In a season like the past, therefore, an application of two hundred pounds of superphosphate per acre, costing three dollars, instead of giving an increase of five or six bushels per acre, may give us an increase of fifteen or twenty bushels per acre. That is to say, owing to the dry weather in the autumn, followed by severe weather in the winter, the weak plants on the unmanured land may either be killed out altogether, or injured to such an extent that the crop is hardly worth harvesting, while the wheat where the phosphate was sown may give us almost an average crop. Sir John B. Lawes has somewhere compared the owner of land to the owner of a coal mine. The owner of the coal digs it and gets it to market in the best way he can. The farmer’s coal mine consists of plant food, and the object of the farmer is to get this food into such plants, or such parts of plants, as his customers require. It is hardly worth while for the owner of the coal mine to trouble his head about the exhaustion of the supply of coal. His true plan is to dig it as economically as he can, and get it into market. There is a good deal of coal in the world, and there is a good deal of plant food in the earth. As long as the plant food lies dormant in the soil, it is of no value to man. The object of the farmer is to convert it into products which man and animals require. Mining for coal is a very simple matter, but how best to get the greatest quantity of plant food out of the soil, with the least waste and the greatest profit, is a much more complex and difficult task. Plant food consists of a dozen or more different substances. We have talked about them in the pages of this book, and all I wish to say here is that some of them are much more abundant, and more readily obtained, than others. The three substances most difficult to get at are: nitric acid, phosphoric acid and potash. All these substances are in the soil, but some soils contain much more than others, and their relative proportion varies considerably. The substance which is of the greatest importance, is nitric acid. As a rule, the fertility of a soil is in proportion to the amount of nitric acid which becomes available for the use of plants during the growing season. Many of our soils contain large quantities of nitrogen, united with carbon, but the plants do not take it up in this form. It has to be converted into nitric acid. Nitric acid consists of seven pounds of nitrogen and twenty pounds of oxygen. It is produced by the combustion of nitrogen. Since these “Talks” were published, several important facts have been discovered in regard to how plants take up nitrogen, and especially in regard to how organic nitrogen is converted into nitric acid. It is brought about through the action of a minute fungoid plant. There are several things necessary for the growth of this plant. We must have some nitrogenous substance, a moderate degree of heat, say from seventy to one hundred and twenty