Why don’t plants get nitrogen from the air?
Plants get carbon dioxide they need from the air. So why don’t plants also get the nitrogen they need from the air, instead of from fertilizer?
You are quite correct that plants need carbon dioxide to live, and they do indeed get it from the air. They also need nitrogen, and the atmosphere is about 78 percent nitrogen by volume, but that nitrogen is quite useless to plants.
The catch is that the word is used to refer both to single atoms of nitrogen, which might be bound to other atoms, and to molecules made of two nitrogen atoms attached to each other.
It is the first form that living things can use to make whatever nitrogen compounds they need. The molecular form, the form in which nitrogen occurs in the atmosphere, is almost useless. The nitrogen atoms form such strong bonds between each other that it’s all but impossible for most living things to pull them apart. The nitrogen-nitrogen bond is so strong that even a forest fire won’t pull many apart and allow them to form usable nitrogen compounds. Among the most important processes that can do the job are lightning, and some clever bacteria that often hang around on the roots of legumes and have developed very subtle, gentler ways to get nitrogen molecules to split.
Things that can serve as nitrogen sources in fertilizer include any sort of dead material, such as compost (all proteins contain nitrogen) and animal feces and urine. In the early 1900s, humans found ways to make nitrogen compounds from atmospheric nitrogen, leading to artificial fertilizers.
Ironically, many nitrogen compounds can be used as explosives. The same process used to break apart nitrogen bonds for artificial fertilizers, which has done much to alleviate world hunger, also made possible the manufacture of explosives on the scale needed for world wars I and II.
Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail him at drknowledge@globe.com or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819. 