Extensive research has been conducted to investigate the nutritional requirements of different crops and ways to enhance soil fertility, which has greatly benefited agricultural production.
Even though people have known for more than two thousand years that adding mineral elements, such as plant ash or lime, to the soils can improve plant growth, the systematic study of plant nutrition is a relatively young science, considering humanity’s long history of cultivating crops. About 250 years ago, farmers and gardeners started to ask the question, “What makes plants grow?”
It was widely believed that soil humus, a brown or black organic substance resulting from the partial decay of plant and animal matter, provided plants with carbon for making sugar and starch, and substances such as saltpeter, lime, and phosphates simply helped the humus to be more useful.
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It was not until around 1840 that the German chemist Justus von Liebig (1803-1873) helped to compile and summarize the scattered information on the importance of mineral elements for plant growth, and plant nutrition began to be established as a scientific discipline. Since then, great progress has been made in the study of plant nutrition.
It is now known that, aside from carbon, hydrogen, and oxygen, which plants get from air and water, about a dozen other nutrients are needed for plant growth. They can be divided into three classes.
The primary (or major) nutrients, including nitrogen, phosphorus, and potassium, are needed in larger quantities than are the secondary nutrients, calcium, magnesium, and sulfur.
These in turn are required in greater quantity than the trace (or minor) nutrients iron, boron, manganese, copper, zinc, molybdenum, chloride, and nickel. These nutrients are contained in the minerals and organic matter in the soil.
Many more elements are found in both soils and plants—for instance, aluminum, cobalt, fluorine, iodine, and sodium. They may not be needed by all plants and may be either beneficial or toxic to plant growth. Silicon, sodium, and cobalt are beneficial to some plants.
Soil and Plant Nutrients
Soil is the natural medium in which crops grow. The nutrient content of a soil and the availability of the nutrients to crops are important factors that determine a soil’s productivity. Soil nutrients are mostly present in forms not immediately available to plants, such as being adsorbed onto or as constituents of soil mineral particles, or in organic matter.
They become available through slow processes, such as biological decomposition of organic matter called mineralization, chemical reactions on soil minerals called weathering, and release from soil particles.
Soil nutrients in agricultural land may be gradually lost with the removal of harvested products, such as grain and straw, which take with them considerable amounts of all nutrients. In addition, nutrients may be lost from the soil through leaching and erosion.
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Therefore, it is common for field crops to suffer from nutrient deficiencies. On the other hand, certain soils may present the crops with problems of mineral toxicity, that is, excess amounts of particular elements. To maintain healthy growth of crops, it is necessary to correct these problems.
Soil testing, visual diagnosis of the plant, and plant tissue analysis can all be used to evaluate the fertility status of the soil and detect deficiencies and toxicities in soil nutrients. The results of such tests provide the basis for recommendations on the application of fertilizer and soil amendments.
Coping with Nutrient Deficiencies
Based on knowledge of plant nutrition and experience accumulated over a long period of time, nutrient management practices have been established to enhance soil fertility and overcome crop nutritional problems by balancing the use of mineral fertilizers combined with organic and biological sources of plant nutrients. In practice, different methods may have both advantages and disadvantages, depending on the particular set of local conditions.
Organic sources of plant nutrients include farm yard manure (animal waste products), green manure (plant products), and compost. They contain small amounts of nutrients and are often bulky in nature.
When added to the soil, their main value is to provide organic matter that promotes microbial activity and improves soil structure, aeration, and water-holding capacity, enabling the soil to respond better to fertilizers and irrigation.
The organic matter may also supply micro nutrients and help to make the phosphate in the soil more available to crops. The disadvantages of applying organic manures are that they may be expensive, difficult to handle, or likely to release excess nutrients into the environment.
Nitrogen availability is one of the most limiting factors in crop productivity. Although nitrogen is abundantly available in the air, this form of it is not directly usable by the plants.
Some crops, such as legumes, can form a beneficial relationship called symbiosis with certain bacteria capable of converting atmospheric nitrogen to ammonia, a process known as biological nitrogen fixation.
The bacteria supply the plant with nitrogen (ammonia), while the plant provides the bacteria with organic compounds for use as energy. Legumes can be used as a source of nitrogen when planted with cereals. Fast-growing legumes can be grown early in the season and then ploughed under to provide nitrogen for the main crop.
Recent research has identified the mechanisms controlling the expression of the nitrogen-fixation genes at the molecular level, and one of the goals of ongoing research is to explore various approaches to constructing a viable nitrogen-fixing system for use with nonlegumes.
Low soil phosphorus availability is another constraint on plant growth. Phosphorus is progressively lost from soil through weathering, and reactions with various soil constituents substantially reduce phosphorus available to plants.
One research effort has been directed toward investigating various root characteristics that are useful in phosphorus uptake by plants. The findings may help farmers to breed for crops that are better able to grow in soils with low phosphorus.
Nutrient removal from cultivated land usually exceeds the natural rate of nutrient input. One remedy is to add appropriate and balanced fertilizers back to the soil. Mineral fertilizers are widely used to supply either single nutrients or multiple nutrients in combination. Foliar spray is effective for correcting deficiencies in micronutrients.
Timing and dosage of applications is important, because the nutritional requirements of a crop vary with its stage of growth. Insufficient supply reduces crop yields, but applying too much or at a wrong time is not only wasteful but also potentially harmful to the environment.
Overcoming Nutrient Toxicities
Soil can become acidified as a result of its own physical properties, microbial activity, climate, vegetation, and the addition of acidifying fertilizers. As a result, important nutrients can be lost, and toxicities from aluminum and manganese may occur.
Liming, the application to land of a material containing calcium, usually chalk or limestone, is often used as a standard measure in order to reduce problems of soil acidity.
Salts introduced in irrigation water, blown inland from oceans, or produced by weathering may accumulate in topsoil and cause toxicity to crops. This is normally controlled by applying more water than the crop can use, so that excess salts are leached downward below the root zone.
It is important, however, that the irrigation water does not have high salt content and that drainage is not a problem. The addition of calcium salts, such as calcium sulfate, together with organic manures, is also effective in treating salt-affected soils.
One focus of research in the field of plant nutrition has been the study of the mechanisms plants employ in avoiding or tolerating toxic elements, such as aluminum, manganese, and salts, and accessing scarce nutrients, such as phosphorus, in the soil.
Knowing how plants cope with various nutrient stresses will help the effort of breeding crops that are better able to withstand adverse soil nutrient conditions and achieve high yields by making use of their own genetic potential.