Nitrogen utilization in crop production

About 5% of soil organic matter or humus consists, on the average, of nitrogen combined with the other elements that are essential for crop growth. Nitrogen is usually the element that is of primary importance in the determination of crop yields and quality. It is required in comparatively large amounts and is likely to be deficient in soils unless the best farm management practices are used. 

Nitrogen is the one element above all others that we associate with growth. As pointed out in Chapter 14, it is a constituent of proteins, enzymes, chlorophyll, deoxyribonucleic acids that make up the genetic code, amino acids and many intermediates invplved in the synthesis of plant substances. It is characteristically present in comparatively large amounts in the growing tips of plants, hut it can move about readily from one part of the plant to another to meet the primary needs at the time. 

The presence of too much available nitrogen can result in a very succulent plant of excessive height and low sugar content. Tall plants and heavy grain yields may sometimes result in lodging and difficulty in harvesting. Occasionally abundant nitrogen results in 

The trend in the use of fertilizer nitrogen in recent years in the United States is shown in Fig. 23.1, supplied through the courtesy of L.B. Nelson of the Tennessee Valley 

Authority (also see Nelson, 1965, for discussion of these data). There has been a rapid increase in the use of anhydrous ammonia and a moderate increase in the consumption of ammonium nitrate and nitrogen solutions. These three nitrogen sources represent the bulk of the nitrogen used. The older sources, such as ammonium sulfate and sodium nitrate, have now become comparatively minor nitrogen sources. 

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The efficiency with which nitrogen is utilized in crop production is affected by so many factors that few general unqualified statements can be made regarding its use. Some of the more important factors are discussed below, as well as in other chapters. Extensive discussions of the subject have beeif published by many authors including Harmsen and Van Schreven (1955), Allison (1964, 1966), Bartholomew and Clark (1965), Nelson and Hauck (1965) and Donald et al. (1963), Parr (1967). 

There is a paucity of information regarding the environmental formation or stability of N-nitrosopesticides. For example, the extensive use of atrazine in crop production programs utilizing heavy application of nitrogen fertilizers has raised the possibility of its N-nitrosation in soils (65). 

Relatively high levels of potassium are utilized by growing plants. Potassium activates some enzymes and plays a role in the water balance in plants. It is also essential for some carbohydrate transformations. Crop yields are generally greatly reduced in potassium-deficient soils. The higher the productivity of the crop, the more potassium is removed from soil. When nitrogen fertilizers are added to soils to increase productivity, removal of potassium is enhanced. Therefore, potassium may become a limiting nutrient in soils heavily fertilized with other nutrients. 

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