The unfertilized soil at the Rothamsted station contains, in 2,000,000 pounds--corresponding to about 6-2/3 inches to the acre--1000 pounds of phosphorus and 35,000 of potassium, while an acre of plowed soil of the same weight at State College, Pennsylvania, contains 1100 pounds of phosphorus and 50,700 of potassium. In a word, normal soils are deficient in phosphorus, and the application of phosphorus in good systems of farming produces marked and profitable increases in crop yields. But what form of phosphorus shall we apply? This is a very important question in agricultural economics, for we have many different kinds of fertilizing materials that contain phosphorus, and one may cost ten times as much as another as a source of phosphorus. Thus 250 pounds of phosphorus in a ton of finely ground natural rock phosphate can be purchased at the mines in Tennessee and delivered at the farmer's railway station in the heart of the Corn Belt for $8. Or the ton of raw phosphate may be mixed with a ton of sulfuric acid in the fertilizer factory, and the two tons of acid phosphate may be sold to the same farmer for $32. Or the fertilizer manufacturer may mix the two tons of acid phosphate with two tons of "filler," containing a little nitrogen and potassium, and then sell the same farmer the four tons of so-called "complete" fertilizer for $80; and the farmer gets no more phosphorus in the four tons of "complete" fertilizer for $80 than in the one ton of natural phosphate for $8. The Pennsylvania State College conducted an experiment for twelve years--1884 to 1895--in which $1.05 an acre was invested in ground raw rock phosphate with a rotation of corn, oats, wheat and hay (clover and timothy), and the value of the increase produced by the phosphorus amounted to $5.85 as an average for the twelve years, and to $8.41 as an average for the last four years. Thus the profit was from about 560 to 800 per cent on the investment, counting corn at 35 cents a bushel, oats at 30 cents, wheat at 70 cents, and hay at $6 a ton. These figures represent the increase produced by phosphorus over and above the value of the crops grown without phosphorus fertilizer. In this case no farm manure was used on either part of the field; but commercial nitrogen and potassium were applied alike on both parts, and clover was grown in the rotation.