solution before the plant can take it from the soil. For the
present, space is taken only to emphasize the value of decaying
organic manures in the important matter of making plant food
available; and attention is also called to the fact that the
decomposition of the organic matter of the soil--including both
fresh materials and old humus--is hastened by tillage and by
underdrainage, which permit the oxygen of the air to enter the soil
more freely, oxygen being a most active agent in nitrification and
other decomposition processes of organic matter, as well as in the
more common combustion of wood, coal, and so forth.

The Renewal of Fertility

In rational systems of general farming the supply of any element
which is normally very abundant may be renewed from the subsoil by
even the very slight erosion which occurs on all ordinary lands in
humid sections. This statement applies to iron and potassium, and
often to magnesium.

If two million pounds of normal surface soil contain 30,000 pounds
of potassium, one inch an acre would contain 4500 pounds of that
element; and if a third of this--1500 pounds--were removed by
cropping and leaching before its removal by surface washing, then
two-thirds of a century could be allowed for the erosion of one inch
of soil, with crop yields of 50 bushels of wheat, 100 bushels of
corn and oats, and 4 bushels of clover seed to the acre, provided
the stalks, straw and clover hay were returned to the land, either
directly or in farm manure. This amount of surface washing is likely
to occur on land sufficiently undulating for good surface drainage,
provided the land is plowed and cultivated as frequently as would be