Nitrogen and Plants
Nitrogen, present in amino acids which fill vascular systems, is a basic requirement for the growth of plant tissue. Nitrogen, however, exists in the atmosphere as a locked molecule of two atoms. In order for plants to use nitrogen as fertilizer, these molecules must first be broken or "fixed" to provide "free" nitrogen atoms. When plants decay, the nitrogen in their systems is released into the soil or atmosphere with oxygen as nitrates. Bacteria and fungi work on these nitrates to fix nitrogen, effectively recycling the element for new plants to use.
Fixed nitrogen is created by very living few plants; the soybean is one. Before humans cleared land to cultivate crops, composted plants and dead fish and animal matter provided enough nitrogen for new plants but with the 18th century Industrial Revolution and its more intensive agricultural practices, soil fertility became a problem. At the beginning of the 20th century, German scientists perfected a method of fixing nitrogen by combining nitrogen and hydrogen to form ammonia. The process, called the Haber-Bosch Process, provided most of the manufactured fertilizer for the 20th century "Green Revolution" that repaired depleted soils and improved agriculture in developing nations.
Inorganic Nitrogen Fertilizer
The Haber-Bosch Process forces nitrogen and hydrogen over iron oxide at high pressure in a chamber heated to between 750 and 1000 degrees Fahrenheit. The resulting ammonia molecule contains one atom of nitrogen and three atoms of hydrogen. Leftover gas is re-cycled over the metal to produce more ammonia. Most commonly, ammonia (or anhydrous ammonia) is liquefied for direct agricultural use or combined with dry potassium and phosphate in varying proportions.
A second type of nitrogen fixer, urea, has become more popular because it is easier to transport and apply than ammonia fertilizers. The Bosch- Meiser Process combines ammonia and dry ice to form ammonium carbamate then heats it, producing urea and water. The larger urea molecule contains two sets of fixed nitrogen atoms, connected by a carbon and oxygen bond. Urea fertilizers must be worked into the soil or watered in on cool days to keep their nitrogen from dispersing.
Organic Nitrogen Fertilizers
At the beginning of the current century, scientists at University of Illinois' Morrow Plots undertook a study of the effectiveness of inorganic nitrogen fertilizers and came to the conclusion that much of the nitrates contained in ammonia and urea fertilizers leech through the soil or end up in waterways as runoff. Their work suggests that improved hybrids and agricultural practices may have played a larger role in the Green Revolution than manufactured fertilizer.
It turns out that the backyard compost bin may be all that is needed to replace nitrogen lost by most soil. Soil still needs nitrogen if planted with non-native species (like lawn grass) or cleared; but a top dressing of well-rotted manure, compost, bone meal or other "natural" source of slow-release nitrogen makes all the nitrogen fertilizer most soil needs.