Consider, for example, the common three-year rotation, corn, wheat, and clover. A fifty-bushel crop of corn removes twelve pounds of phosphorus from the soil; the twenty-five bushel wheat crop draws out eight pounds; and then the two-ton crop of clover withdraws ten pounds, making thirty pounds required for this simple rotation. The most common type of land in St. Mary county, Maryland, after two hundred years of farming, contains phosphorus enough in the soil for five rotations of this simple sort. Mathematically that is all the further traffic in rotations that soil can bear. Agriculturally that soil has refused to bear any sort of traffic, whether single or in rotations, and has been abandoned for farm use except where fertilized.
These crops would remove from the soil one hundred and twenty-four pounds of nitrogen in the corn and wheat, and the roots and stubble of the clover would contain forty pounds of nitrogen. Now, if the soil furnishes seventy-six pounds of nitrogen to the corn crop and forty-eight pounds to the wheat crop, will it furnish forty pounds to the clover crop, or as much as remains in the roots and stubble? If so, how does the rotation actually enrich the soil in nitrogen?
You will be interested to know that there are many exact records of the effect upon the soil of the rotation of crops. This particular three-year rotation has been followed at the Ohio Agricultural Experiment Station for thirteen years, and the average yield of wheat has been, not twenty bushels, not sixteen bushels, but eleven bushels per acre, where no plant food was applied; although where farm manure was used the wheat yielded twenty bushels, and with manure and fine-ground natural rock phosphate added the average yield of wheat for the thirteen years has been more than twenty-six bushels per acre. The corresponding yields for corn are thirty-two, fifty-three and sixty-one bushels, and for clover they are one and two-tenths, one and six-tenths and two and two-tenths tons of hay per acre.
You will wish to know also that the Ohio Station has conducted a five-year rotation of corn, oats, wheat, clover, and timothy for the last fifteen years, both with and without the application of commercial plant food. As an average of the fifteen years the unfertilized and fertilized tracts have produced, respectively:
30 and 48 bushels of corn
32 and 50 bushels of oats and 27 bushels of wheat .9 and 1.6 tons of clover
1.3 and 1.8 tons of timothy
In 1908 the unfertilized land produced nine-tenths ton of clover, while land treated with farm manure produced three and two-tenths tons per acre.
You will welcome the information that the average yield of wheat on an Illinois experiment field down here in "Egypt," in a four-year rotation, including both cowpeas and clover, has been eleven and one-half bushels on unfertilized land, fourteen bushels where legume crops have been plowed under, and twenty-seven bushels where limestone and phosphorus have been added with the legume crops turned under; and that the aggregate value of the four crops, corn, oats, wheat, and clover, from another "Egyptian" farm, has been $25.97 per acre on unfertilized land, and $54.24 where limestone and phosphorus have been applied.
In your very busy and very successful railroad experience, you may have overlooked the reports of the Pennsylvania Agricultural Experiment Station, showing the results of a four-year rotation of crops that has been conducted with very great care for more than a quarter of a century. These, you will agree, are exactly such absolute data as we sorely need just now when facing the stupendous problem of changing from an agricultural system whose equal has never been known for rapidity of soil exhaustion to a system which shall actually enrich the land. By averaging the results from the first twelve years and also those from the second twelve years, in this rotation of corn, oats, wheat, and hay (clover and timothy), we find that the yields have decreased as follows: