lcium 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.

Measuring Fertility Losses

The plant food required for one acre of wheat yielding 50 bushels,
one acre each of corn and oats yielding 100 bushels, and one acre of
clover yielding four tons, includes for the total crops:

Potassium 320 pounds
Magnesium 68 pounds
Calcium 168 pounds
Phosphorus 77 pounds

If only the grain, including a yield of 4 bushels an acre of clover
seed, is considered, the straw, stalks and hay being returned to the
soil--either directly or in farm fertilizer--then the loss per acre
from four years of cropping as above would be as follows:

Potassium 51 pounds
Magnesium 16 pounds
Calcium 5 pounds
Phosphorus 42 pounds

T