Fertilizing the Earth with NitrogenPlants need nitrogen. It is a nutrient that allows them to grow larger and faster. Plants are not able to make use of the nitrogen gas in the atmosphere, two nitrogen atoms bonded together. It is not a useable form of the nutrient. So they get the nitrogen they need from the soil where its form has been converted by soil bacteria. In natural conditions, plant growth is limited in many areas by the amount of usable nitrogen available in the soil. In an effort to grow more crops, people have been transforming nitrogen from the atmosphere into nitrogen fertilizers for crops. This has been very successful over the past century, allowing people to farm on lands that had not been as productive in the past, and allowing areas of the world with less fertile lands to produce enough food for growing populations. However, fertilizers are often overused, and that can cause problems.
Nitrogen from fertilizers sinks into soils, often creating conditions that favor the growth of weeds rather than native plants. Nitrogen then washes into waterways causing a surplus of nutrients, a situation called eutrophication. In freshwater lakes, rivers, and streams eutrophication causes aquatic weeds to grow unchecked up from the bottom. They sometimes fill the entire lake, river, or stream. Algae cloud the water green and slimy algal scum coats shallow rocks.
As the nitrogen-rich waters make their way downstream to the ocean, they cause even more problems. For example, every summer for more than 30 years high nitrogen levels at the Mississippi River Delta have caused a dead zone in the Gulf of Mexico where the water empties into the ocean. This dead zone, in which oxygen levels are too low for animals to survive, covered more that 8000 square miles (more than 20,000 km2) of ocean in 2001. The dead zone forms when excess nitrogen causes algae to grow and reproduce very rapidly. As the huge algae population begins to die and decompose, oxygen that is usually dissolved in the seawater is used up. Animals can not survive without oxygen. They flee to another part of the ocean if they can, or they die. Compounding the low oxygen problem, some of the algae that are multiplying rapidly are toxic. Known as “red tides”, blooms of toxic algae can cause major fish kills, and render clams, scallops and other shellfish highly dangerous for human consumption.
Although it is one of the larger dead zones and is relatively well-studied, the dead zone in the Gulf of Mexico (see image at left) is not the only one of its kind. There are about 150 dead zones in the world’s oceans. Almost all of them are located at the mouths of rivers where nitrogen fertilizers and other nutrient sources like sewage and livestock waste are added to the seawater. Scientists who have studied the Gulf of Mexico dead zone say that if the amount of fertilizers used on American crops were decreased by 12-14%, the dead zone would shrink to about a quarter of its current size without impact on the growth of the crops.