"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." "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?"