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Dept. of Natural Resources and Environmental Sciences
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An essential element for plants, nitrogen is crucial to plant growth and development. Most nitrogen-78 percent of the total-occurs as a relatively inert gas in the atmosphere that is not directly available to plants. Nitrogen must be reduced to ammonium or oxidized to nitrate before plants can use it. This process, called nitrogen fixation, occurs naturally. In recent history, humans have greatly altered the nitrogen cycle to increase crop production, which can cause environmental problems in Illinois and elsewhere.
The natural cycle is the formation of organic nitrogen in soils and its slow release to plants. Humans fix large amounts of nitrogen by producing fertilizer, which takes nitrogen gas from the atmosphere and uses energy to make a plant-available form. We grow millions of acres of crops, like soybeans, that fix nitrogen from the atmosphere. We also add nitrogen by burning fossil fuels, which can release nitrogen compounds that are then deposited by rainfall. Human fixation of nitrogen now equals the natural fixation and allows us to feed a growing population.
|Figure 1. Planting date response
in Northern Illinois.
The use of nitrogen fertilizers and nitrogen-fixing plants on 80 percent of the Illinois landscape has contributed to high concentrations of nitrate in our state's rivers and reservoirs. We add about 1.9 billion pounds of nitrogen in fertilizer each year, and soybeans fix another 1 billion pounds. Most nitrogen (1.9 billion pounds) is shipped away as grain, leaving 1 billion pounds in the soil. Because more than 35 percent of the state is tile drained, this excess nitrate easily can be transported to rivers and about 0.5 billion pounds of it ultimately enters the Mississippi River each year.
High nitrate in streams and rivers is a problem for drinking water suppliers and is thought to be linked to the hypoxic (low-oxygen) zone in the Gulf of Mexico. Nutrients such as nitrogen increase the growth of marine organisms like algae, and when these organisms die, oxygen is used up in the water. Recent estimates show that Illinois contributes about 15 percent of the nitrogen found at the mouth of the Mississippi River, demonstrating that environmental problems can originate far from the place where they become evident.
|Figure 2. Fertilizer nitrogen,
shown being applied here,
is the largest input of nitrogen to Illinois.
What can we do about this problem? Following current nitrogen fertilizer recommendations and applying fertilizer closer to when the crop needs it (at planting or side-dressing following planting) would help reduce losses and river concentrations without reducing yields. It is not possible to grow corn and soybeans without some loss of nitrate.
We need fixed nitrogen in Illinois to maintain crop yields and produce food for the state, our nation, and the world. But responsible stewardship urges us to consider the environmental effects of nitrogen, so that losses to rivers are decreased, drinking water supplies are improved, and the amount of nitrogen we add to the Mississippi River is minimized.
|Department of Crop Sciences
College of Agricultural, Consumer and Environmental Sciences
University of Illinois Extension
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