"Practically the entire stock or store of all of the elements in the soil is insoluble and consequently unavailable for the use of growing plants; and, as I said, some of the chief plans and efforts of the farmer should be directed to the business of making plant food available. "The nitrogen contained in the insoluble organic matter of the soil is made soluble and available by the process called nitrification. Three different kinds of bacteria are required to bring about the complete change." "Are these bacteria different from the nitrogen fixing bacteria?" asked Mr. Thornton. "Entirely different," Percy replied, "and there are three distinct kinds, one for each of the three steps in the process. "The first may be called ammonia bacteria. They have power to convert organic nitrogen into ammonia nitrogen; that is, into the 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. Johnston makes everything so plain that I can mighty near see what he saw when he looked through the microscope."