Micronutrients in Crop Production
International seminar on minor elements and their problems
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.
Micronutrient Deficiencies
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 to farmers.
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.
Current Status of Micronutrients in the Region
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 chicken manure.
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.
How to Correct Micronutrient Deficiencies
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 condition.
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.
Problems in Micronutrient Use
There are two main problems: diagnosis of existing deficiencies, and toxicity brought about by over-correction when a deficiency has been identified.
Diagnosis of Micronutrient Deficiencies
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 or nickel.
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.
Micronutrient Toxicity
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.
Future Research Needs
During the final discussion, participants identified important topics for future research. These were:
- - The physiological effects of micronutrient deficiencies, including their effect on the flowering and fruiting of perennial fruit trees;
- - Low-cost diagnostic techniques that can be used in the field by extension staff or farmers;
- - Sampling strategies for both soils (soil testing) and crops (leaf analysis);
- - Improved laboratory methods of analysis. In particular, it was suggested that the hot-water method of boron extraction can give variable results. A new method was presented at the workshop which uses synthetic resins.
- - Movement of micronutrients in soil and groundwater;
- - Micronutrient requirements of tropical fruit crops such as durian and litchi.
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.
Conclusion
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.
International Seminar on Micronutrients in Crop Production
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
Index of Images
Figure 1 Guava with Iron Deficiency
Figure 2 Lumpy Papaya Fruit Caused by Boron Deficiency
