“Grow more legumes, and plow more under, either directly or in manure.”

“That sounds easy, but can you suggest some practical system?”

“I think so.  I know too little of your conditions to think I could suggest the best system for you to adopt; but I can surely suggest one that will supply nitrogen for such crop yields as we have considered:  Suppose we change the order of the crops and grow wheat, corn, oats, and cowpeas, and grow clover with the wheat and oats, plowing the clover under in the spring as green manure for corn and cowpeas.  If necessary to prevent the clover or weeds from producing seed, the field may be clipped with the mower in the late summer when the clover has made some growth after the wheat and oats have been removed.  Leave this season’s growth lying on the land.  As an average it should amount to more than half a ton of hay per acre.  The next spring the clover is allowed to grow for several weeks.  It should be plowed under for corn on one field early in May and two or three weeks later the other field is plowed for cowpeas.  The spring growth should average nearly a ton of clover hay per acre.  In this way clover equivalent to about three tons of hay could be plowed under.  Clover hay contains 40 pounds of nitrogen per ton; so this would supply about 120 pounds of nitrogen in addition to the 173 pounds possible to be supplied in the manure.  This would make possible a total return of 293 pounds, while we figured some 200 pounds removed.  Of course if you save only 100 pounds in the manure the amount returned would be reduced to 220 pounds.”

“There are two questionable points in this plan,” said Mr. Thornton, " one is the impossibility, or at least the difficulty, of growing clover on this land.  The other point is, How much of that 120 pounds of nitrogen returned in the clover is taken from the soil itself?  I remember you figured 86 pounds of nitrogen in two tons of cowpea hay, but you also assumed that about 29 pounds of it would be taken from the soil.”

“Yes, that is true,” Percy replied, “at least 29 pounds and probably more.  You see the cowpeas grow during the same months as corn and on land prepared in about the same manner.  If the soil will furnish 75 pounds of nitrogen to the corn crop, and 48 pounds to the oats and wheat, it would surely furnish 29 pounds to the cowpeas.  Of course this particular amount has no special significance, but the other definite amounts removed in corn, oats, and wheat aggregate 171 and the 29 pounds were added to make the round 200 pounds.  Perhaps 210 pounds would be nearer the truth, in which case the soil would furnish about half as much nitrogen to the cowpea crop as to the corn crop.  This is reasonable considering that corn is the first crop grown after the manure is applied.  You will remember that only one-tenth of the total nitrogen of the cowpea plant remains in the roots and stubble?”

“Yes, that’s what we figured on.”