calcium--and probably magnesium--is of much greater significance
than potassium, from the standpoint of the maintenance of usable
plant food in the soil. It should be noted, too, that certain crops
which are exceedingly important for economic systems of permanent
agriculture require very large amounts of calcium as plant food.
Thus a four-ton crop of clover hay takes about 120 pounds of calcium
from the soil, or the same amount as of potassium; while such a crop
of alfalfa requires about 145 pounds of calcium, but only 96 pounds
of potassium. When it is known that the abandoned "Leonardtown loam"
still contains in two million pounds of surface soil 18,500 pounds
of potassium and only 1000 pounds of total calcium, the significance
of these chemical and mathematical data must be apparent.

The Liberation of Fertility

Probably there has never been a greater waste of time and effort in
the name of science than in the endeavor to determine the
"available" plant food in soils. The almost universal assumption has
been that the plant food in the soil exists in two distinct
conditions, "available" and "unavailable," and that the
determination of the "available" plant food would reveal both the
crop-producing power of the soil and the fundamental fertilizer
requirements for the improvement of the soil for crop production.

After ascertaining the total stock of plant food in the plowed soil,
the next important question is not how much is "available," but
rather how much can be made available during the crop season, year
after year. In other words we must make plant food available by
practical methods of liberation, by converting it from insoluble
compounds into soluble and usable forms; for plant food must be in