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:
Potassium 36,250 pounds
Magnesium 8,790 pounds
Calcium 11,450 pounds
Phosphorus 1,190 pounds
The older gray silt loam prairie, the most extensive soil of Southern Illinois, contains in two million pounds of soil:
Potassium 24,940 pounds
Magnesium 4,690 pounds
Calcium 3,420 pounds
Phosphorus 840 pounds
These data represent averages involving hundreds of soil analyses, and they emphasize the fact that normal soils are rich in potassium and poor in phosphorus. This is to be expected, for most soils are made from the earth's crust, and normal soils should bear some relation in composition to the average of the earth's crust, which contains in two million pounds 49,200 pounds of potassium and 2,200 pounds of phosphorus, as shown by the weighted averages of analyses involving about two thousand samples of representative rocks, reported by the United States Geological Survey.