"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." "Well, that wouldn't go far toward replacing the 171 pounds removed from the soil by the corn, oats, and wheat, that's sure," was Mr. Thornton's comment. "It is worse than that," Percy repeated. "Land that will furnish 48 pounds of nitrogen for a crop of oats or wheat will furnish more than 10 pounds for a crop of cowpeas. At the end of such a four-year rotation such a soil would be about 200 pounds poorer in nitrogen per acre than at the beginning, if all crops were removed and nothing returned." "How much would it cost to put that nitrogen back in commercial fertilizer?" asked Mr. Thornton. "That depends, of course, upon what kind of fertilizer is used." "Well, most people around here who use fertilizer buy what the agent calls two-eight-two, and its costs about one dollar and fifty cents a hundred pounds; but it can be bought by the ton for about twenty-five dollars." "'Two-eight-two' means that the fertilizer is guaranteed to contain two per cent. of ammonia, eight per cent. of available 'phosphoric acid,' and two per cent. of potash." "Ammonia is the same as nitrogen, is it not?" "No, it is not the same," replied Percy. "Ammonia is a compound of nitrogen and hydrogen. In order to have a clear understanding of the relation between ammonia and nitrogen we only need to know the combining weights of the elements. The smallest particle of an element is called an atom. Hydrogen is the lightest of all the elements and the weight of the hydrogen atom is used as the standard or unit for the measure of all other atomic weights; thus the atom of hydrogen weighs one." "One what?" interrupted Mr. Thornton. "No one knows," replied Percy. "The atom is extremely small, much too small to be seen with the most powerful microscope; but you know all things are relative and we always measure one thing in terms of another. We say a foot is twelve inches and an inch is one-twelfth of a foot, and there we stop with a definition of each expressed in terms of the other, and both depending upon an arbitrary standard that somebody once adopted; and yet, while the foot is known in most countries, it is rare that two countries have exactly the same standard for this measure of length.