The Functions and Critical Concentrations of Micronutrients in Crop Production
Function and Role of Micronutrients
The seven micronutrients or trace elements discussed in this book are as important to plant growth as the levels of macronutrients in the soil. Micronutrients are required only in very small quantities. Their concentrations in plant tissues are only a small proportion of the concentrations of macronutrients.
The functions of soil micronutrients for crops are listed in Table 1 (from Brady and Weil 1999). Micronutrients play many complex roles in plant nutrition, but most of them are used in the functioning of a number of enzyme systems. However, there is considerable variation in the specific functions of the various micronutrients in plants and in microbial growth processes. For example, copper, iron, and molybdenum are an essential part of the complex reactions which make up photosynthesis and many other metabolic processes. Zinc and manganese function in many plant enzyme systems as bridges. They connect the enzyme with the substrate upon which it is meant to act.
Since most micronutrients are relatively immobile in the plant, they are not readily transferred from older leaves to younger ones. Therefore the concentration of the nutrient tends to be lowest in younger leaves. Symptoms of deficiency are most pronounced in young leaves which develop after the supply of the nutrient has run low.
Sources of Micronutrients
Sources of the seven micronutrients vary markedly from one area to another. The wide variability in the content of these elements in soils is shown in Table 2 (Brady and Weil 1999).
All of the micronutrients are found in varying quantities in igneous (volcanic) rocks. Two of them, iron and manganese, are common in silicate minerals, such as biotite and hornblende. Others, such as zinc, may also replace some of the major constituents of silicate minerals, including clays.
Chlorine, by far the most soluble of the group, is added to soils in considerable quantities each year through rainwater. It is also added to soil in fertilizers and in other ways. These natural sources help prevent chlorine deficiency under field conditions.
Organic matter is an important secondary source of some trace elements. Several tend to be present as complex combinations in humus. Copper, in particular, is often tightly held by organic matter, to the extent that its availability can be very low in organic peat soils (Histosols). Soil profiles of uncultivated soil tend to have higher concentrations of micronutrients in the surface soil, much of them presumably in the organic matter. There is a certain connection between levels of soil organic matter and the content of copper, molybdenum, and zinc. Although the elements present in the organic matter are not always readily available to plants, their release through decomposition is undoubtedly an important fertility factor. Animal manure is a good source of micronutrients, many of them being present in organic forms.
Deficiencies and Toxic Concentrations of Micronutrients in Soils and Plants
Research has indicated that background levels of micronutrients in soils and plants vary widely. So do the levels likely to cause symptoms of deficiency or toxicity, as shown in Table 3 and Table 4.
Index of Images
Table 1 The Functions of Micronutrients<BR>
Table 2 Major Sources of Seven Micronutrients, and Usual Content of These Nutrients in Soils, and in Harvested Crops
Table 3 Concentration of Micronutrients in Soils and Plants
Table 4 Deficiency, Normal, and Critical Toxicity Levels of Micronutrient Elements in Plants
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