In the most depleted soils nitrogen is usually the most deficient element, although it may not be the only deficiency. Thus in the depleted “Leonardtown loam,” which occupies such extensive areas of land in Southern Maryland, near the District of Columbia, and which has been to a large extent agriculturally abandoned after one or two centuries of farming, only 900 pounds of nitrogen are found in the plowed soil of an acre—that is, in 2,000,000 pounds of surface soil, corresponding to about 6-2/3 inches an acre. This total amount if made available would be sufficient for only six such crops of corn as are actually produced on our best land in good seasons, and yet it is four times as much as is contained in an equal weight of the subsoil.
The average prairie land of the Corn Belt contains only 5000 pounds of nitrogen in the plowed soil of an acre 6-2/3 inches deep, whereas a 100-bushel crop of corn removes 150 pounds of nitrogen from the soil. A simple computation shows the supply in the plowed soil to be sufficient for only 33 such crops. Even the 100-bushel crop of corn per acre is known to have been produced in many places on exceptionally rich land, and yet the ten-year average yield in the United States is only 25 bushels to the acre.
200 Per Cent for Nitrogen
On Broadbalk Field at Rothamsted, England, wheat has been grown on the same land every year for about two-thirds of a century. As an average of the sixty years, 1852 to 1911 the yield was 12.6 bushels an acre on unfertilized land, 14.6 where mineral plant food was annually applied, 20.3 where nitrogen salts alone were used, and 37 where both nitrogen and mineral plant food were applied.
During the thirty years, 1882 to 1911 the average yields were 11.7 bushels an acre on the unfertilized land, 14 with minerals, 18.7 where only nitrogen salts were used, and 38 where both nitrogen and minerals were regularly supplied.
These absolute data from the oldest agricultural experiment station in the world should help us to understand why the ten-year average yield of wheat is 33 bushels an acre for all of Great Britain, 37-1/2 for England alone, and only 14 for the United States.
The application of nitrogen increased the yield of wheat by 24 bushels an acre—from 14 to 38 bushels—as an average of the last thirty years, following an average increase of 26.3 for the nitrogen applied during the previous thirty years. It is true that the cost of the fertilizers used exceeded the value of the increase in yield; but let us bear in mind that this truth does not destroy the other truth.
Prove all things, and hold fast that which is good. It is a good fact that 1218 bushels of wheat were produced by the application of nitrogen to an acre of land during a period of sixty years, over and above the produce of another acre which differed only by not receiving nitrogen; whereas the total produce from an acre of unfertilized land was only 756 bushels during the same sixty years. It is a good fact that the increase alone from the nitrogen applied is more than twice the total yield of the unfertilized land during the last thirty years, and he does well who holds fast this fact.