nd add their dead bodies to it. This
vegetable addition to the soil is generally known as _humus_.

[Illustration: FIG. 2. GROUND ROCK AT END OF A GLACIER]

In due course of time low forms of animal life came to live on these
plants, and in turn by their work and their death to aid in making a
soil fit for the plowman.

Thus with a deliberation that fills man with awe, the powerful forces of
nature splintered the rocks, crumbled them, filled them with plant food,
and turned their flinty grains into a soft, snug home for vegetable
life.

SECTION II. TILLAGE OF THE SOIL

A good many years ago a man by the name of Jethro Tull lived in England.
He was a farmer and a most successful man in every way. He first taught
the English people and the world the value of thorough tillage of the
soil. Before and during his time farmers did not till the soil very
intelligently. They simply prepared the seed-bed in a careless manner,
as a great many farmers do to-day, and when the crops were gathered the
yields were not large.

Jethro Tull centered attention on the important fact that careful and
thorough tillage increases the available plant food in the soil. He did
not know why his crops were better when the ground was frequently and
thoroughly tilled, but he knew that such tillage did increase his yield.
He explained the fact by saying, "Tillage is manure." We have since
learned the reason for the truth that Tull taught, and, while his
explanation was incorrect, the practice that he was following was
excellent. The stirring of the soil enables the air to circulate through
it freely, and permits a breaking down of the compounds that contain the
elements necessary to plant growth.

You have seen how the air helps to crumble the stone and brick in old
buildings. It does the same with soil if permitted to circulate freely
through it. The agent of the air that chiefly performs this work is
called carbonic acid gas, and this gas is one of the greatest helpers
the farmer has in carrying on his work. We must not forget that in soil
preparation the air is just as important as any of the tools and
implements used in cultivation.

[Illustration: FIG. 3. SLOPE TO WATER SHOWS SOIL WEATHERED FROM FACE
OF CLIFF]

If the soil is fertile and if deep plowing has always been done, good
crops will result, other conditions being favorable. If, however, the
tillage is poor, scanty harvests will always result. For most soils a
two-horse plow i