fail because the clover will cease to
make a thrifty growth when grown so nearly continuously. It succeeds
best on fertile land.

Potatoes and Crimson Clover.--In some potato-producing sections in warm
latitudes it is a not uncommon practice to grow potatoes year after
year on the same land, seeding to crimson clover after the removal of
the crop in August, and plowing the clover down early in the spring.
Rye has been similarly used farther north. In each instance available
plant-food must be freely supplied. The practice is a temporary
expedient of value, but probably cannot be pursued indefinitely with
profit. This is likewise true of similar close rotations.

CHAPTER XVI

THE NEED OF COMMERCIAL FERTILIZERS

Loss of Plant-food.--The soil is composed chiefly of material that
never will enter into the structure of plants, but that serves us by
affording a congenial place for plant-roots. It anchors the plants,
holds moisture for them, and offers opportunity for all the processes
necessary to the preparation of plant-food and to its use. In this
material are the abundant supplies of such plant-food as silica, but,
as has been previously stated, their very abundance leads us rightly to
disregard them in our thinking. Our interest is only in the very small
percentage of material that is composed of the four constituents which
may be lacking in available form in the soil: nitrogen, phosphoric
acid, potash, and lime. We believe that the only consideration that now
need be given lime is as a soil-corrective and, when there is no
acidity, we may assume that there is plenty of lime present. When
yields of crops tend to decrease, the only plant-foods with which we
are concerned are nitrogen, phosphoric acid, and potash.

The materials were stored in all agricultural land, and much of the
supply is in inert forms. They help to make what we call the natural
strength of the land. The rotting of organic matter, tillage, and many
other agencies bring about some availability. The removal of crops,
leaching, etc., reduce the supply. The right use of commercial
fertilizers involves the addition of some plant-food when the available
supply in a particular soil is inadequate.

Prejudice against Commercial Fertilizers.--The owner of land that was
made very fertile by nature, and that has not been cropped long enough
to reduce the supply of available fertility to the danger-point, rarely
fails to entertain a prejudice agains