_The Work of Plants._--Living plants sometimes send their roots into rock crevices; there they grow, expand, and split off rock fragments. Certain kinds of plants live on the surface of rocks. They feed on the rocks and when they die and decay they keep the surface of the rocks moist and also produce carbonic acid which dissolves the rocks slowly just as the vinegar dissolved the limestone in our experiment.
Dead decaying roots, stems, and leaves of plants form largely the organic matter of the soil. When organic matter has undergone a certain amount of decay it is called humus, and these soils are called organic soils or humus soils. The black soils of the woods, swamps and prairies, contain large amounts of humus.
_Work of Animals._--Earth worms and the larvæ of insects which burrow in the soil eat soil particles which pass through their bodies and are partially dissolved. These particles are generally cast out on the surface of the soil. Thus these little animals help to move soil, to dissolve soil, and to open up passages for the entrance of air and rain.
We have seen that the soil particles vary in size and that for the best development of the plant the particles of the soil must be so arranged that the delicate rootlets can readily push their way about in search of food, or, in other words, that the soil must have a certain texture. By the texture of the soil we mean the size of its particles and their relation to each other. The following terms are used in describing soil textures: Coarse, fine, open, close, loose, hard, stiff, compact, soft, mellow, porous, leachy, retentive, cloddy, lumpy, light, heavy. Which of these terms will apply to the texture of sand, which to clay, which to humus, which to the garden soil, which to a soil that plant roots can easily penetrate? We find then that texture of the soil depends largely on the relative amounts of sand, silt, clay and humus that it contains.
We learned in a previous paragraph that plant roots take moisture from the soil. What becomes of this moisture? We will answer this question with an experiment.
=Experiment.=--Take a pot or tumbler in which a young plant is growing, also a piece of pasteboard large enough to cover the top of the pot or tumbler; cut a slit from the edge to the centre of the board, then place it on top of the pot, letting the stem of the plant enter the slit. Now close the slit with wax or tallow, making it perfectly tight about the stem. If the plant is not too large invert a tumbler over it (Fig. 21), letting the edge of the tumbler rest on the pasteboard; if a tumbler is not large enough use a glass jar. Place in a sunny window. Moisture will be seen collecting on the inner surface of the glass. Where does this come from? It is absorbed from the soil by the roots of the plant and is sent with its load of dissolved plant food up through the stem to the leaves. There most of the moisture is passed from the leaves to the air and some of it is condensed on the side of the glass.