compounds suitable for plant food.]

The modern agriculturist realizes that the soil is a laboratory for the
production of plant food and he ordinarily takes more pains to provide a
balanced ration for it than he does for his family. Of course the
necessity of feeding the soil has been known ever since man began to
settle down and the ancient methods of maintaining its fertility, though
discovered accidentally and followed blindly, were sound and
efficacious. Virgil, who like Liberty Hyde Bailey was fond of publishing
agricultural bulletins in poetry, wrote two thousand years ago:

But sweet vicissitudes of rest and toil
Make easy labor and renew the soil
Yet sprinkle sordid ashes all around
And load with fatt'ning dung thy fallow soil.

The ashes supplied the potash and the dung the nitrate and phosphate.
Long before the discovery of the nitrogen-fixing bacteria, the custom
prevailed of sowing pea-like plants every third year and then plowing
them under to enrich the soil. But such local supplies were always
inadequate and as soon as deposits of fertilizers were discovered
anywhere in the world they were drawn upon. The richest of these was the
Chincha Islands off the coast of Peru, where millions of penguins and
pelicans had lived in a most untidy manner for untold centuries. The
guano composed of the excrement of the birds mixed with the remains of
dead birds and the fishes they fed upon was piled up to a depth of 120
feet. From this Isle of Penguins--which is not that described by Anatole
France--a billion dollars' worth of guano was taken and the deposit was
soon exhausted.