Work of the Air.—­The air has helped in the work of wearing down the rocks and making soils.  If a piece of iron be exposed to moist air a part of the air unites with part of the iron and forms iron rust.  In the same way when moist air comes in contact with some rocks part of the air unites with part of the rock and forms rock rust which crumbles off or is washed away by water.  Thus the air helps to break down the rocks.  Moving air or wind picks up dust particles and carries them from one field to another.  On sandy beaches the wind often blows the sand along like snow and piles it into drifts.  The entire surface of sandy regions is sometimes changed in this way.  Sands blown from deserts sometimes bury forests which with their foliage sift the fatal winding sheet from the dust-laden winds.

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 larvae 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.

SOIL TEXTURE

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.

CHAPTER IV

RELATION OF SOILS TO WATER