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 farmyard 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 micronutrients 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.
See also: Overcoming Nutrient Toxicities, Soil and Plant Nutrients
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