By a fertile soil is meant one that has an abundance of plant food in the proper proportions. The soil contains all the elements that are needed to support life, but they are in an inorganic form, that is, they are lifeless. Plants alone can take these inorganic substances from the soil, and change them into starch, sugar, fats, and protein. All animals, including man, must get these substances through plants, or through other animals that have already absorbed them from plants. The soil contains ten elements that are absorbed or assimilated by plants. These are: (1) lime, (2) magnesia, (3) iron, (4) sulphur, all of which are found in most plants in very small proportions, and are present in most soils in quantities far beyond the needs of crops for ages to come; (5) carbon, which is obtained by plants through their leaves directly from the air and the sunshine; (6) hydrogen and (7) oxygen, which are taken from the water in the soil and carried to the leaves, where they also help to take the carbon from the atmosphere. With none of these elements, then, does the farmer need to concern himself in regions where the water supply is abundant, as they are, and will continue to be, plentifully supplied by nature. But the other three, (8) nitrogen, (9) potassium, and (10) phosphorus, are needed by plants in large quantities, and are taken from the soil far more rapidly than nature can replace them. All these elements are necessary to plant life, but some plants require a large amount of one element, others a small proportion of that, but a large amount of some of the others. No two varieties of plants require exactly the same proportions, so it is easy to see that the plant that takes out of the soil any one element makes the soil less capable each year of producing a good crop of the same kind. In the early days of farming in this country, it was the custom to grow a single crop, which had been found to give good results, year after year in the same field. In Virginia and other near-by states nearly all the best land was given every year to the cultivation of tobacco, which exhausts the soil rapidly. In the states farther north other crops were planted in the same way. As a result, some of the most fertile soil in Virginia, the Carolinas, Massachusetts, and other eastern states has been so exhausted that it is no longer worth cultivating. Everywhere throughout the New England states are to be found these worn-out farms, and, while they were never so fertile as the lands of the Mississippi Valley, each one was rich enough to support a family in comfort, with something left to sell; but because they were required to produce the same crops, and so take the same element from the soil, year after year, they have become so lacking in one of the essential elements that they are unfit for cultivation, and have been abandoned.