Wind drifts a seed from the parent plant until it settles to the ground, perhaps in a field or by the roadside, or even in the schoolyard. There it remains through the long winter; but with the return of spring, encouraged by the warm sunlight, the seed awakens from its dormant condition, breaks open the seed-cover and sends leaves into the air and roots into the ground. No one planted the seed; yet the plant has made its way in the world and it thrives until it has given to other seeds the same opportunity to start in life.
Had the seed fallen upon a board or a stone it might have sent out leaves and roots; but it could never have developed into a plant, for something necessary would have been lacking. What is there in the soil that is so necessary to the success of plant life? How has it come to be there? What is this soil that the plants need so much? These are some of the questions which we will try to answer.
One readily sees that the soil furnishes a place in which the plants may fix themselves,—an anchorage, as it were. It is also easy to see that from the soil the plants obtain a supply of water; and, moreover, that this water is very necessary, for the vegetation in a moist country suffers greatly in time of drought, and few plants are able to grow in a desert region because there is so little water. You can make a desert in the schoolroom and contrast it with moist soil by planting seeds in two dishes of soil, watering one, but furnishing no water to the other.
That water is necessary to plants is also proved by the plant itself. The sap and the moisture which may be pressed out of a grass stem or an apple are principally water taken from the soil by the roots. But there is more than water, for the juice of an apple is sweet or sour, while the sap and juice of other plants may be sweet or bitter. There are substances dissolved in the water.
It is these dissolved substances that the plants need for their growth, and they find them ready for use in the soil. There is a plant-food which the roots seek and find, so that every plant which sends roots into the soil takes something from it to build up the plant tissue. The sharp edges of some sedges, which will cut the hand like a dull knife, and the wood ashes left when a wood fire is burned, represent in part this plant-food obtained from the soil.
Let us take a handful of soil from the field, the schoolyard, or the street and examine it. We find it to be dirt that "soils" the hands; and when we try to brush off the dirt, we notice a gritty feeling that is quite disagreeable. This is due to the bits of mineral in the soil; and that these are hard, often harder than a pin, may often be proved by rubbing soil against a piece of glass, which the hard bits will often scratch, while a pin will not.
Fig. 21. A boulder-strewn soil of glacial origin with one of the large erratics on the right similar to those which early attracted attention to the drift. See page 105.
Study this soil with the eye and you may not see the tiny bits, though in sandy soils one may easily notice that there are bits of mineral. Even fine loamy and clay soils, when examined with a pocket lens or a microscope, will be found to be composed of tiny fragments of mineral. It is evident that in some way mineral has been powdered up to form the soil; and since the minerals come from rocks, it is the rocks that have been ground up. That powdered rock will make just such a substance as soil may be proved by pounding a pebble to bits, or by collecting some of the rock dust that is made when a hole is drilled in a rock. Much the same substance is ground from a grindstone when a knife is sharpened on it, making the water muddy like that in a mud hole.
It will be an interesting experiment to reduce a pebble to powder and plant seeds in it to see whether they will grow as well as in soil; but in preparing it try to avoid using a sandstone pebble, because sandy soils are never very fertile.