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