Micronutrients may be minor in terms of the amounts needed by the
crop, but they can be major in terms of their
impact on crop growth. In order to maximize yields, all nutrients must
be optimized. If one nutrient is lacking, it negates the
value of the others. The response of crops to supplementation, in cases
where deficiency exists, can be very marked.
Deficiency is shown in various kinds of physiological damage, all of which affect the quality and quantity of
produce. There may also be latent deficiency, in which there are no visible symptoms but the crop does not respond as expected
to applied fertilizer.
The Workshop held in 1999 had four main aims: To assess the current status of micronutrients in crop production
in the region; to develop recommendations for extension purposes; to identify research gaps and the need for future work;
and to discuss other relevant issues in micronutrient usage.
Critical levels of micronutrients which are sufficient to allow good growth of plant tissues vary, but are generally
small. Areas where crops suffer from boron deficiency, for example, have a B content (ppm or mg/kg of soil) of less than
0.15 (topsoil) or 0.10 (subsoil). Critical levels in reference leaves are usually in the range 10-20 ppm for most crops.
However, a lack of this very small amount of boron or some other micronutrient in the crop is likely to cause serious
physiological damage, thus retarding plant growth and reducing the crop yield substantially.
The symptoms of deficiency vary according to the nutrient and type of crop. In Thailand, boron deficiency in
black gram, a legume crop very sensitive to boron, causes a 40-50% drop in yield but no visible symptoms in the seed. In
peanut or soybean, boron deficiency often induces an internal empty space in the pods known as `hollow heart". Legumes
sown in boron deficient soil have a poor rate of germination. More seed must be sown, and seedlings are stunted. In apples
grown in Korea, boron deficiency causes internal corking, while shoot tips form a rosette shape. Papaya with boron deficiency
have lumpy fruit. In general, a common consequence of boron deficiency in all crops is an interruption in flowering and
fruiting, so that yields are poor and the fruit or grain is deformed or discolored.
Zinc deficiency tends to result in stunted growth and small leaves.
Fruit trees deficient in zinc often have a
rosette-like growth at the end of shoot tips, while citrus trees also
show inter-veinal chlorosis ("mottle-leaf"). Rice grown in
zinc-deficient soils in Taiwan showed small brown spots which appeared a
few weeks after transplanting. Root development was poor,
and yields were low. Deficiencies of manganese, magnesium and copper
show similar variation in different crops. In the
case of iron deficiency, a universal symptom in all crops is the
chlorosis of young leaves.
Critical levels vary, not only from one crop species to another, but
even in different varieties of the same crop
species. It seems that critical levels of micronutrients should be
established every time a new crop variety is bred and extended
Early detection is important. Otherwise the plant cells may be
irreparably damaged and the yield reduced, even if
the deficiency is corrected at a later stage of growth. In short-term
crops such as vegetables, usually by the time a deficiency
is detected, the damage to plant cells is already done. It is too late
to save the crop, although countermeasures will
save subsequent crops. In the case of perennial crops such as fruit
trees, corrective measures can be applied. The farmer may
lose a harvest, but will usually be able to save the crop.
Field surveys have shown that boron deficiency is widespread in a number of countries, including the
Philippines, Thailand, Korea, Malaysia, Taiwan ROC and Japan. It is interesting that no boron deficiency seems to occur in the
intensively cropped vegetable farms of the Cameron highlands in Malaysia, where farms are fertilized with heavy applications of
The micronutrient content of soils is largely related to the parent
material. Boron deficiency, is more common
in volcanic soils or soils derived from igneous rocks, than in soils
derived from sedimentary rocks such as limestone.
Apart from boron, volcanic soils are usually fairly rich in
micronutrients. In Japan, a volcanic island chain, severe
micronutient deficiencies are rare except for boron, and manganese
which is easily leached. Typically, tropical soils are acidic and
highly leached. Micronutrient deficiencies in tropical countries are
common, and often severe.
Corrective measures once a micronutrient deficiency has been
diagnosed are usually simple and cheap. The
missing nutrient is applied as a fertilizer, either to the soil or as a
foliar spray. Only a small amount is needed. Farmers in Japan
are getting good results from boron in the form of slow-release
fertilizers. Iron deficiency may be difficult to correct, and
there has been some success in selecting rhizobium which are effective
in soil with a low iron content, and inoculating either
soil or young seedlings with them.
Sometimes deficiencies are corrected by amending the soil conditions
which cause them. In the case of iron
deficiency of peanut in Taiwan, foliar applications of ferrous sulfate
were effective, but the best remedy was to apply sulfur well
before planting, in order to lower the pH. Tests of rice with brown spot
disease in Taiwan showed low available manganese,
silica and potassium. Silicate slag was the best amendment for this
In the Philippines, where field surveys showed widespread boron and zinc deficiency, there was a clear response
to applications of boron, zinc sulfate and chicken manure. The general improvement in yields often seen after applications
of composted manure may be partly because of the improvement to soil properties, but may also reflect the correction of
latent micronutrient deficiencies. Perhaps chicken manure could even be seen as an effective micronutrient fertilizer,
although analysis is needed to determine its total nutrient value.
Does this mean that chicken manure should be recommended as a remedy
for micronutrient deficiencies? This
depends on the costs and returns. In some countries, chicken manure is
the best solution. In other countries, foliar fertilizers
are cheaper and more effective.
An important issue is whether micronutrients should be premixed into compound fertilizers. For some crops in
some countries, this might be beneficial. For example, coconut have a high chloride requirement. Many scientists believe
that compound fertilizers for coconut in the Philippines should contain chloride, and also zinc, sulfur and boron. In this
situation, where three million hectares are planted in a single perennial crop, smallholders might benefit from premixed
fertilizers which included micronutrients.
In other situations, premixing of micronutrients would be likely to
lead to toxicity problems. There is also the
economic aspect. Fertilizer companies manufacture fertilizer in bulk,
in order to make a profit. If they have to produce a wide
range of premixed fertilizers with different micronutrient levels for
different crops, the cost of fertilizer would rise.
Smallholders buying fertilizer would have to pay extra for
micronutrients they might not need.
Proper management of plant residues, and good soil and water conservation, are all part of good nutrient
management. Most micronutrients are lost as the result of erosion and leaching.
There are two main problems: diagnosis of existing deficiencies, and toxicity brought about by over-correction
when a deficiency has been identified.
It is not easy to diagnose micronutrient deficiencies from the visible symptoms. Symptoms vary in different soils
and with different crops. Moreover, the symptoms of a micronutrient deficiency are very similar to those of virus
diseases. Experience is needed to distinguish the two. Often, in fact, they cannot be distinguished on the basis of the
symptoms themselves, but only according to the pattern of symptom development. Virus symptoms tend to begin at one spot and
then spread throughout the field. Deficiency symptoms tend to develop over the whole field.
A further problem is that many crops are suffering from multiple micronutrient deficiency, rather than a lack of
only one element. Multiple deficiencies of this kind are very difficult to diagnose. Often too, micronutrient deficiencies
are combined with virus infection. One participant suggested that micronutrient deficiencies may weaken perennial crops,
and make them more susceptible to virus infection.
Laboratory testing for micronutrients, whether of soil or plant tissues, is a more reliable indicator of a deficiency
state than visible symptoms. Laboratory tests can also identify deficiency states which do not produce visible symptoms, but
inhibit fertilizer response and depress yields. However, testing for micronutrients is expensive. Most small-scale
farmers cannot afford it.
Furthermore, a high level of a particular micronutrient in an
agricultural soil does not mean there is enough for
the crop. Sometimes a minor nutrient is present, but soil conditions
mean that it is not available to plants. The condition
most affecting availability is the soil pH. Calcareous or alkaline soils
have poor availability of iron, magnesium, copper and zinc.
In soils with a low pH, molybdenum is less available to plants, while
on soils with a high pH, boron is less available.
Iron deficiencies can also be induced by an excess of magnesium, copper
As well as a high pH, boron availability also decreases if soils are coarse in texture, or if soils are dry. One
example was given at the Workshop from Nagasaki Prefecture in Japan of carrots which developed a rough, blackened skin as a
result of boron deficiency after two years of relatively low rainfall. Carrots have a relatively high boron requirement, and
those in Nagasaki were unable to absorb enough boron because the soil was too dry. Similarly, the lumpy fruit typical of
boron deficiency in papaya are more common on latosols and old slate soils in Taiwan if young trees are planted in dry soil
over the summer.
Production of horticultural crops in Asian countries with a cold
winter is often carried out in
greenhouses. Greenhouse soils generally receive heavy fertilizer
applications. There is a common problem of salt accumulation,
particularly phosphate. This reduces the availability of
micro-nutrients such as iron, manganese, copper, zinc and boron,
which are converted to an insoluble form.
A new field of study is the manipulation of contaminated soils, in order to make heavy metals less available
to plants. Most industrialized countries have areas where the soil has been contaminated by industry. In Taiwan, scientists
are studying the treatment with manganese oxide of soils contaminated with cadmium and lead, in order to convert
the contaminants to less available forms.
Most micronutrient deficiencies are easily corrected by the
application of small quantities of fertilizer.
Molybdenum deficiency in legume crops in Thailand, for example, was
corrected by application rates of only 0.25 kg/ha. In cases of
boron deficiency, borax is usually applied at a rate of just over one
kilogram per hectare.
However, once a deficiency has been diagnosed, farmers tend to apply the needed nutrient repeatedly each year,
often at higher than recommended rates. The aim is to protect the crop from future deficiencies. The result, however, can be
toxicity problems which may do even more damage than the original deficiency.
In Korea, some apple orchards are suffering from boron deficiency,
while others are suffering from excess
applied boron. In general, boron levels in Korea are not so high as to
cause toxicity problems, but some orchards do show
the characteristic symptoms of chlorotic leaves, leaves arched
backwards, and necrosis of the shoot epidermis of scions
above the graft. Fruit are smaller and have a shorter storage life,
often developing internal browning after harvest.
Manganese toxicity is also seen, especially in soils with a high calcium
content. The main symptom in apple trees is necrosis of the bark.
Boron toxicity has been found in rice in the Philippines, where the
main symptom is brown necrotic spots on the leaf
tip and margins. The boron in this case seems to originate in
geothermal springs rather than applied boron fertilizer.
Toxicity problems may be even more serious than micronutrient
deficiencies. A state of deficiency can be
corrected quickly. Toxicity may take a long time to correct. It may take
years for an excess nutrient to be slowly leached from the soil.
During the final discussion, participants identified important topics for future research. These were:
Participants also recommended that the Center publish an extension manual on the diagnosis and correction
of micronutrient problems. The Center has now begun to prepare this.
Micronutrient problems are a serious constraint to productivity in the Asian and Pacific region, and are tending
to become more serious year by year. Most farmers now depend on mineral fertilizers as a nutrient source. They are
applying macronutrients in fertilizer, but not micronutrients. High-yielding varieties are removing large quantities of trace
elements in the harvest which are not being replaced.
Furthermore, micronutrient deficiencies are difficult to diagnose.
Symptoms vary from crop to crop, and
overlap with the symptoms of virus disease, while laboratory tests are
expensive. Latent deficiencies have no visible symptoms
at all, but crops will not respond well to fertilizer treatments. Yields
will be suppressed until the deficiency state is corrected.
Although deficiencies are usually easily corrected by a small application of the missing nutrient, there is not
much difference between the levels needed to correct a micronutrient deficiency, on the one hand, and the levels which
produce toxic symptoms in plants.
Held at National Taiwan University on November 8-13
No. of countries participating: 6 (Japan, Korea, Malaysia, Philippines, Taiwan ROC, Thailand)
No. of papers: 11
No. of participants: 20 plus 50 observers
Co-sponsors: National Taiwan University, Council of Agriculture, Executive Yuan
Figure 1 Guava with Iron Deficiency
Figure 2 Lumpy Papaya Fruit Caused by Boron Deficiency