“Experience in agriculture shows that the production of vegetables on a given surface increases with the supply of certain matters, originally part of the soil which had been taken up from it by plants--the excreta of man and animals. These are nothing more than matters derived from vegetable food, which in the vital processes of animals, or after their death, assume again the form under which they originally existed as parts of the soil. Now we know that the atmosphere contains none of those substances, and therefore can replace none; and we know that their removal from a soil destroys its fertility, which may be restored and increased by a new supply. Is it possible, after so many decisive investigations into the origin of the elements of animals and vegetables, the use of the alkalies of lime and the phosphates, that any doubt can exist as to the principles upon which a rational agriculture depends? Can the art of agriculture be based upon anything but the restitution of a disturbed equilibrium? Can it be imagined that any country, however rich and fertile, with a flourishing commerce, which for centuries exports its produce in the shape of grain and cattle, will maintain its fertility if the same commerce does not restore, in some form of manure, those elements which have been removed from the soil, and which cannot be replaced by the atmosphere? Must not the same fate await every such country, which has actually befallen the once prolific soil of Virginia, now in many parts no longer able to grow its former staple productions--wheat and tobacco? In the large towns of England the produce both of English and foreign agriculture is largely consumed. Elements of the soil indispensable to plants, do not return to the fields; contrivances resulting from the manners and customs of the English people, and peculiar to them, render it difficult, perhaps impossible, to collect the enormous quantity of the phosphates which are daily, as solid and liquid excreta, carried into the rivers. These phosphates, although present in the soil in the smallest quantity, are its most important mineral constituents. It was observed that many English fields exhausted in that manner, immediately doubled their produce as if by a miracle when dressed with bone earth imported from the Continent. But if the export of bones from Germany is continued to the extent it has now reached, our soil must be gradually exhausted, and the extent of our loss may be estimated by considering that one pound of bones contains as much phosphoric acid as a hundredweight of grain.”