All of these differences may be overcome by the use of the proper means. Sometimes it could be done at an expense which would be justified by the result; and, at others, it might require too large an outlay to be profitable. It becomes a question of economy, not of ability, and science is able to estimate the cost. Soil cannot be cultivated understandingly until it has been subjected to such an examination as will tell us exactly what is necessary to render it fertile. Even after fertility is perfectly restored it requires thought and care to maintain it. The ingredients of the soil must be returned in the form of manures as largely as they are removed by the crop, or the supply will eventually become too small for the purposes of vegetation. FOOTNOTES: [T] It is due to our country, as well as to Prof. Mapes and others, who long ago explained this absorptive power of clay and carbon, to say that the subject was perfectly understood and practically applied in America a number of years before Prof. Way published the discovery in England as original. SECTION THIRD. MANURES. CHAPTER I. CHARACTER AND VARIETIES OF MANURES. [What must a farmer know in order to avoid failures? Can this be learned entirely from observation? What kind of action have manures? Give examples of each of these. May mechanical effects be produced by chemical action? How does potash affect the soil?] To understand the science of _manures_ is the most important branch of practical farming. No baker would be called a good practical baker who kept his flour exposed to the sun and rain. No shoemaker would be called a good practical shoemaker, who used morocco for the soles of his shoes, and heavy leather for the uppers. No carpenter would be called a good practical carpenter, who tried to build a house without nails, or other fastenings. So with the farmer. He cannot be called a good practical farmer if he keeps the materials, from which he is to make plants, in such a condition, that they will have their value destroyed, uses them in the wrong places, or tries to put them together without having every thing present that is necessary. Before he can avoid failures _with certainty_, he must know what manures are composed of, how they are to be preserved, where they are needed, and what kinds are required. True, he may from observation and experience, _guess_ at results, but he cannot _know_ that he is right until he has learned the facts above named. In this section of our work, we mean to convey some of the information necessary to this branch of _practical farming_.