"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?"
"That is exactly what I mean. I see that you do not like percentage any better than I do. Really the acre is the best agricultural unit. We buy and sell the land itself by the acre; we report crop yields at so many bushels or tons per acre; we apply manure at so many loads or tons per acre; we apply so many hundred pounds of fertilizer per acre; sow our wheat and oats at so many pecks or bushels per acre; and we ought to know the invoice of plant food in the plowed soil of an acre and the amounts carried off in the crops removed from an acre.
"Now, referring again to these figures from the forty acres of clover at two tons per acre. If the eighty tons were burned and the ashes mixed with the surface soil on a tenth of an acre the increase per acre would be as follows:
4,000 pounds of phosphorus 24,000 pounds of potassium 6,200 pounds of magnesium 23,400 pounds of calcium.