It is wisdom and good business policy for farmers to study carefully this question of plant food and to learn what each crop is taking from the soil, so that it may be replaced. It has been found by long and careful experiments, that when land has been "single cropped," as this abuse of the land is called, for a long time, the soil has been almost entirely deprived of its nitrogen. As you know, nitrogen is one of the elements of the air, so that there is a never-ending supply, but most plants are unable to take it from the air, and until the last few years the task of replacing nitrogen in the soil was considered impossible. Recent discoveries, however, have shown that there are two ways in which it may be done. By means of electricity, nitrogen may be directly combined with the other elements of the soil. The other method is nature's own plan, and so is easier and cheaper. It has been found that while most plants exhaust the nitrogen from the soil, one class of plants, the legumes, of which beans, peas, clover, and alfalfa are the best known, have the power of drawing large stores of nitrogen from the air, and, by means of bacteria attached to their roots, restoring it to the ground. So farmers have learned that if they plant corn one year, it is wiser not to plant corn in the same field the next year, but to sow wheat, which requires less nitrogen, and the following year to sow clover, so that the nitrogen which the corn and wheat have taken from the soil, may be put back into it. If the land be naturally fertile, and has been well cared for, the soil is then ready to produce a good crop of corn again. If the soil has become worn-out and the farmer is trying to improve its general condition, he can gain better results by keeping the field in clover a second year, when a profitable crop of clover seed may be had from the land. This system of changing each year, and alternating cereal crops, which take the nitrogen from the soil, with leguminous plants, which restore it to the soil again, is called "rotation of crops," and if regularly followed will preserve a proper balance of nitrogen in the soil.