“It is a remarkable fact,” said Percy, “that of the ten essential elements of plant food, nitrogen is the most abundant, measured by crop requirements, and at the same time the most expensive.  The air above an acre of land contains enough carbon for a hundred bushels of corn per acre for two years, and enough nitrogen for five hundred thousand years; and yet the nitrogen in commercial fertilizers costs from fifteen to twenty cents a pound.  At commercial prices for nitrogen, every man who owns an acre of land is a millionaire.

“You mean he has millions in the air,” amended Mr. Thornton.

“Yes, that is the better way to put it,” Percy admitted, “but the fact is he can not only get this nitrogen for nothing by means of legume crops, but he is paid for getting it, because those crops are profitable to raise for their own value.  Clover, alfalfa, cowpeas, and soy beans are all profitable crops, and they all have power to use the free nitrogen of the air.

“There are a few important facts to be kept in mind regarding nitrogen: 

“A fifty-bushel crop of corn takes 75 pounds of nitrogen from the soil.  Of this amount about 50 pounds are in the grain, 24 pounds are in the stalks, and 1 pound in the cobs.  A fifty-bushel crop of oats takes 48 pounds of nitrogen from the soil, 33 pounds in the grain, and 15 in the straw.  A twenty-five bushel crop of wheat also takes 48 pounds of nitrogen from the soil, 36 pounds in the grain and 12 in the straw.

“These amounts will vary to some extent with the quality of the crops, just as the weight of a bushel of wheat varies from perhaps 56 to 64 pounds, although as an average wheat weighs 60 pounds to the bushel.”

“You surely remember figures well,” remarked Mr. Thornton as he made some notations.

“It is easy to remember what we think about much and often,” said Percy; “as easy to remember that a ton of cowpea hay contains 43 pounds of nitrogen as that Blairville is 53 miles from Richmond.”

“I have added those figures together,” continued Mr. Thornton, “and I find that the three crops, corn, oats, and wheat, would require 171 pounds of nitrogen.  Now suppose we raise a crop of cowpeas the fourth year, how much nitrogen would be added to the soil in the roots and stubble?”

“Not any.”

“Do you mean to say that the roots and stubble of the cowpeas would add no nitrogen to the soil?  Surely that does not agree with the common talk.”

“It is even worse than that,” said Percy.  “The cowpea roots and stubble would contain less nitrogen than the cowpea crop takes from a soil capable of yielding thirty bushels of corn or oats.  Only about one-tenth of the nitrogen contained in the cowpea plant is left in the roots and stubble when the crop is harvested.  Suppose the yield is two tons per acre of cowpea hay!  Such a crop would contain about 86 pounds of nitrogen, and about 10 pounds of nitrogen per acre would be left in the roots and stubble.”