“I do not know what you mean by natural manure,” said the Deacon, “unless it is the droppings of animals.” --“To distinguish them, I suppose,” said the Doctor, “from artificial manures, such as superphosphate, sulphate of ammonia, and nitrate of soda.” --“No; that is not how I used the term. A few years ago, we used to hear much in regard to the ‘exhaustion of soils.’ I thought this phrase conveyed a wrong idea. When new land produces large crops, and when, after a few years, the crops get less and less, we were told that the farmers were exhausting their land. I said, no; the farmers are not exhausting the _soil_; they are merely exhausting the accumulated plant-food in the soil. In other words, they are using up the _natural manure_. “Take my own farm. Fifty years ago, it was covered with a heavy growth of maple, beech, black walnut, oak, and other trees. These trees had shed annual crops of leaves for centuries. The leaves rot on the ground; the trees also, age after age. These leaves and other organic matter form what I have called natural manure. When the land is cleared up and plowed, this natural manure decays more rapidly than when the land lies undisturbed; precisely as a manure-pile will ferment and decay more rapidly if turned occasionally, and exposed to the air. The plowing and cultivating renders this natural manure more readily available. The leaves decompose, and furnish food for the growing crop.” EXHAUSTION OF THE SOIL. “You think, then,” said the Doctor, “that when a piece of land is cleared of the forest, harrowed, and sown to wheat; plowed and planted to corn, and the process repeated again and again, until the land no longer yields profitable crops, that it is the ‘natural manure,’ and not the soil, that is exhausted?” “I think the _soil_, at any rate, is not exhausted, and I can easily conceive of a case where even the natural manure is very far from being all used up.” “Why, then,” asked the Deacon, “is the land so poor that it will scarcely support a sheep to the acre?” “Simply because the natural manure and other plant-food which the soil contains is not in an available condition. It lies dead and inert. It is not soluble, and the roots of the plants cannot get enough of it to enable them to thrive; and in addition to this, you will find as a matter of fact that these poor ‘exhausted’ farms are infested with weeds, which rob the growing crops of a large part of the scanty supply of available plant-food.” “But these weeds,” said the Deacon, “are not removed from the farm. They rot on the land; nothing is lost.” “True,” said I, “but they, nevertheless, rob the growing crops of available plant-food. The annual supply of plant-food, instead of being used to grow useful plants, is used to grow weeds.”