=EXERCISE= 1. What tools are used in tillage? 2. How should a poor and shallow soil be treated? 3. Why should a poor and shallow soil be well compacted before sowing the crop? 4. Explain the value of a circulation of air in the soil. 5. What causes iron to rust? 6. Why is a two-horse turning-plow better than a one-horse plow? 7. Where will clods do the least harm--on top of the soil or below the surface? 8. Do plant roots penetrate clods? 9. Are earthworms a benefit or an injury to the soil? 10. Name three things that tillage does. SECTION III. THE MOISTURE OF THE SOIL Did any one ever explain to you how important water is to the soil, or tell you why it is so important? Often, as you know, crops entirely fail because there is not enough water in the soil for the plants to drink. How necessary is it, then, that the soil be kept in the best possible condition to catch and hold enough water to carry the plant through dry, hot spells! Perhaps you are ready to ask, "How does the mouthless plant drink its stored-up water?" The plant gets all its water through its roots. You have seen the tiny threadlike roots of a plant spreading all about in fine soil; they are down in the ground taking up plant food and water for the stalk and leaves above. The water, carrying plant food with it, rises in a simple but peculiar way through the roots and stems. The plants use the food for building new tissue, that is, for growth. The water passes out through the leaves into the air. When the summers are dry and hot and there is but little water in the soil, the leaves shrink up. This is simply a method they have of keeping the water from passing too rapidly off into the air. I am sure you have seen the corn blades all shriveled on very hot days. This shrinkage is nature's way of diminishing the current of water that is steadily passing through the plant. A thrifty farmer will try to keep his soil in such good condition that it will have a supply of water in it for growing crops when dry and hot weather comes. He can do this by deep plowing, by subsoiling, by adding any kind of decaying vegetable matter to the soil, and by growing crops that can be tilled frequently. The soil is a great storehouse for moisture. After the clouds have emptied their waters into this storehouse, the water of the soil comes to the surface, where it is evaporated into the air. The water comes to the surface in just the same way that oil rises in a lamp-wick. This rising of the water is called _capillarity_. [Illustration: FIG. 5. AN ENLARGED VIEW OF A SECTION OF MOIST SOIL, SHOWING AIR SPACES AND SOIL PARTICLES]