that plant if the climate is unfavorable. A soil which produces oranges in Florida could not produce them in Illinois. About ten elements are necessary for plant life. They are: carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, sulphur, magnesium, and iron. Which of these come in part from the air?

Fertility is lost by the growing of plants because the plants need to take certain elements from the soil. If, then, we remove or harvest the plant, the ground gets nothing in return. This loss of fertility cannot be avoided. However, soil loses fertility in some ways which can be avoided. Thus, improper draining on sloping land allows rains to wash away valuable materials. Then, too, if certain crops are grown too long upon a piece of land, its soil becomes acid, or sour, and thus loses fertility.

158. How Can Soil be Kept Fertile ? — - While we cannot help removing fertility when we remove a crop, yet the loss can be restored. We must put everything back on the land that is not actually sold or used. That is why the farmer uses manure, the most common of fertilizers; it restores plant food to the soil. The farmer does not profit if he sells the fertility of his soil at too low a price. Thus, when grains are cheap, it may be most profitable to use them as cattle food, because the price received for farm fertility as meat is often greater than that received for it as grain. Besides, when cattle are raised, much of the farm's fertility can be restored as manure.

Dairying is profitable, because dairy products (What are they?) bring a good price, and because they do not remove much fertility. If we sell butter, for example, we are selling chiefly fat. Now, fat is composed of

carbon, oxygen, and hydrogen (cf. § 189), which are elements coming from water and from the carbon dioxide of the air. If the skim milk is used on the farm, the soil need not lose its fertility.

159. Why Should Crops Be Rotated? - Rotation, or changing of crops, on a certain piece of land, is now used by all good farmers. Different crops use different soil elements. Corn removes much of all three of the elements nitrogen, phosphorus, and potassium, and should not be planted every year. Grains, such as wheat, which take up phosphorus, and grass, which takes up little phosphorus, are often grown for two years and then corn in the third year.

Rotation is good in other ways, for roots of different plants differ from one another and one plant will often break up the ground for another. The diseases and pests of plants differ, too, and one plant will not be affected by those of another.

Have you seen artificial fertilizers? They are usually powdered substances which contain the elements, or some of the elements, needed to give back to the soil what has been taken from it by crops. Fertilizers are spread over the ground, or "plowed under." Generally they supply one or more of the three elements: phosphorus, potassium, and nitrogen.

160. Exercises. 1. Make definitions for these terms: fossil, bed rock, outcrop, stratified rock, weathering, soil, glacier, irrigation, soil fertility.

2. Why are beach pebbles rounded?

3. Report some case of weathering you saw on the way to school.

4. Find out from a gardener what kind of soil is best for the growing of potatoes.

5. Ask a florist how he prepares soil for the growing of flowers. 6. Why do farmers "cultivate" the soil between rows of corn? 7. Of what use are song birds to the farmer?

8. What are some of the tools and implements used on the farm? 9. What advantages has a child that grows up on the farm? What disadvantages?

10. Do you think that a farmer ought to burn up his fields, or let it rot and become part of the soil?

reasons.

11. Find out what substances are in skim milk. skim milk be used?

the stubble in Find out the

For what can

12. Bring to school some samples of stratified and of unstratified rocks.

CHAPTER XVII

MINERALS AND METALS

- We have already looked

161. What are Minerals? at our earth as made up of several layers: the air, the water, the crust, and the interior, or core; we have also looked upon it as made up of elements (cf. § 113). We may now look upon it as made up of animal, vegetable, and mineral material. When we realize that rocks are made up of minerals, we can understand that minerals make up much more of the earth than the other two put together. Some of these minerals, such as granite, seem never to have had anything to do with life, while others, such as coal, are the remains of former living things. Those substances which are, or have been, part of a living thing, or organism, we call organic substances. Name some of them.

Quartz, clay, sand, flint, chalk, gold, copper, and even water are minerals, so you see that minerals differ greatly in their qualities. Most of the metals are found united with other materials, just as hydrogen and oxygen are united in water. Those minerals from which we may take out, or "extract," a metal are called ores of that metal. Most of our common, useful metals are found in ores. Lead, iron, tin, zinc, silver, gold, and copper are taken from the ground as ores, although gold and copper are sometimes found in nature in a pure form. To start

with the lead ore and to turn out a lead pipe takes a great deal of time, machinery, and labor. Many huge factories all over our country are working to get the useful metals out of the ores in which nature placed them.

162. Is Iron Necessary to Man? If we were to give up everything in our homes or in the food we eat that is made of iron or is made by means of iron machinery, we should have practically nothing left. Can you name anything which would remain? A race which does not use iron has few tools and conveniences and cannot be highly civilized. Most ancient peoples used iron to some extent the old Egyptians employed it; it is found in the ruins of Babylon. Could our modern inventions, such as the automobile, telephone, victrola, and steam engine, exist without iron?

163. How Is Iron Found? The commonest ore of iron, which furnishes half of all the iron used, is called hematite. Hematite is a compound of iron and oxygen, a rough, ordinary-looking rock varying in color from gray to reddish brown. The ore is often in great ledges and has to be blasted out. Then it is sent to the smelters, or furnaces, to be smelted, or turned into metal. The smelter is usually built near some industrial center that has good coal within easy reach; for coal is needed to remove the oxygen of the ore. Much of the iron ore from the deposits in Minnesota, Wisconsin, and Upper Michigan is sent down the Great Lakes in large steamers. Did you ever see one?

164. How Is Iron Prepared? How do you suppose early man learned to smelt iron? The ancients did it