"THE subject is somewhat complicated," Percy replied, "yet it involves no more difficult problems than have been solved in many other lines. The chief trouble is that we have done too little thinking about our own real problems. Even in the country schools we have learned something of banking and various other lines of business, something of the history and politics of this and other countries, something of the great achievements in war, in discovery and exploration, in art, literature, and invention; but we have not learned what our soils contain nor what our crops require. Not one farmer in a hundred knows what chemical elements are absolutely required for the production of our agricultural plants, and one may work hard on the farm from four o'clock in the morning till nine o'clock at night for forty years and still not learn what corn is made of. "All agricultural plants are composed of ten chemical elements, and the growth of any crop is absolutely dependent upon the supply of these plant food elements. If the supply of any one of these plant food elements is limited, the crop yield will also be limited. The grain and grass crops, such as corn, oats, wheat, and timothy, also the root crops and potatoes, secure two elements from the air, one from water, and seven from the soil. "The supply of some elements is constantly renewed by natural processes, and iron, one of the ten, is contained in all normal soils in absolutely inexhaustible amount; while other elements become deficient and the supply must be renewed by man, or crop yields decrease and farming becomes unprofitable. "Matter is absolutely indestructible. It may change its form, but not a pound of material substance can be destroyed. Matter moves in cycles, and the key to the problem of successful permanent agriculture is the circulation of plant food. While some elements have a natural cycle which is amply sufficient to meet all requirements for these elements as plant food, other elements have no such cycle, and it is the chief business of the farmer to make these elements circulate. "Take carbon, for example. This element is well represented by hard coal. Soft coal and charcoal are chiefly carbon. The diamond is pure crystallized carbon, and charcoal made from pure sugar is pure, uncrystallized carbon. This can easily be made by heating a lump of sugar on a red hot stove until only a black coal remains. Now these different solid materials represent carbon in the elemental form or free state. But carbon may unite with other elements to form chemical compounds, and these may be solids, liquids, gases.