compound of nitrogen and hydrogen; and this step in the process is
called ammonification.

"The other two kinds are the true nitrifying bacteria. One of them
converts the ammonia into nitrites, and the other changes the
nitrites into nitrates. These two kinds are known as the nitrite
bacteria and the nitrate bacteria.

"Technically the last two steps in the process are nitrification
proper; but, speaking generally, the term nitrification is used to
include the three steps, or both ammonification and nitrification
proper.

"Now, the nitrifying bacteria require certain conditions, otherwise
they will not perform their functions. Among these essential
conditions are the presence of moisture and free oxygen, a supply of
carbonates, certain food materials for the bacteria themselves, and
a temperature within certain limits.

"You may remember, Mr. Thornton, that more soil nitrogen is made
available for cowpeas during the summer weather than for clover
during the cooler fall and spring?"

"Yes, I remember that distinction."

"I declare," said Miss Russell, "Tom talks as though he had been
there and seen the things going on. I haven't seen you using any
microscope."

"Well, I tell you, I've mighty near seen 'em," was the reply. "Mr.