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

The average annual loss by leaching from good soils in humid sections is known by the results of many analyses to be about as follows per acre: 

     Potassium 10 pounds
     Calcium 300 pounds
     Phosphorus 2 pounds

The average annual loss of magnesium in drainage water from good soils is probably 30 pounds or more an acre, but the data thus far secured are inconclusive with respect to that element.

A careful consideration of the trustworthy data clearly reveals the fact that potassium is very abundant in normal soils, while phosphorus is relatively very deficient; and, all things considered, 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