen. Molybdenum
is needed in a very low quantity. In bold-seeded legumes
like French bean, pea, cowpea and garden pea, the seed
itself can be enriched, since seed legumes have the capacity
to accumulate molybdenum to a very high level. When
garden pea had 0.17 ppm molybdenum (which is low), it
responded to soil application of Mo, whereas, it failed to
respond to external application of Mo when the seed had
0.65 ppm of Mo. Gurley and Gidden (1969) corrected Mo
deficiency in soybean by seed enrichment to 20-30 times
the normal seed-Mo level, thereby avoiding the deficiency.
Cobalt too has been involved in enhancing nitrogen fixation
and improving N economy.
Micronutrients and disease resistance / tolerance in
plants
Importance of adequate nutrition for disease
resistance in humans is something most people accept from
personal experience. Although this vital principle is also well-
recognized in plant science, it is often ignored in practical
agriculture. This is especially true of micronutrients. Of the
many reviews dealing with plant nutrition and disease
(Graham, 1983), few have seriously considered
micronutrients. However, the role of Mn was treated at length
(Huber and Wilhelm, 1988), and the flurry of research on
siderophores in disease control (Swinburne, 1986) has
brought Fe to prominence in plant pathological literature.
Manganese
Of the micronutrients, Mn may prove to be the
most important in development of resistance in plants to
both root and foliar diseases of fungal origin. Availability of
Mn to plant roots and soil microorganisms varies mercurially
over time, depending on many environmental and soil biotic
factors. Consequently, Mn availability is subject to
manipulation by both higher plants and microorganisms. As
Mn is required in larger concentrations by higher plants than
by fungi and bacteria, there is an opportunity for the pathogen
to exploit this difference in requirement. The role of Mn in
mitigating diseases in horticultural crops has been presented
in Table 8. Whereas, effects of nutrition on disease are
normally limited to the deficiency range (Graham, 1983),
there are a few indications in literature that suppressive
effects of Mn operate well into the sufficiency range of the
host plant. This would appear to indicate either that (i) Mn
requirement of the host plant for disease resistance is higher
than for yield (ii) that Mn is somehow involved in lowering
the inoculum potential of soil-borne, pathogens.
Several mechanisms have been proposed for the
role of Mn in disease resistance, but lignification was found
to be the most prominent. Some of the possible roles of
manganese are outlined below:
◆ Manganese is involved elsewhere in the biosynthetic
pathway of phenols and lignin. Mn deficiency leads
to a decrease in soluble phenols (Brown et al, 1984),
J. Hortl. Sci.
Vol. 4 (1): 1-27, 2009
Edward Raja
15
Table 8. Effects of Mn deficiency reported on plant diseases in
horticultural crops
Horticultural plant Disease Pathogen Effect
Grapes Phylloxera Phylloxera Decrease
Palm Leaf spot Excerohilum Decrease
rostratum
Cucumber Mildew Erysiphe Decrease
Cow pea Mildew Decrease
Onion Rot Storage fungi Decrease
Erysitre polygone
Legume vegetables Canker Rhizoctonia Decrease
Potato Late Blight Phytophthora Decrease
infestans
Stem Canker Rhizoctonia Decrease
solani
Swede Mildew Erysiphe Decrease
cruciferarum
Tomato Bacterial Pseudomonas Decrease
speck syringae
Wilt Fusarium Decrease
oxysporum
Wilt Verticillium Decrease
asbo-abum
Virus Tomato Decrease
Mosaic virus
Pumpkin Mildew Erysiphe Decrease
sclerotiorum
Scab Streptomyces Decrease
scabies
Late Blight Phytophthora Decrease
infestans
Sugar beet Insect Root borer Decrease
Sugar beet Leaf spot Cercospora sp. Decrease
Source: Huber and Wilhelm (1988)
which are frequently implicated in disease resistance
(Bell, 1981)
◆ Photosynthesis is severely inhibited by Mn deficiency.
It has been argued that decrease in root exudation of
organic materials may follow and result in weaker
rhizofloral population, less able to compete with
potential root pathogens in the rhizosphere (Graham
and Rovira, 1984). However, while photosynthetic
capacity in Mn-deficient leaves responds quickly to
foliar-applied Mn, evidence of ineffectiveness of foliar
applied Mn in controlling take-all (Reis et al, 1982;
Huber and Wilhelm, 1988) suggests that this
mechanism is not important in this disease
◆ Direct inhibition of the pathogen is commonly
suggested as a mechanism of Mn action. Although
Mn is essential for microbial growth (Bertrand and
Javillier, 1912), the requirement is nearly 100 times
lower than requirement in higher plants. Both
organisms exploit this marked difference in
requirement between host and pathogen
Copper
Control of foliar pathogens by topical applications
of Cu salts was been well-established by the turn of the
20th Century. This was almost 30 years before Cu was
recognized as an essential nutrient in both higher and lower
plant life. Whereas Cu is required by lower plant forms in
minute amounts (Bortels, 1927), it is particularly toxic in
higher concentrations (Keast et al, 1985). Copper has been
used extensively as a fungicide at concentrations 10 to 100
times greater than those normally needed as a foliar spray,
to cure Cu deficiency (0.1-0.2kg ha-1). Most of its fungicidal
properties have been used against foliar pathogens, since
Cu added to soil is quickly adsorbed, and only a low
concentration remains in the soil solution.
Zinc
Zinc is important for integrity and stability of
biological membranes (Chvapil, 1973; Bettger and O’Dell,
1981). In plant root membranes specifically, it has been
suggested that Zn may be important in preventing root
membranes from leaking (Graham et al, 1987). This
hypothesis has particular relevance to the finding that
zoospores of Phytophthora cinnamomi were attracted to
Zn-deficient Eucalyptus marginata and E. sieberi roots
than to Zn-adequate roots. Many studies showed that Zn
reduced plant diseases, which probably may be related to
the toxic effects of Zn directly on the pathogen, rather than
through the plant’s metabolism. Thus, studies on artificial
media (usually agar) in the absence of host plants have shown
that high concentrations of Zn can inhibit growth or
development of microorganisms. Somashekar et al (1983)
demonstrated that 50 ppm of Zn resulted in growth reduction
in Penicillium citrinum by 28%, Cachliobolus
miyabeanus by 89% and Cladosporium cladosporoides
by 12%. Cripps et al (1983) showed that 3 ppm of ZnSO
4
inhibited growth in Trametes versicolor and Stereum
strigosazonatum by 100%, Trichoderma and Alternaria
by 64%. Epicoccum by 43%, but did not inhibit the growth
of Curvularia or Penicillium. Hooley and Shaw (1985)
determined that more than 7.5m M Zn was required to inhibit
one strain (6500P) of Phytophthora dreschsleri by 50%,
but only 5.5 mM Zn was required to inhibit strain 6503IMI
to the same degree. All these results showed that Zn could
have a similar effect on pathogens that attack horticultural
crops. In a survey, it was observed that level of Zn in the
Micronutrients in horticultural crops
J. Hortl. Sci.
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16
soil was lower in soils conducive to root rot
(Phymatotrichopsis omnivorum) of cotton (Smith and
Hallmark, 1987) and infection of ginseng by Pseudomonas
cichorii (Gvozdyak and Pindus, 1988). Although these
reports are correlative, the interpretation is that Zn is
important in reducing Fusarium root rot of chickpea, Cicer
arietinum (Gaur and Vaidya, 1983) and Rhizoctonia
bataticola rot of groundnut, Arachis hypogea (Murugesan
and Mahadevan, 1987).
Boron
Boron has also been reported to have beneficial
effects in reducing plant disease; many of these effects have
previously been reported by Graham (1983), as have the
lack of effects. These are (i) its role in formulation of
carbohydrate-borate complexes controlling carbohydrate
transport and cell membrane permeability or stability (ii) its
role in metabolism of phenolics, with its primary role in
synthesis of lignin (Lewis, 1980). Since then, B has also
been shown to reduce diseases such as clubroot of cabbage,
caused by Plasmodiaphora brassicae in Sweden
(Vladimirskaya et al, 1982) and other crucifers (Dixon and
Webster, 1988); Fusarium solani in bean Phaseolus
vulgaris L. (Guerra and Anderson, 1985); Verticillium albo-
atrum in tomato (Dutta and Bremner, 1981); Rhizoctonia
solani in mungbean, pea and cowpea (Kataria, 1982;
Kataria and Grover, 1987); Rhizoctonia bataticola in
groundnut (Murugesan and Mahadevan, 1987) and tomato
yellow leaf curl virus in tomato (Zaher, 1985). Boron has
been shown to decrease expression of the potato wart
disease (Synchytrium endobioticum) of potato (Hampson
and Hard, 1980) and club root of crucifers. In both cases,
the disease is expressed by formation of a tumor or a gall;
and in both reports, B decreased the severity of disease-
expression. Boron did not, however, diminish initial infection
of the host.
One consequence of B deficiency is increase in
indoleacetic acid (IAA) concentration because of inhibition
of IAA oxidase activity (Coke and Wittington, 1968),
presumably by accumulation of phenolics. Similar conditions
may occur in tumors and galls. Potato wart tumors have
elevated levels of auxin-like substances (Reingard and
Pashkar, 1959). This increase in auxin (or auxin-like) activity
has been explained by an increase in auxin protectors
(Tandon, 1985). It is worthwhile to note that, in a variety of
tomato in which tumors were not found following successful
infection, tomatine, an auxin antagonist, was present
(Hampson and Haard, 1980). The auxin protectors from
potato wart have been isolated, and shown to contain ferulic
acid and caffeic acids covalently bound to a protein and
chlorogenic acid. Interestingly, both chlorogenic and caffeic
acid have been identified in the necrotic areas of B-deficient
celery, Apium graveolens L (Perkins and Aronoff, 1956),
and ferulic and vanillic acid in B-deficient oil palm (Elaeis
guineensis) (Rajaratnam and Lowry, 1974). In addition, it
is common to use borate as a buffer during plant tissue
extraction to prevent conjugation of phenolics with proteins.
These observations suggest that B may suppress gall or
tumor formation by suppressing high concentrations of auxin
or auxin-like substances. Gall formation itself is not a defense
mechanism of the plant i.e., galls are incited by the pathogen
(Dixon, 1984). Thus B, by suppressing high levels of auxin
or auxin-like substances, may also suppress gall formation.
There is no conclusive evidence that explains how
B decreases disease caused by vascular pathogens. The
association of B with lignin synthesis (Lewis, 1980) makes
it tempting to suggest that B may suppress infection of the
stele by a lignified physical barrier at the endodermis. It
may be argued that this barrier would be of little consequence
if, as suggested by Mai and Abaci (1987), Fusarium enters
just behind the root cap, a region of the root with little
lignification. However, if B deficiency weakens the
lignification of other parts of the root system, successful
infection may be more likely at other locations on the root
axis. Indeed, Dutta and Bremner (1981) observed that B
depressed the symptoms of Verticillium wilt in tomato, and
roots of B-supplied plants showed no vascular discoloration.
This suggests that B inhibited invasion of xylem by the
pathogen.
Iron
Older literature sheds little light on the role of Fe in
disease resistance, since it is relatively sparse compared
with that of Cu, Mn and B. However, the sophistication of
microbial Fe-acquisition systems suggests that microbes
have a high requirement compared to higher plants and
higher utilization efficiency. In this respect, Fe appears to
stand in contrast to Cu, Mn and B, for which microbial
requirements are relatively low. Addition of Cu, Mn and B
to deficient soils generally benefits the host, whereas the
effect of Fe fertilization in disease resistance cannot be
predicted. Copper was antagonistic to Fe in the F.
oxysporum f. sp. lycopersici, although lycomarismin has
no known function in Fusarium wilt disease. Fe decreased
tolerance of Fusarium wilt in tomato without affecting
development of the fungus (Waggoner and Dimond, 1953).
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Vol. 4 (1): 1-27, 2009
Edward Raja
17
Iron stimulated and Cu inhibited spore germination (Strakhov
and Yaroshenko, 1959; Halsall and Forrester, 1977; Vedie
and Le Normand, 1984). Though its key role in oxidative
phosphorylation is known, Fe is directly or indirectly involved
in all plant synthesis, but especially high Fe requirement for
syntheses of the phytoalexin wyerone is of interest in the
present context (Swinburne, 1986). Iron is essential for
production of the host-attacking exoenzymes of fungi, such
as pectin methylesterase of Fusarium oxysporum
(Sadasivam, 1965) and endo- and exo-glucanase by Phoma
herbarum, a leaf spot pathogen of peanut (Shinde and
Gangawane, 1987).
Molybdenum
There have been few reports associating Mo with
response of plants to disease and no reports have been found
to specifically address effects of Mo deficiency (Graham
1983). However, Dutta and Bremner (1981) demonstrated
that Mo applied to tomato roots reduced the symptoms of
Verticillium wilt. Miller and Becker (1983) also reported
that Mo suppressed Verticillium wilt in tomato. Molybdenum
had a direct effect by reducing production of roridin E, a
toxin produced by Myrothecium roridum (Fernando, 1986)
and in slightly inhibiting zoosporangia formation by
Phytophthora cinnamomi and P. dreschleri (Halsall and
Forrester, 1977). Soil-application of Mo decreased nematode
populations (Haque and Mukhopadhyua, 1983). It is not
known whether Mo within the host plays any specific role
in protecting plants from disease. Because of the
requirement for Mo by the enzymes nitrogenase and nitrate
reductase, any effect of Mo deficiency on pathogenesis may
be indirect through an effect on N metabolism (Shkolnik,
1984).
Silicon (Si)
Although Si is not regarded as a full-fledged
essential element for growth of higher plants, it is evident
from recent work that it plays a critical role in biochemical
pathways leading to resistance to certain pathogens. Adatia
and Besford (1986) observed that cucumber powdery
mildew that could not be controlled by repeated application
of fungicide, could be controlled by silica. Meyer et al (2008)
observed that silicon helped in powdery mildew control in
grape, strawberry and cucumber, but in gerbera, the reduced
uptake limited its role in disease. In view of this, judicious
use of silicon (separately, or along with biopesticides) and
balanced nutrition can control diseases in horticultural crops
in an integrated way.
Role of micronutrients in improving post-harvest life
and marketability of horticultural produce
Low storability of horticultural produce is well-
known in a tropical country like India, but, the energy
intensive cold-storage units escalate cost of storage.
However, the capabilities of some micronutrients in
complementing these techniques are well-known. Boron
has a synergistic role with Ca in fruit-tree nutrition, since
both are needed for fruit quality. Brown heart disease of
pear is a serious storage disorder affecting the storage-
life when controlled atmosphere storage is done. Studies
conducted in South Africa indicated that storage life of
mango, citrus, avacodo and grape improved with
increased Ca and B content in the fruit (Kruger et al, 2003).
Wojcik and Wojick (2003) indicated that pre- and post-
harvest B spray on the fruit supplemented with soil B,
resulted in better Ca status in pears and increased storage
life, higher firmness and titrable acidity. The fruits also had
lower membrane permeability and were less sensitive to
internal browning than control fruits. Poor storability of
melons in Brazil was solved by pre harvest spray of Ca and
B from fruit-set to harvest. Storage life could be increased
by higher Ca binding to the cell wall which, increased
methoxylated pectin in cells of the melon skin (Chitara and
Praca, 2004).
India paid a heavy price by losing export market
for Alphonso mango to spongy tissue. Micronutrients can
prevent occurrence of some of the disorders, in that cause
reduction in marketable yield, while simultaneously
increasing the profit of farmers and exporters. Zinc and B
play a direct role in reducing physiological disorders. Zinc
stabilizes membrane permeability and B (by increasing the
mobility of Ca to the fruits. Mn, Cu and Fe) also plays a
positive role by increasing photosynthesis and providing
carbohydrates supply for good Ca uptake. Internal browning
(chocolate) of grape in Brazil was associated with plants
having abnormal yellowing and malformed clusters, with
small berries and dark- brown pulp. This was corrected by
supplying B (0.1% spray at flowering). Edward Raja (2009)
observed that spongy tissue in mango cv. Alphonso, a
calcium-related disorder observed in acid soils of Konkan,
India, could be rectified by correcting severe B and Zn
deficiencies by foliar and soil application. Improving root
health by dolomite application (to eliminate Al toxicity), and
facilitating better light-penetration by pruning, resulted in
enhanced carbohydrate accumulation, and translocation and
better Ca uptake by roots.
Micronutrients in horticultural crops
J. Hortl. Sci.
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18
Bitter pit, a serious physiological disorder in apple,
was reduced by sprays of Zn and Cu (Schmitz and Engel,
1973). The immobile calcium oxalate in fruits is solubilised
by Zn and Ca is released to the fruit. Chvapil (1973) reported
that Zn is tightly bound to cell membranes in leaves and it
has the greatest affinity for cell membranes, followed by
Cu, Fe, Ca and Co. This points out to the possibility of Zn
and Cu sprays releasing Ca from various chelating and
complexing agents. Smith and Green (1982) observed fewer
cork spots in apples with higher B in the fruit-flesh and
more of higher quality fruits. According to Shear (1975),
both soil and foliar spray of B can increase fruit Ca and
reduce Ca-related physiological disorders. Shear and Faust
(1971) showed increased movement of Ca into leaves
sprayed with B. Proper and timely application of
micronutrients can help correct several physiological
disorders of fruits, thereby increasing their marketability.
Role of micronutrients in enhancing nutritional
security through horticultural crops
Horticulture products are protective tools which
have to be nutrient-dense, since, nutritive value of the cereals
is deteriorating due to growth-dilution and declining soil-
health. Horticultural crops have minerals in better available-
form compared to cereals like wheat and rice. Enhancement
of nutritional security by biofortification and organic farming
is gaining ground in the present agricultural paradigm. Fe
and Zn deficiencies are becoming public- health issues.
These can be addressed to a great extent by consumption
of fruits and vegetables. Seventeen minerals are needed
for human health and even boron has qualified for 5 out of
6 criteria for essentiality in human nutrition (3rd International
conference on Boron, 2005). Phloem-immobile
micronutrients like Fe, B, V and Cr cannot be increased in
food crops by spray-application. According to Welch (1997),
macronutrient treatment can influence concentration of beta-
carotene and micronutrients in carrot. Root vegetables from
acid soils have adequate Fe, Zn, N, Cu, Mo and Se for better
human health. Biotechnology can play a major role in human
nutrition by producing tangerines rich in micronutrients.
Horticulture must change in ways that will closely link food
production to human health and nutritional requirements.
Holistic food system models hold promise in providing
sustainable interventions to these complex nutrition and
health problems. Sustainable solutions to micronutrient
malnutrition can only be found in forming a nexus between
agricultural production and human health. The magnitude
of the problem is so great that we must use every tool at
our disposal to eliminate this scourge from the world.
Crop phenology and micronutrients
The mean crop removal of all micronutrients does
not exceed 600 g/ha, whereas the total micronutrients exceed
more than several tons/meter depth, the maximum feeding
zone of any horticultural crop. This opens up great potential
for solving the problem of micronutrient disorders of
horticultural crops in India, since micronutrient reserves in
Indian soils are adequate for thousands of crops. When 5
ppm Zn can be adequate for 2500 crops of wheat in
Australia, it is definitely adequate for 10000 crops of mango
(considering its very low removal at 44 g/ha). It should be
clear from Table 9 that perennial fruit crops like mango and
grape remove much less micronutrients than to vegetables
which, in turn, remove much less than a wheat crop does,
since, dry matter produced by a cereal crop is much higher
than that by fruits or vegetables.
Though all micronutrients are essential for the entire
crop growth, each nutrient is needed at some phenological
stage of growth in larger quantity, to maintain crop
productivity. Boron is needed both at the time of planting or
sowing for root growth, at the reproductive stage for
pollination and, at maturity stage, to avoid fruit-drop, cracking
and, also, for mobilization of calcium for better shelf-life.
Since it is highly immobile in the plant, it is continuously
needed. But, reproductive parts need more B than do
vegetative parts (Rerkasem et al, 1996). It is better to give
a foliar spray at pre-bloom for pome fruits and, a pre-bloom
and post-bloom spray for other fruits. Since B is phloem-
mobile, in some fruits like apple, one spray at flowering is
sufficient, whereas, for B immobile crops like mango, pre-
bloom and post-bloom sprays are essential (Edward Raja et
al, 2005).
Though Fe is needed throughout the plants life, Fe
nutrition becomes a problem at flowering due to poor
photosynthate supply to roots. Root-health is very important
in iron and calcium nutrition, since these are taken up in the
Table 9. Micronutrient removal (g/ha) by major horticultural crops
Crop Zn M n Fe Cu B M o
Mango 44 68 150 13 28 2
Papaya 68 110 140 22 48 3
Banana 110 380 190 14 68 4
Grape 130 240 180 22 62 4
Tomato 110 180 210 48 64 7
Cabbage 140 220 240 22 68 7
Beans 95 128 120 48 48 4
Mean 99 189 176 27 55 4
Source: Cakmak (1993), South Africa Mango Growers Handbook (2002,
2003)
J. Hortl. Sci.
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Edward Raja
19
root-tip region only. Spray of Fe as 0.5% FeSO
4
at flowering,
along with B, is always helpful. For leguminous vegetables,
it is needed at seed-sowing for nodulation (Hemantranjan,
1998). Since Mn is also phloem- immobile, it needs to be
continuously available to the plant, more so, at fruit-set (since
it is essential for photosynthesis). Leaves adjacent to fruits
are important for fruit growth and their Mn level needs to
be at an optimum. An acidified rhizosphere always ensures
enough Mn. Nitrate N should be avoided at flowering. Since
Mn is important for disease resistance, a continuous supply
keeps the plant healthy. Copper is also needed, more at the
reproductive stage than at the vegetative stage. It is immobile.
Protection of plants with copper fungicide at flowering
ensures copper availability for reproductive growth.
Molybdenum is partially mobile and is needed for nodulation
in legume vegetables. It is needed more in the early stage
of crop-growth and in crops that need high N, like banana/
tomato that need it more in acid soils. If seeds are enriched
with Mo or seed-dressing is done, there is less need for Mo
at the late stage of a crop. Zinc is also a partially mobile; it
is required at an early stage of crop growth or during early
establishment of tree crops. In sensitive crops like grape,
mango and citrus, a spray (0.3% ZnSO
4
) at pre-bloom,
followed by a spray at the reproductive phase, is helpful,
since it can protect leaves and fruits from reactive oxygen
species (Cakmak, 2000). In situations where topsoil is
removed by leveling during cold season, Zn deficiency
affects crop establishment. Hence, Zn is needed more at
the early and late stages for fruit membrance stability and
to mobilize Ca for preventing physiological disorders.
Micronutrient toxicity
Micronutrient excess is as much a problem as
deficiency and skill is needed in micronutrient correction.
Since farmers are prone to using excess micronutrients, this
creates a problem rather than solving a problem. Some of
the common toxicity problems encountered with
micronutrients are discussed below. Boron toxicity occurs
due to saline irrigation water and saline soils. Citrus and
beans are extremely sensitive to B toxicity. Copper toxicity
occurs due to natural pollution by mine ores or
anthropogenical reason due to use of fertilizers and
fungicides. Copper accumulates more in the root and
damages it more than the shoot. It induces K, Ca, Mg and
Fe deficiencies and causes Fe chlorosis.
New approaches in micronutrient nutrition
According to Marshner (1995), rhizosphere
modification in some crop species by Type I and Type II
mechanisms is a Fe-stress response for better iron nutrition.
In this process, a plant spices very susceptible to Fe
deficiency may be grown adjacent to a Fe efficient genotype,
to benefit from the rhizosphere modified with higher
available iron. A banana cultivar with high available Mn in
its rhizosphere can benefit an acid lime crop susceptible to
Mn-deficiency. Thus, rhizosphere modification can help in
micronutrient nutrition of crops by complementary existence.
Bio-fortification in horticultural crops
Nearly 40 – 50% of world population suffers from
Fe and Zn deficiencies (Welch, 1999, 2002). Bio-fortification
of food crops by breeding is one of the priority research
areas of breeders (Welch and Graham, 2004). But
bioavailability of Fe and Zn in grains is low due to the
presence of phytic acid, which reduces their uptake in the
digestive system (Cakmak, 2002). Due to low phytic acid
content in fruits and vegetables, horticultural crops lend
themselves for bio-fortification. Iyengar and Edward Raja
(1988) observed some vegetables like okra getting enriched
with Zn in pods (edible portion) when zinc level in soil was
high. Hence, exploiting fruits and vegetables not only for
vitamins but also for critically deficient Fe and Zn, has a
great potential in addressing nutritional security of the nation.
It is well- known that plants differ in their efficiency for
uptake of nutrients from soils and in certain situation, options
other than fertilizer-application (to correct micronutrient
disorders) are considered. In tackling iron chlorosis
(especially, induced chlorosis), breeding resistant varieties
in soybean and sorghum is on for the past 25 years. Breeding
is considered in the following situations :
1. When the cost of correction is high
2. When the method of correction is difficult
3. When the deficiency affects yield and quality very
severely (Liebig stress)
4. When agronomic correction may result in environmental
pollution
5. When agronomic correction produces produce with low
nutritional value
Discussing the present technologies and future
prospects it can be observed that sustainable and cost
effective correction of Fe deficiency is possible by
developing Fe efficient cultivars or rootstocks for sensitive
crops. Breeding for tolerance to zinc deficiency has been
well-identified for managing Zn deficiency in beans
(Hacisalihoglu et al, 2004). Another alternative is
development of transgenic crops. The Zn transporter protein
Micronutrients in horticultural crops
J. Hortl. Sci.
Vol. 4 (1): 1-27, 2009
20
of Arabidopsis was transferred to barley, which resulted in
correction of zinc deficiency (Ramesh et al, 2004). For
enriching Fe content of rice seeds, transgenic rice was
developed with ferritin gene, but increase in Fe content was
inadequate (Qu et al, 2005).
Micronutrient management in Acid Soils
The western coast of peninsular India and parts of
eastern and northeastern India has acidic soils, which need
to be managed differently for micronutrient disorders. In
these soils, aluminium and manganese are present at toxic
levels (Pandey et al, 1994), while Mo and B are usually
deficient (Edward Raja et al, 2005). Other micronutrients
like Zn, Fe, Cu, are adequately available in these soils. Since
B is the only mobile micronutrient in soil, it gets lost by
leaching (like nitrogen). Since acid soils are distributed in
high-rainfall regions, its deficiency is a perennial problem.
But, the fact is that B-uptake by plants at identical water-
soluble B content was greater at lower soil solution pH
(Wear and Patterson, 1962). Hence, plants can manage with
lower soil B in acid soils, than in high pH soils (calcareous
soils). But, due to loss by leaching of fertilizer, slow release
B sources like colemanite have a large potential for B-loving
crops like, mango. Vegetables like cauliflower, carrot and
turnip need to be supplied with adequate B. Deficiency of
molybdenum is a problem in acid soils. But, liming for
eliminating Al-toxicity can increase available Mo in soil and
solve the problem. Use of molybdenum-rich seeds of legume
vegetables, or seed treatment at sowing, also mitigates the
problem.
Foliar nutrition in micronutrient: An option or
management compulsion in horticultural crops
Hamilton et al (1943) were the first to establish
potential of foliar nutrition in field- level nutrition
management, by proving the influence of urea spray the N-
nutrition in apple. Initial research on the potential of this
technique was confined to supplying macronutrients like
NPK, since, deciduous crops like apple (whose root systems
are inactive throughout winter and early spring) have to be
compensated for the time lost. This became all the more
important when high-yielding clones were introduced and
advances in horticultural technology doubled the yield of
apple and there was a need for providing the extra nutritional
requirement. Controlling physiological disorders by directly
spraying on the fruits is also widely followed in apple. But,
in India, foliar sprays are still optional as their vast potential
is yet to be recognized in terms of increased yield, quality
and post-harvest life.
Crop-specific micronutrient foliar formulations
Each crop is specific in its micronutrient
requirement, based on its metabolic requirement, capacity
to modify its root-soil interface, exploit the rhizosphere-soil
nutrients, better root geometry, faster specific rate of
absorption at low concentration (low Kw), improved internal
root-distribution and superior utilization or low functional-
requirement for the nutrient (Graham et al, 1992). In the
alfsoils of IIHR, Bangalore, abundant in available Mn, three
crops growing side-by-side exhibited contrasting response
to Mn application. While acid lime exhibited deficiency,
guava exhibited sufficiency and banana exhibited toxicity
for Mn (Edward Raja, unpublished data). The most scientific
approach would be to identify micronutrient disorder at the
micro-farming level by leaf and soil analysis and suggest a
farm/site specific recommendation. In the Indian context,
with more than 2 million farm holdings in horticulture, it is
impossible to provide farm-specific advice, due to lack of
infrastructure for leaf/soil analysis and lack of manpower
to interpret it. Hence, crop-specific foliar micronutrient
strategy was proposed as one of the strategies to overcome
this problem (Edward Raja, 2009) This involves identification
of the predominant micronutrient disorder of a crop in agro-
ecologically similar regions, developing a micronutrient
formulation, incorporating the deficient nutrient in proportion
to intensity of the deficient nutrition. This is similar to iodine-
fortified common-salt promoted for public health. This
concept is totally different from the existing market for
micronutrient foliar formulations in India, which have
following basic inadequacies :
1. All the existing market micronutrient formulations in
India are meant to correct Zn deficiency, which is the
predominant disorder in rice, wheat and maize, whereas,
the predominant micronutrient disorder affecting both
vegetative and reproductive growth in horticultural
crops is that of Boron deficiency. This is a basic and
fundamental flaw in the existing foliar micronutrient
correction strategy in India
2. Reproductive parts need 2-3 times more B than do
vegetative parts. Hence, foliar spray of B is a must
3. In rainfed crops like mango, foliar spray is the only
efficient method for correction of micronutrient
disorders
4. In view of chemical, physical and biological soil
degradation, root health will become a problem in the
future and foliar nutrition will gain significance
5. Soil-applied micronutrients like Zn, Fe, Cu and Mn have
J. Hortl. Sci.
Vol. 4 (1): 1-27, 2009
Edward Raja
21
low efficiency (3-5%), whereas, foliar nutrients have
an efficiency of 20-40%. In view of this foliar nutrients
having micronutrients are more of a compulsion than
an option, especially in crop- specific foliar formulations
Micronutrient management in organic farming of
horticultural crops
Organic farming has an in-built advantage of
providing balanced nutrition especially for micronutrients,
since, presence of adequate organic matter makes them
available to the plant. Except copper, all micronutrients are
available adequately in organically-farmed soils.
Why micronutrient disorders are not common in organic
farming ?
1. Moderate and severe micronutrient disorders are
uncommon in organic farming since crop growth
rate is not as fast as in conventional farming. In the
latter, application of fertilizers of high nutrient content
(urea: 44% N; DAP: 46% N) results in accelerated
growth rate.
2. High organic-manure (FYM+ Vermicompost)
application results in many organic acids, which
complexes micronutrients in the soil (especially Fe, Mn,
Cu, and Mn) and makes available to plants. Humic acid
and fulvic acid levels are very high, resulting in adequate
available micronutrients.
3. In organic farming, balanced nutrition is achieved by
avoiding extreme nutrient deficiencies like P-induced
Zn deficiency, N-induced B deficiency and heavy-metal
induced Fe deficiency. Due to crop residue recycling
and application of composts like vermicompost, nutrient
reserves are recycled and made available to the plant.
Soils have Zn, Cu, Mo Fe and Mn in abundant quantity.
But since crop residue recycling is the basic credo of
organic farming, micronutrient depletion does not occur.
Deficiency of B is likely to be encountered in organic
farming practiced in high-rainfall areas, with coarse soil
texture and an undulating topography.
4. The rhizospheric pH is maintained near neutral (<>) in
organic farms due to crop rotation, avoiding some
organic inputs which form acid in soils, thereby resulting
in better micronutrient nutrition.
5. Microbial activity, is very high and this also releases
minerals from soil.
6. Root health is very good in organic farming, and
therefore, nutrient uptake is higher.
7. Toxic elements like Al, Mn and Na are generally absent.
8. Less leaching-loss or run-off loss of B due to better
soil structure.
9. Root-penetration upto deep sub-soils, which thereby
supplies micronutrients.
Techniques to enhance micronutrient uptake in
organic farming
1. Use of mycorrhiza (VAM) for mobilization of Zn
and P
2. Enriching FYM with rock phosphate releases Zn, Ca
and Mg/ s into soil
3. Use of gypsum for supplying calcium and sulphur
4. Use of dolomite provides Ca and Mg wherever needed
and increases availability of molybdenum
5. Since mango is highly susceptible to Fe deficiency, use
13-1, Fe-deficiency resistant rootstock from Israel when
mango is grown in calcareous soils
6. Use of Boradeaux mixture/Burgundy mixture should
be encouraged for controlling diseases in soils of high
pH, so that copper deficiency is eliminated
7. By leaf analysis, the limiting micronutrient is identified
and a spray of such micronutrients can be given (Zn,
Mn, Fe, Ca, B and Mo) in consultation with an organic
certifying agency
8. Use of neem cake is recommended in high pH
calcareous soils, since it can recycle soil Fe, Mn, Zn,
and Cu and make them available to the plant by
rhizosphere acidification. Fe deficiency in fruits and
flower crops has been corrected by this method. Neem
decoction can be used for drenching, if chlorosis is seen
When a farm is converted from a conventional one
to organic-farming system certain modifications have to be
made in nutrient management to overcome problems caused
by the earlier system. A 3-year period is required to correct
the system. Conventional farms have depleted organic
matter and high available-phosphorus, which creates
problems of availability, uptake and translocation of
micronutrients. Increasing N and K to the level of P is one
method; another is to encourage availability, uptake and
translocation of micronutrients by increasing organic-matter
status. The key for exploiting the enormous micronutrient
reserves of Indian soils is to increase the organic matter
status through on-farm and off-farm bulky, organic inputs
viz., crop residues and green manures. Crop residues like
fallen leaves, pruned crop-wastes etc., are to be used for
increasing soil organic matter status. Instead of adding
inorganic P fertilizer to soil, FYM can be enriched with rock
phosphate at 20% ratio (so that excess P in the soil is
Micronutrients in horticultural crops
J. Hortl. Sci.
Vol. 4 (1): 1-27, 2009
22
avoided) thereby reducing the risk of P-induced zinc
deficiency. The increase in organic-matter status itself
increases availability of soil micronutrients owing to the
chelating ability of humic and fulvic acids in the compost.
Micronutrients and future challenges in horticulture
production
Though some more essential micronutrient is added
to the existing list, it is doubtful if they will have as much
importance as the already identified ones. Hence, it is
essential to tap expertise in diagnosis and treatment of
micronutrient deficiencies and toxicities. Experimental
techniques for detecting micronutrient disorders need to be
refined. Analysis of leaves for manganese and boron
deficiency detection presents a severe problem, since old
leaves accumulate B and Mn in the margins and give a
wrong picture. Delayed sampling of deficient leaves also
presents a problem in diagnosis. Fertilizers and additives
affect availability of micronutrients indirectly and these
problems are attributed to other factors. The present strategy
on micronutrients revolves only around increasing the yield
of horticultural crops. Farmers’ interest will be taken care
of when micronutrients are used not only to increase
biological yield, but also marketable yield, by improving
quality and post- harvest life, ultimately bringing profits to
the farmer. As discussed earlier, the role of B. Zn, Mn and
Fe is paramount in realizing this objective. Public interest
will be adequately taken care of if it receives horticultural
produce of high quality, since worldwide, clinical and sub-
clinical deficiencies of these micronutrients have been
noticed (Cakmak, 2008). Besides, pesticide residues are
increasing to harmful levels due to exclusive dependence
on curative management by chemical pesticides. A shift to
preventive management, using balanced lignin biosynthesis
and preventing oxidative stress nutrition and using Mn, B,
Zn and Cu, as part of Integrated Disease and Pest
Management will go a long way in implementing “value
addition” at the farm level. To operationalize the strategy,
following action is required:
1. Micronutrient correction should be done by mobilizing
soil reserves of Fe, Zn, Mn, Cu and B by humidified
organic manures (vermicompost)
2. Use of Zn-enriched NPK fertilizers (like iodized
common salt) as was done in Turkey (Cakmak et al,
1999). This will also simultaneously enhance the use-
efficiency of NPK fertilizers due to removal of Zn
deficiency on mild or moderate stress [Mitscherlich
type or Bevere stress (Lieberg type)]
3. Use crop-specific foliar formulations for correction
of predominant micronutrient disorders as a
complementary strategy to supply from soil and also
as disease tolerance strategy due to balanced nutrition
4. Agronomic bio-fortification by foliar spray and
increasing soil availability of Fe and Zn so that
consumers get fortified value-added food at reduced
cost (Cakmak, 2002)
5. Increased shelf-life/post-harvest life by directly
enriching fruits with Ca and B to reduce dependence
on energy consuming cold-storage systems. It can
be also part of integrated post-harvest management
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