Let the pupils each present a system of rotation that includes the crops raised at home. The system presented should as nearly as possible meet the following requirements:
1. Legumes for gathering nitrogen.
2. Money crops for cash income.
3. Cultivated crops for tillage and weed-destruction.
4. Food crops for feeding live stock.
CHAPTER III
THE PLANT
SECTION XII. HOW A PLANT FEEDS FROM THE AIR
If you partly burn a match you will see that it becomes black. This black substance into which the match changes is called carbon. Examine a fresh stick of charcoal, which is, as you no doubt know, burnt wood. You see in the charcoal every fiber that you saw in the wood itself. This means that every part of the plant contains carbon. How important, then, is this substance to the plant!
You will be surprised to know that the total amount of carbon in plants comes from the air. All the carbon that a plant gets is taken in by the leaves of the plant; not a particle is gathered by the roots. A large tree, weighing perhaps 11,000 pounds, requires in its growth carbon from 16,000,000 cubic yards of air.
Perhaps, after these statements, you may think there is danger that the carbon of the air may sometime become exhausted. The air of the whole world contains about 1,760,000,000,000 pounds of carbon. Moreover, this is continually being added to by our fires and by the breath of animals. When wood or coal is used for fuel the carbon of the burning substance is returned to the air in the form of gas. Some large factories burn great quantities of coal and thus turn much carbon back to the air. A single factory in Germany is estimated to give back to the air daily about 5,280,000 pounds of carbon. You see, then, that carbon is constantly being put back into the air to replace that which is used by growing plants.