"There are five plots in each of four different fields that have received no applications of plant food from the beginning. Thus, every year the crops are carefully harvested and weighed from twenty measured plots of ground that receive no treatment except the rotation of crops. The difference between the average of the first twelve years and the average of the second twelve years should represent the actual change in productive power during a period of twelve years. These averages show that the yield of corn has decreased from forty-one and seven-tenths bushels to twenty-seven and seven-tenths bushels; that the yield of oats has decreased from thirty-six and seven-tenths bushels to twenty-five bushels; that the yield of wheat has decreased only from thirteen and three-tenths bushels to twelve and eight-tenths bushels; and that the yield of hay has decreased from three thousand seventy pounds to two thousand one hundred and eighty pounds. "As a general average of these four crops the annual value of produce from one acre has decreased from $11.05 to $8.18. Here we have information which is almost if not quite equal in value to that from the Agdell rotation field at Rothamsted. While the Rothamsted experiments cover a period of sixty years, each crop was grown but once in four years; whereas, in the Pennsylvania experiments, there have been four different series of plots, so that in twenty-four years there have been twenty-four crops of corn, twenty-four crops of oats, twenty-four crops of wheat, and twenty-four crops of hay. "Under this four-year rotation the value of the crops produced has decreased twenty-six per cent. in twelve years. What influence will impress that fact upon the minds of American landowners? A loss amounting to more than one-fourth of the productive power of the land in a rotation with clover seeded every fourth year! This one fact is the mathematical result of four hundred and eighty other facts obtained from twenty different pieces of measured land during a period of twenty-four years. "As an average of these twenty-four years, the addition of mineral plant food produced increases in crop yields above the unfertilized land as follows: Corn increased forty-five per cent. Oats increased thirty-two per cent. Wheat increased forty-two per cent. Hay increased seventy-seven per cent. "As a general average of the four crops for the twenty-four years, the produce where mineral plant food is applied, was forty-nine per cent. above the yields of the unfertilized land, although the same rotation of crops was practiced in both cases." "Those are some of the absolute facts of science secured from practical application in the adoption and development of definite systems of permanent prosperous agriculture, and they should be made to serve this greatest and most important industry just as the established facts of mathematical and physical science are made to serve in engineering."