of
plant-food, which, if the soil needed it, would, if supplied in
sufficient quantity, and in an available condition, produce, according
to soil, season, climate, and variety, a maximum crop."

CHAPTER IV.

NATURAL MANURE.

We often hear about "natural" manure. I do not like the term, though I
believe it originated with me. It is not accurate; not definite enough.

"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