“The cowpea is an annual plant.  It is planted, produces its seed, and dies the same season.  It has no need to store up material in the roots for future use.  Consequently the substance of the root is largely taken into the tops as the plan approaches maturity.  It is different with the clover plant.  This is a biennial with some tendency toward the perennial plant.  It lives long and develops an extensive root system, and its stores up material in the roots during part of its life for use at a later period.  About one-third of the total nitrogen content of the clover plant is contained in the roots and stubble.  This means that the roots and stubble of a two-ton crop of clover would contain about forty pounds of nitrogen, or more than we assumed was taken from the soil by the cowpeas.  But there is still another point in favor of the clover.  The cowpeas make their growth during the summer months when nitrification is most active, whereas the clover growth we have counted on occurs chiefly during the fall and spring when nitrification is much less active, consequently the clover probably takes even a larger proportion of its nitrogen from the air than we have counted on.”

“That is rather confusing,” said Mr. Thornton, “you say the cowpea grows when nitrification is most active, and yet you say that it takes less nitrogen from the air than clover.  Isn’t that somewhat contradictory?”

“I think not,” said Percy.”  Let me see.—­Just what do you understand by nitrification?”

“Getting nitrogen from the air, is it not?”

“No, no.  That explains it.  Getting nitrogen from the air is called nitrogen fixation.  This action is carried on by the nitrogen-fixing bacteria, such as the clover bacteria, the soy bean bacteria, the alfalfa bacteria, which, by the way, are evidently the same as the bacteria of sweet clover, or mellilotus.  Then we also have the cowpea bacteria, and these seem to be the same as the bacteria of the wild partridge pea, a kind of sensitive plant with yellow flowers, and a tiny goblet standing upright at the base of each compound leaf,—­the plant called Cassia Chamaecrista by the botanist.”

“Nitrification is an altogether—­”

“Well, I declare!  Excuse me, Sir, but that’s Charlie calling the cows.  Scotts, I don’t see where the time has gone!  You’ll excuse me, Sir, but I must look after separating the cream.  You will greatly oblige me, Mr. Johnston, if you will have dinner with us and share our home to-night.  In addition to the pleasure of your company, I confess that I am mightily interested in this subject; and I would like especially to get a clear understanding of that nitrification process, and we’ve not had time to discuss the potash and ‘phosphoric acid,’ which I know cost some of our farmers a good part of all they get for their crops, and still their lands are as poor as ever.”

“I appreciate very much your kind invitation, Mr. Thornton.  I came to you for correct information regarding the agricultural conditions here, and you were very kind and indulgent to answer my blunt questions, even concerning your own farm practice and experience.  I feel, Sir, that I am already greatly indebted to you, but it will certainly be a great pleasure to me to remain with you to-night.”