Nitrogen fertilizer created a “Green Revolution” after World War II that promised to feed hungry Third World populations. It also created tons of agricultural runoff that has contributed to groundwater pollution and greenhouse gases. The science behind the mixed blessings of the fertilizer used on lawns around the world lies in the compound known as ammonia.
The Nitrogen Cycle
Nitrogen, a basic building block of plant growth, comprises 80 percent of the atmosphere, but it is locked in molecules by strong bonds which must be broken for plants to be able use it. Rain and minerals in the soil accomplish this slowly but steadily. When plants or animals that consume plants die or eliminate waste, nitrogen joins with hydrogen or oxygen molecules to form ammonia or nitrates; even the amino acids that help build organisms leave nitrogen as they decay. Because molecular bonds in ammonia and urea-based compounds are weak, nitrogen is freed as a gas as matter decays and disperses into the atmosphere, completing the nitrogen cycle.
Ancient farmers discovered that they could use the urea-rich manure of farm animals to improve agricultural yields. The United Nations Industrial Development Organization traces the development of fertilizer back to Mesopotamia. Justus von Liebig, a 19th century German chemist, identified nitrogen as an essential need for growth, but until Fritz Haber developed a process using heat, pressure and a sheet of osmium metal to decouple nitrogen molecules to form a new compound, nitrogen fertilizers were too expensive to produce on a large scale. The new compound was liquid ammonia.
Ammonia contains one nitrogen and three hydrogen atoms. Anhydrous ammonia – ammonia in a liquid solution with water – is injected into the ground to prevent its evaporation. Other compounds -- ammonium nitrate or sulfate or ammonium phosphate -- bind ammonia with other nutrients. Most lawn fertilizers use urea, which is the result of a chemical reaction between carbon and ammonia at high temperatures. The urea is formed into a pellet called a “prill,” and coated with one or more compounds containing ammonia.
Manufacturers manipulate the composition of lawn fertilizers to address fertility needs of turf grasses in specific soils, but all lawn fertilizer contains ammonia compounds because nitrogen is the primary nutrient needed by lawns with average soils. Louisiana State University’s Ag Center says that the ammonia compounds in lawn foods are composed so that some nitrogen is released immediately and others require water and time to make the nitrogen available, or to “fix” it in the soil.
Manure and compost contain urea and ammonia that form naturally as organic matter decays; these natural sources release nitrogen more slowly and their action is controlled by temperature and moisture. Lawn fertilizer manufacturers may incorporate organic sources or add coatings to prills to moderate and control the rate of nitrogen release from the built-in ammonia compounds.