Fertility in Normal Soils

Of the four important mineral elements, potassium is by far the most
abundant in common soils. Thus, as an average of ten residual soils
from ten different geological formations in the eastern part of
United States, two million pounds of subsurface soil were found to
contain:

Potassium 37,860 pounds
Magnesium 14,080 pounds
Calcium 7,810 pounds
Phosphorus 1,100 pounds

Even the depleted, and to some extent abandoned, gently undulating
upland "Leonardtown loam," which was farmed for generations and
which, according to the surveys of the Federal Bureau of Soils,
covers 41 per cent of St. Mary's County, Maryland, and more than
45,000 acres of Prince George's County--still contains in two
million pounds of surface soil--corresponding to the plowed soil of
an acre about 6-2/3 inches deep:

Potassium 18,500 pounds
Magnesium 3,480 pounds
Calcium 1,000 pounds
Phosphorus 160 pounds

The brown silt loam prairie soil of the early Wisconsin glaciation
is the most common type of the greatest soil area in the Illinois
Corn Belt. Two million pounds of this surface soil contain as an
average: