(b) Chemical Agents.—The cracks formed by mechanical agents make it easy for chemical agencies to convert the rock into soil. (1) The water that percolates into the ground is sure to contain impurities. From the air it gathers some of the carbonic acid gas given off by animals when they breathe. On the surface of the ground it seeps among decaying leaves, roots and other organic matter, and there dissolves humic acid, ammonia, and other chem icals. Thus the water becomes a weak chemical solution, usually acid, and is ablAb dissolve some minerals and weaken the rest.
(2) The air itself, especially when moist, produces similar results. The water, oxygen, carbonic acid gas, ammonia, and other chemicals which the air contains in minute quantities cause decay. The proc ess is like the rusting or oxidizing of iron, which sometimes goes on so rapidly that a bright blade may become red when left out of doors overnight. Oxidation is the commonest method by which rocks are converted into soil. The red, yellow, or brown coating on the out side of rocks is the result of oxidation.
(c) Organic Agents.—Anything which helps to expose bits of rock to the attack of air or water helps to make soil. (1) A lichen helps when it attaches itself to the side of a bare, solid rock. (2) The higher plants help when they send rootlets into cracks. As the roots grow the cracks are forced open. (2) Animals such as wood chucks and prairie dogs expose bits of rocks to the air when they dig their burrows. (3) The patient ant&in the same way bring up innumerable tiny bits of rock and place them within reach of sun and rain. (4) Angle worms get their food by eating the fine soil. In the process of digestion they take out the decaying organic matter, while the soil passes through them and is subjected to chemical action.
If all of the soil of a given region were swept into the ocean the mechanical, chemical, and organic processes here described would in time break up the exposed rocks and form a new cover of soil, but it would take hundreds of thousands of years.
Kinds of Soil.—(1) Gravelly Soils.—For most kinds of plants the gravelly soils formed by swiftly running water are generally too coarse. They allow air and water to penetrate freely to the roots, but do not retain the water, and the crops are apt to dry up.
Moreover, although the roots can find their way easily among the particles, the soil does not furnish soluble chemicals in sufficient quantities. Gravelly soil is also hard to plow and cultivate because of the stones. When a flood in the Miami River spread four or five inches of gravel over some of the farms in Ohio the farmers were completely discouraged at first. Then they went to work with tip carts and laboriously cleared off the gravel acre by acre.
(2) Sandy Soils.—Sand, which usually consists largely of quartz grains, has similar disadvantages. It is, indeed, easy to plow and cultivate, but it furnishes little nourishing material for plants, and the water runs through it quickly. In the sandy " pine barrens " of Carolina, Georgia, and Florida the water escapes so fast that only a few grasses can grow, and the country is almost a desert. In Florida the orange grower must each year give his groves tons and tons of fertilizer, because the sand in which the orange trees grow contains such small supplies of the essential chemicals.
(3) Clayey Soils.—Clay has the opposite faults from sand and gravel. Although it contains a fair supply of the essential chemicals in relatively available form it is so sticky and compact that plowing is difficult. Even the strongest plants can barely send their roots into it, and the well-digger dreads "hard pan," as he calls a layer of clay, almost as much as solid rock. In Chinese Turkestan certain streams that are used for irrigation bring down large quantities of clay and spread it on the fields to a depth of two or three inches in a single season. When the clay dries it forms a solid cake so hard that it must be left two or three years before it can be cultivated.
(4) Loamy Soils.—Many soils consist of loam, a mixture of sand and clay. The best loam also contains humus, or decayed vegetable matter. Peat and coal were originally pure humus. Since humus consists largely of carbon it gives a dark or even black color to the soil. Although humus is of great value as a fertilizer, plants do not flourish in it alone. Thus neither pure sand, pure clay, nor pure humus is good for agriculture. What the farmer most desires is a rich loamy mixture of the three which has the good qualities of each.