Micronutrients in Crop Production
Micronutrient deficiciencies are easy to treat but difficult to diagnose. The symptoms mimic those of disease, or may even be caused by disease. The young citrus tree shown above suffers from zinc deficiency induced by citrus greening disease. Greening bacteria have blocked the circulation of sap, so that zinc taken up by the roots cannot reach the rest of the plant.
At the same time, yields are higher than they were in the past. With each cropping season, micronutrients are removed in the harvested crop and are not being replaced. The result of this mining of nutrients is widespread micronutrient deficiency problems all over Asia. These include shortages of zinc, iron, molybdenum, boron, manganese and copper. Some common forms are as follows:
Deficiency Symptoms
Boron Deficiency
This is widespread in many countries, including the Philippines, Thailand, Korea, Malaysia and Japan. It is more common in volcanic soils, or in soils derived from igneous rocks. It is not often found in soils derived from sedimentary rocks such as sandstone.
Some of the symptoms of boron deficiency in fruit trees are described on the opposite page. In annual crops, the symptoms vary from one species to another. Black gram deficient in boron does not show any visible symptoms in the seed, but the yield may fall by as much as 50%. In peanut or soybean, boron deficiency often results in an empty space within the seed known as "hollow heart". A common result of boron deficiency in all crops as an interruption in flowering and fruiting. Yields are poor, and the fruit or grain is deformed or discolored.
Boron deficiency in fruit trees
The production of orchard fruits is a growing industry all over the Asian and Pacific region. Boron deficiency is common. In apples this produces the symptom of corking in the fruit. Affected trees show die-back of shoots, or the leaf tips may be distorted into a rosette shape.
The problem of boron deficiency can be easily solved by applying boron, usually in the form of borax, a white crystalline salt. The quantities needed are very small. Farmers must be careful not to apply too much, or trees may suffer from boron toxicity. If growers of perennial crops such as fruit trees keep applying boron every year, there is a danger that boron will build up in the soil and poison the plant.
A typical symptom of boron toxicity is that the leaves curve backwards. Chlorosis of leaves and yellow leaf veins is common. Fruit from apple trees suffering from boron toxicity may become brown inside during storage.
Iron Deficiency
Iron deficiency is common in leached tropical soils, particularly in calcareous soils derived from limestone. Legumes are particularly sensitive. The main symptom of iron deficiency is chlorosis or yellowing between the veins of new leaves.
Zinc Deficiency
Zinc may be present in the soil, but not available to plants. A high soil pH means that zinc is less soluble, so that crops may suffer from zinc deficiency. This tends to result in stunted growth and small leaves. Rice grown in zinc-deficient soils in Taiwan showed small brown spots, and root development was poor. Fruit trees deficient in zinc may have a growth at the end of shoot tips which looks rather like a rosette. Citrus trees often show chlorosis between the veins of the leaves, a condition sometimes known as "mottle-leaf" (see page 1).
Diagnosis Is Difficult
These assorted symptoms show that it is difficult to diagnose a micronutrient deficiency from the symptoms alone. In fact, sometimes there may be no symptom at all except a reduction in yield. This type of latent or hidden deficiency may be suspected if crops do not respond to applied fertilizer. All nutrients must be optimized for the best yields. If one nutrient is lacking, it negates the value of all the others. To make diagnosis more difficult, crops often suffer from multiple micronutrient deficiencies, not just one.
Moreover, the symptoms often mimic those of diseases, especially virus diseases. One difference between virus and deficiency symptoms is the way they appear. Virus disease tends to appear in one part of the field and spread out from there. Deficiency symptoms appear and develop all over the whole field fairly evenly.
Farmers need laboratory tests to be sure of a diagnosis, and these are expensive. However, farms in a region which share the same kind of soil tend to share the same plant nutrient problems. There is no need to test on a farm-by-farm basis, although this is ideal if farmers can afford it.
Remedial Action
How can farmers protect their crops from micronutrient deficiencies? Certainly, they should never apply a good dressing of assorted micronutrients "Just in case". Toxicity problems from a surplus of micronutrients in the soil can be even more damaging than a deficit.
The symptoms of toxicity may be as severe, and toxicity is much more difficult to correct. It may take years for leaching to gradually wash the excess nutrients from the soil.
Farmers should apply only the nutrient needed, in the quantity needed by the crop. Application rates are very low. A typical boron application, for example, is one kilogram per hectare. Critical levels in leaves of most micronutrients are only 10-20 ppm (or mg/kg). However, a lack of this very small amount is likely to cause serious damage to the plant. For this reason, if there are micronutrient deficiencies, it is important that they are diagnosed and treated without delay.
In short-term crops such as vegetables, by the time symptoms are marked enough to diagnose deficiency, it may be too late to save most of the crop. In the case of perennial crops such as fruit trees, once corrective measures are applied, the farmer may still lose his harvest. However, he will usually be able to save the crop.
Nutrient Uptake by Plants
A high level of a particular micronutrient in the soil does not always mean that there is enough for the crop. Soil conditions may determine whether a nutrient is available to plants. The condition most affecting availability is the soil pH. Calcareous soils with a high pH have poor availability of iron, magnesium, copper and zinc. In acid soils with a low pH, molybdenum is less available to plants.
Deficiency states may sometimes be corrected, not just by applying the missing nutrient, but by changing soil conditions to enhance availability. In one case of iron deficiency of peanut in Taiwan, applying iron to the soil in the form of ferrous sulfate was not very effective. Foliar applications were more effective, but the best remedy was to apply sulfur well before planting, to lower the soil pH. In cases of boron deficiency, boron applications are sometime more effective if there is also a lime dressing to increase the soil pH.
The Wrong Kind of Microelements
Pollution of Soil with Heavy Metals
Just as soil conditions can be manipulated to increase the availability of nutrients, so they can be manipulated in the other direction, to decrease the availability of nutrients to plants. This is sometimes done with soils which are contaminated with heavy metals such as lead or cadmium.
A number of experiments have shown that amending soils with lime to increase the soil pH can reduce the concentration of lead or cadmium dissolved in the soil solution. This reduces the uptake by plants, and hence the level of lead or cadmium present in the harvested crop. Similarly, the application of phosphate to polluted soils can reduce the solubility of zinc in the soil solution.
Soil treatments of this kind are only an answer if contamination is slight. They do not remove the pollutants, but they may bring the heavy metal content of food crops grown on the soil down to acceptable levels. If contamination is severe, the only remedy is to grow non-food crops such as flowers or high-value timber trees.
Index of Images
Figure 1 Pummelo Tree Deficient in Zinc, Manganese and Iron
Figure 2 Lumpy Papaya Fruit Caused by Boron Deficiency
Figure 3 Boron Toxicity in Citrus
