Tyrone Camerata, age 13, of Dover, Del., for his question:

WHAT IS THE NITROGEN CYCLE?

Nitrogen, like other elements used by plants and animals, passes through a cycle of chemical changes and eventually returns to its original form.

Although nitrogen makes up almost four fifths of the air, ordinary green plants cannot use it. They absorb nitrogen from the soil as nitrates. The nitrates are used in making proteins and other substances.
If an animal eats the plant, some of the proteins and other nitrogenous compounds are changed into animal proteins and similar compounds. The animal then excretes nitrogenous wastes.

Decayed bacteria act upon the wastes and upon the dead bodies of plants and animals. They produce ammonia as a waste product. Ammonium compounds are used by nitrifying bacteria, which release nitrites as waste products.

Another group of bacteria change the nitrites to nitrates, which again can be used by green plants. The nitrogen cycle is then complete.