If all plants liked and used the same food, our food would be too much of one kind. But as plants eat many kinds of minerals, through them we receive material to build all parts of our bodies. We need lime for our bones. We need sugar. We need salt in our bodies. Plants eat from the soil lime, salt, chalk, iron, and many other minerals. Suppose you go into the country. Why, here is corn in this field! Last year clover was here! You say to the farmer: “Why do you change things in this way? Why does not your wheat grow where it grew last year?” “Oh,” says the farmer, “my crops will not grow every year in the same place. Next year I shall have this cornfield put down in grass for a while.” “But why, why, Mr. Farmer? Why not have the grass-field always the grass-field? Why not have the wheat-field always in wheat?” “It is because each plant has some chief kind of mineral food that it needs most. It takes more of that kind out of the ground. After a time it eats up all of that sort of mineral that is within reach of its roots in one field. Then the crop must be grown in a new place, out of which the mineral has not been eaten. “By and by the sun, rain, frost, acting on the ground, bring up more of the kind of mineral that was used. In the meantime some other plant that needs more of something else will grow in that field. “So, after a time, the wheat, or corn, can be planted in its old field again. But if crops grow year after year in the same field they will be sickly for lack of food. “New, rich earth that has never been tilled, will feed the same crops many years.” While we are in the country, let us look around at what the farmer does. Here he brings cart-loads of stuff from the stables. He scatters it over his fields. Here he brings barrels of a kind of dust. He puts that too over the ground. “O, Mr. Farmer, what are you doing now?” “I am putting food on the ground for my crops.” The plants have eaten much of the food from the earth. These things contain more of the same kind of food. Men have studied plants until they know what each kind needs to eat. They put this kind of food on the ground and it is carried to the plant by means of water. What are these things that the farmer puts on his field? Bones are ground into bone dust. Fish are also ground into a powder or dust. Shells and sea-weeds and lime are scattered over the ground. Dead horses and other animals buried in what is called a muck heap, until they are decayed and fall into pieces, are among the material that the farmer brings to enrich the soil. Do you not see that all this food would be of no use to the plant were it not for the green leaves? The green leaves turn the raw sap into food for plants and men. The green leaves also take other things from the air and build them up into plant stuff. Our breath, the smoke from our fires, become food for plants, and so food for men. I shall tell you of this in another lesson.