“On the other hand we were taught in college that the plowed soil of an acre of our most common Illinois corn belt land contains only 1200 pounds of phosphorus, and that a hundred-bushel crop of corn takes twenty-three pounds of phosphorus out of the soil.  Furthermore that about one pound of phosphorus per acre is lost annually in drainage water in humid regions.  By dividing 1200 by 24 it is easy to see that fifty corn crops such as we ought to try to raise would require as much phosphorus as the present supply in our soil to a depth of about seven inches.  Of course there is some phosphorus below seven inches, but it is the plowed soil we must depend upon to a very large extent.  The oldest agricultural experiment station in the world is at Rothamsted, England.  On two plots of ground in the same field where wheat has been grown every year for sixty years, the soil below the plow line has practically the same composition, but on one plot the average yield for the last fifty years has been thirteen bushels per acre, while on the other the yield of wheat has averaged thirty-seven bushels for the same fifty years.”

“The same kind of wheat?” inquired Mr. Thornton.

“Yes, and great care has always been taken to have these two plots treated alike in all respects, save one.”

“And what was that?”

“Plant food was regularly incorporated with the plowed soil of the high-yielding plot.”

“You mean that farm manure was used?”

“No, not a pound of farm manure has been used on that plot for more than sixty years; and, furthermore, the two plots were very much alike at the beginning; but, to the high-yielding plot, nitrogen, phosphorus, potassium, magnesium, calcium, and sulfur have all been applied in suitable compounds every year.”

“That is to say,” observed Mr. Thornton, “that the land itself has produced thirteen bushels of wheat per acre and the plant food applied has produced twenty four bushels, making the total yield thirty-seven bushels on the fertilized land.”

“That is certainly a fair way to state it,” replied Percy.

" Well, that sounds as though something might be done with run-down lands.  About what part of the twenty-four bushels increase would it take to pay for the fertilizers?”

“About 150 per cent. of it,” Percy replied.

“One hundred and fifty per cent!  Why, you can’t have more than a hundred per cent. of anything.”

“Oh, yes, you can.  The twenty-four bushels are one hundred per cent. of what the fertilizers produced, and the land itself increased this by fifty per cent., so that the fertilized land produced one hundred and fifty per cent. of the increase from the plant food applied.

“Well, that’s too much college mathematics for me; but do you mean to say that it would take the whole thirty-seven bushels to pay for the plant food that produced the increase of twenty-four bushels?”