such Both of the two chief constitu ents of the soil thus have their origin in the breaking up of solid rock. The chief agents by which nature accomplishes the immense task of grinding huge masses of rock into sand and clay are water, the atmosphere, and the roots of plants.
Oxygen unites with iron to rust it, and carbon dioxide with lime, potash, and other substances, to form bicarbonates. Water aids in the crum bling process in two ways. It dissolves the cem ent which had held together the particles of the rock, performing this work with especial ease where it has taken up certain gases from the air, or in its passage through the earth. It enters the crevices of the rocks, and in freezing expands with irresistible force, breaking up any rocks into which it can gain entrance in frosty weather. Water aids in chemical decomposition by carrying the chemical agents of the air to places where the atmosphere does not otherwise circulate. Moreover, on exposed surfaces, moist 1 Most of the igneous rocks, such as granite and gneiss, contain both free quartz and feldspar, and their decomposition produces both sand and clay.
ure is a strong ally of oxygen and the other de structive agents. Vegetable growths cooperate with water and the chemical agents at every step. The seeds of tiny lichens attach them selves to the apparently smooth surface of lava, quartz, or other unbroken rocks. The acid of the roots enables them to eat into the rock until it is roughened, and the rootlets themselves are able to find their way into the openings. Later the effects of growing plants become more obvious. Roots swollen by moisture and in growth force apart pieces of rock which have first been separated by water or the corroding gases. The plants keep moist the rocks upon or near which they grow, and their leaves give off a continual supply of oxygen, which, in turn, acts upon the surrounding rocks. All these processes go on quietly but constantly. They have not only created the soil which we now possess, but they are continually contributing new soil to take the place of that which is washed into the ocean.
Water as we have seen is one of the most active agents in the making of soil, but its ser vice does not end there. Few rocks would of
themselves yield a serviceable soil. In order that it may accomplish its purpose of furnishing the necessary conditions of plant growth, the decomposed rock must be thoroughly mixed. This again requires that parts of the soil should be carried long distances. Water is the great carrier and mixer of soils. Every mountain torrent brings down sand, mud, and silt from the weathered rocks of the mountains. In times of flood, when rivers are swollen beyond their banks, and the water sweeps over places not washed by the ordinary streams, tons of soil are carried down to the valleys and mingled with soils which had been brought by other tribu taries of the same river. Soil is brought from the hillsides and mountain tops where it could not be utilized, to the more level valleys where cultivation is possible. Changes in the beds of rivers, and inundations occasioned by heavy rains, or by such causes as the dams of beavers, spread the soil over wide areas.
Glaciers are also efficient carriers and pulver izers. The rocks over which they pass, and which they carry at their sides and on their under surfaces, are reduced to soil and are washed into the valleys when the glacier is finally transformed at the melting line into a muddy Wind, to some extent, carries sand and dust, and so aids in the grand process of soil mixture which is finally completed by plants and earthworms and by the conscious efforts of man.
There is one element of the soil which, as its name indicates, does not come directly from the disintegration of rocks. This is the vegetable mould or humus. It is not, as was formerly supposed, the chief source of fertility, but it is nevertheless an important constituent. It is formed by the decay of organic matter, and is rich in nitrogen, which plants require for their growth, and which they are apparently unable to take from the air, or from any other source so readily as from the vegetable mould. No soil is naturally fertile unless it contains a cer tain amount of organic matter, and soils in which such matter is abundant are quite apt to be supplied with the other elements of plant food.