e of power is essential
to a successful agriculture. Most soils, no matter how unproductive
their condition to-day, have natural strength that we take into
account, either consciously or unconsciously. Some good farm methods
came into use thousands of years ago. Experience led to their
acceptance. They were adequate only because there was natural strength
in the land. Nature stored plant-food in more or less inert form and,
as availability has been gained, plants have grown. Our dependence
continues.

Plant Constituents.--There are a few technical terms whose use cannot
be evaded in the few chapters on the use of lime and fertilizers. A
plant will not come to maturity unless it can obtain for its use
combinations of ten chemical elements. Agricultural land and the air
provide all these elements. If they were in abundance in available
forms, there would be no serious soil fertility problem. Some of their
names may not interest us. Six or seven of these elements are in such
abundance that we do not consider them. A farmer may say that when a
dairy cow has luxuriant blue-grass in June, and an abundance of pure
water, her wants are fully met. He omits mention of the air because it
is never lacking in the field. In the same way the land-owner may
forget the necessity of any kind of plant-food in the soil except
nitrogen, phosphoric acid, potash, and lime. Probably the lime is very
rarely deficient as a food for plants, and will be considered later
only as a means of making soils friendly to plant life.

Nitrogen, phosphoric acid, and potash are the three substances that may
not be in available form in sufficient amount for a growing crop. The
lack may be in all three, or in any two, or in any one, of these plant
constituents. The natural strength of the soil includes the small
percentage of these materials that may be available, and the relatively
large stores that nature has placed in the land in inert form as a
provision against waste.

The thin covering of the earth that is known as the soil is
disintegrated rock, combined with organic matter. The original rock
"weathered," undergoing physical and chemical change. A long period of
time was required for this work, and for the mixing and shifting from
place to place that have occurred. Organic matter has been a factor in
the making of soils, and is in high degree a controlling one in their
production of food.

Organic Matter.--Nature is resourceful and is constantly al