Air contains certain chemical substances which are present in greater quantities when the atmosphere is moist. These chemicals attack even the hardest rocks. To see how rocks can be affected one only needs to pick up a pebble from the sea or a river. On the outside the pebble is smooth and rounded: this is the result of the work of water and the air’s chemical action. The inside of the pebble is still rough because it has not been reached by the external forces.
The chemical action of the atmosphere is extremely important. It can break down rocks and put oxygen into them by oxidation. The result is farm soil without which we cannot grow our food.
However, we must not think that agricultural soil is merely a collection of tiny particles of rock which have been crumbled away. Such a mixture does not contain many of the ingredients necessary to sustain plant life and only certain primitive plants, such as lichens, can obtain any nourishment from it.
Soil becomes fertile when these pioneer’ plants like lichens die and decay. Their decomposed bodies enrich the soil with their chemical substances to from a material known as humus. It is this humus, which is a mass of dead and decaying vegetation, which makes soil fertile.
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