"Yes, that is the rule under general or average conditions, but it should be stated that the amount of sulfur required by plants is very small as compared with phosphorus, a difference which places a great distinction between them. Besides considerable quantities of sulfur are returned to the air in the combustion of coal and organic matter, and this returns to the soil in rain. The information thus far secured shows that sulfur rarely if ever limits the crop yields under field conditions; and the same may be said of iron, which is required by plants in very small amount and is contained in practically all soils in enormous quantities. "While normal soils contain abundance of potassium, with about half as much calcium and one-fourth as much magnesium; yet, when measured by crop requirements for plant food, the supplies of these three elements are not markedly different. On the other hand, about 300 pounds of calcium are lost per acre per annum by leaching from good soils in humid climates, compared with about 10 pounds of potasssium and intermediate amounts of magnesium; so that, of these three elements, calcium requires by far the most consideration and potassium the least, even aside from the use of limestone to correct or prevent soil acidity. "Among the conditions essential for nitrification may be mentioned the presence of free oxygen and limestone; and of course all bacteria require certain food materials, resembling other plants in this respect." "Are they plants?" asked Mrs. Thornton. "I thought they were tiny little animals." "No, they are classified as plants," replied Percy; "but the scientists have difficulty with some of the lower organism to decide whether they are plants or animals. The college boys used to say that some animals were plants in the botanical department and animals again when they studied zoology. Orton says it is easy to tell a cow from a cabbage, but impossible to assign any absolute, distinctive character which will divide animal life from plant life. "The oxygen is essential for nitrification, because that is an oxidation process. That is, it is a kind of combustion, so to speak. The organic matter is oxidized or converted into substances containing more oxygen than in the original form. In ammonification the carbon is separated or divorced from the nitrogen and united with oxygen. Some of the hydrogen of the organic matter remains temporarily with the carbon, and some is held temporarily with the nitrogen in the form of ammonia. "The nitrite bacteria replace two of the hydrogen atoms in ammonia with one of oxygen, and insert another oxygen atom between the nitrogen and the remaining hydrogen, thus forming nitrous acid; H-O-N=O, or HNO2. "The nitrate bacteria then cause the direct addition of another oxygen atom, which is held by the two extra bonds of the nitrogen atom, which you will remember is a five-handed atom.