"Now the various acids of carbon and nitrogen constitute one of the most important factors in soil fertility. They are the means by which the farmer can dissolve and make available for the growing crops the otherwise insoluble mineral elements, such as iron, calcium, magnesium, and potassium, all of which are contained in most soils in great abundance. These elements exist in the soil chiefly in the form of insoluble silicates. Silicon itself is a four-handed element which bears somewhat the same relation to the mineral matter of the soil as carbon bears to the organic matter. Quartz sand is silicon dioxid (SiO2). Oxygen, which is present in nearly all substances, including air, water, and most solids, constitutes about one-half of all known matter. Silicon is next in abundance, amounting to more than one-fourth of the solid crust of the earth. Aluminum is third in abundance (about seven per cent), aluminum silicate being common clay. Iron, calcium, potassium, sodium, and magnesium, in this order, complete the eight abundant elements, which aggregate about ninety-eight per cent. of the solid crust of the earth. "It is worth while to know that about two and one-half per cent. of the earth's crust is potassium, while about one-tenth of one per cent. is phosphorus; also that when a hundred bushels of corn are sold from the farm, seventeen pounds of phosphorus, nineteen of potassium, and seven of magnesium are carried away. "The acids formed from the decaying organic matter not only liberate for the use of crops the mineral elements contained in the soil in abundance, but they also help to make available the phosphorus of raw phosphate, when naturally contained in the soil, as it is to some extent in all soils, or when applied to the soil in the fine-ground natural phosphate from the mines. "Now the increase or decrease of organic matter in the soil is measured with a very good degree of satisfaction by the element nitrogen, which is a regular constituent of the organic matter of the soil; and you are already familiar, Mr. Thornton, with the amounts of nitrogen contained in average farm manure and in some of our most common crops." "Yes, Sir, I have some of the figures in my note book and I mean to have them in my head very soon. But, say, that organic matter seems to be a thing of tremendous importance, and I'm sure we've got mighty little of it. I think about the only thing we'll need to do to make this old farm productive again is to grow the vegetation and plow it under. As it decays, it will furnish the nitrogen, and liberate the phosphorus, potassium, calcium, and magnesium; and we may have plenty of all of them just waiting to be liberated."