"Our farmers are still producing crops very largely by drawing on
this reserve. Of course most of this great supply of humus is very
old. It represents the organic residues most resistant to
decomposition; and, where corn and oats are grown exclusively, the
soil has reached a condition on many farms under which the
decomposition of the reserve organic matter is so slow that the
nitrogen liberated from its own decay and the minerals liberated
from the soil by the action of the decomposition products are not
sufficient to meet the requirements of large crops, and for this
reason alone some of our lands that are still rich are said to be
run down; but they only require a moderate use of clover or farm
manure or other fresh and active organic matter to at once restore
their productiveness to a point almost equal to the yields from the
virgin soil. Some Illinois farmers who have discovered this apparent
restoration have jumped to the conclusion that they have solved the
problem of permanently maintaining the fertility of the soil; and I
judge from a remark made by the Secretary of Agriculture that some
Iowa farmers have the same mistaken notions.

"These fresh supplies of active organic matter serve primarily as
soil stimulants, hastening the liberation of nitrogen from the
organic reserve and of minerals from the inorganic soil materials.

"Where one of the Eastern farmers has managed a farm under the
rotation system with the occasional use of clover or light
applications of farm manure,--where this has been continued until
the great reserve is largely gone, and the phosphorus supply greatly
depleted, then the land is truly run down, but not until then.