Part 1 — THE SOIL IN FIELDS AND GARDENS

City and country teachers are expected to teach classes about the formation and cultivation of soil. It is surprising how much of the needed materials can be brought in by the children, even in the cities. The beginning is a flowering plant growing in a pot. A window box is a small garden. A garden plot is a miniature farm.

Materials to collect for study indoors. A few pieces of different kinds of rock: Granite, sandstone, slate; gravelly fragments of each, and finer sand. Pebbles from brooks and seashore. Samples of clays of different colors, and sands. Samples of sandy and clay soils, black pond muck, peat and coal. Rock fossils. A box of moist earth with earthworms in it. Keep it moist. A piece of sod, and a red clover plant with the soil clinging to its roots.

What is soil? It is the surface layer of the earth's crust, sometimes too shallow on the rocks to plough, sometimes much deeper. Under deep soil lies the "subsoil," usually hard and rarely ploughed.

What is soil made of? Ground rock materials and decayed remains of animal and plant life. By slow decay the soil becomes rich food for the growing of new plants. Wild land grows up to weeds and finally to forests. The soil in fields and gardens is cultivated to make it fertile. Plants take fertility from the soil. To maintain the same richness, plant food must be put back into the soil. This is done by deep tillage, and by mixing in with the soil manures, green crops, like clover, and commercial fertilizers.

Plants must be made comfortable, and must be fed. Few plants are comfortable in sand. It gets hot, it lets water through, and it shifts in wind and is a poor anchor for roots. Clay is so stiff that water cannot easily permeate it; roots have the same trouble to penetrate it and get at the food it is rich in. Air cannot get in.

Sand mixed with clay makes a mellow soil, which lets water and air pass freely through. The roots are more comfortable, and the tiny root hairs can reach the particles of both kinds of mineral food. But the needful third element is decaying plant and animal substances, called "humus." These enrich the soil, but they do a more important thing: their decay hastens the release of plant food from the earthy part of the soil, and they add to it a sticky element which has a wonderful power to attract and hold the water that soaks into the earth.

What is the best garden soil? A mixture of sand, clay, and humus is called "loam." If sand predominates, it is a sandy loam—warm, mellow soil. If clay predominates, we have a clay loam—a heavy, rich, but cool soil. All gradations between the two extremes are suited to the needs of crops, from the melons on sandy soil, to celery that prefers deep, cool soil, and cranberries that demand muck—just old humus.

How do plant roots feed in soil? By means of delicate root hairs which come into contact with particles of soil around which a film of soil water clings. This fluid dissolves the food, and the root absorbs the fluid. Plants can take no food in solid form. Hence it is of the greatest importance to have the soil pulverized and spongy, able to absorb and hold the greatest amount of water. The moisture-coated soil particles must have air-spaces between them. Air is as necessary to the roots as to the tops of growing plants.

Why does the farmer plough and harrow and roll the land? To pulverize the soil; to mellow and lighten it; to mix in thoroughly the manure he has spread on it, and to reach, if he can, the deeper layers that have plant food which the roots of his crops have not yet touched. Killing weeds is but a minor business, compared with tillage.

Later, ploughing or cultivating the surface lightly not only destroys the weeds, but it checks the loss of water by evaporation from the cracks that form in dry weather. Raking the garden once a day in dry weather does more good than watering it. The "dust mulch" acts as a cool sunguard over the roots.

The process of soil-making. If the man chopping wood in the Yosemite Valley looks about him he can see the soil-making forces at work on a grand scale. The bald, steep front of El Capitan is of the hardest granite, but it is slowly crumbling, and its fragments are accumulating at the bottom of the long slope. Rain and snow fill all crevices in the rocks. Frost is a wonderful force in widening these cracks, for water expands when it freezes. The loosened rock masses plough their way down the steep, gathering, as they go, increasing power to tear away any rocks in their path.

Wind blows finer rock fragments along, and they lodge in cracks. Fine dust and the seeds of plants are lodged there. The rocky slopes of the Yosemite Valley are all more or less covered with trees and shrubs that have come from wind-sown seeds. These plants thrust their roots deeper each year into the rock crevices. The feeding tips of roots secrete acids that eat away lime and other substances that occur in rocks. Dead leaves and other discarded portions of the trees rot about their roots, and form soil of increasing depth. The largest trees grow on the rocky soil deposited at the base of the slope. The tree's roots prevent the river from carrying it off.

When granite crumbles, its different mineral elements are separated. Clear, glassy particles of quartz we call sand. Dark particles of feldspar become clay, and may harden into slate. Sand may become sandstone. Exposed slate and sandstone are crumbled by exposure to wind and frost and moving water, and are deposited again as sand-bars and beds of clay.

The most interesting phase of soil study is the discovery of what a work the humble earthworm does in mellowing and enriching the soil.