"If we compare the average yield of turnips, barley, clover, and
wheat of the last twenty years with the yield of turnips in 1848,
barley in 1849, clover in 1850 and wheat in 1851 we find that on the
unfertilized land in this rotation of crops in fifty years the yield
of turnips has decreased from ten tons to one-half ton, and the
yield of barley has decreased from forty-six to fourteen bushels,
the yield of clover has decreased from two and eight-tenths tons per
acre to less than one-half ton, while the yield of wheat has
decreased only from thirty bushels to twenty-four bushels. As a
general average the late yields are only one-third as large as they
were fifty years before on the same land. Wheat grown once in four
years has been the only crop worth raising on the unfertilized land
during the last twenty years, and even the wheat crop has distinctly
decreased in yield; although where mineral plant food was applied
the yield has increased from thirty bushels, in 18851 to
thirty-eight bushels as an average of the last twenty years. In the
fallow rotation on the unfertilized land the yield of wheat averaged
thirty-four and five-tenths bushels during the first twenty years
(1848 to 1867) and twenty-three and five-tenths bushels during the
last twenty years.

"On another Rothamsted field the phosphorus actually removed in
fifty-five crops from well-fertilized land is two-thirds as much as
the total phosphorus now contained in the plowed soil of adjoining
untreated land.

"In the early 80's the Pennsylvania Agricultural Experiment Station
began a four-year crop rotation, including corn, oats, wheat, and
mixed clover and timothy.