the liberation of plant food, and undoubtedly the extension or
distribution of the root system of the growing plant is another very
potent factor. If the root surfaces come in contact with one per
cent. of the total surface of the soil particles in the plowed soil,
then we might conceive of a relationship whereby one per cent. of
the phosphorus in that soil would be dissolved or liberated from the
insoluble minerals and thus become available as food for the growing
crop. We know that the rate of liberation varies greatly, with
different soils and seasons, and crops also differ in their power to
assist themselves in the extraction of mineral plant food from the
soil. The presence of limestone encourages the development of
certain soil organisms which tend to hasten some decomposition
process. But, all things considered, it may be said, speaking very
generally, that the equivalent of about one per cent. of the total
phosphorus contained in the plowed soil does become available for
the crops under average conditions. On this basis one hundred and
sixty pounds of phosphorus would furnish about one and one-half
pounds for the crops during one season. But in such a soil the
phosphorus still remaining may be the most difficultly soluble, and
the supply of decaying organic matter may be extremely low, so that
possibly less than one pound per acre would become available, and
this would meet the needs of less than four hundred pounds per acre
of clover hay. Furthermore, the supply grows less and less with
every crop removed.

"With your ordinary soil, carrying twelve hundred and seventy pounds
of phosphorus, perhaps you may be able by a liberal use of decaying
organic matter to liberate ten or fifteen pounds of phosphorus, or
sufficient for a crop of forty to sixty bushels of corn; and, with a
subsoil richer in phosphorus than the surface, and with more or less