In the prosecution of his art, two distinct classes of substances engage his attention—the _living_ crops he raises, and the _dead_ earth from which they are gathered. If he examine any fragment of an animal or vegetable, either living or dead, he will observe that it exhibits pores of various kinds arranged in a certain order—that it has a species of internal structure—that it has various parts or _organs_—in short, that it is what physiologists term _organized_. If he examine, in like manner, a lump of earth or rock, he will perceive no such structure. To mark this distinction, the parts of animals and vegetables, either living or dead—whether entire or in a state of decay, are called _organic_ bodies, while earthy and stony substances are called _inorganic_ bodies. Organic substances are also more or less readily burned and dissipated by heat in the open air; inorganic substances are generally fixed and permanent in the fire. But the crops which grow upon it, and the soil in which they are rooted, contain a portion of both of these classes of substances. In all fertile soils, there exists from 3 to 10 per cent. of vegetable or other matter of _organic_ origin; while, on the other hand, all vegetables, as they are collected for food, leave, when burned, from one-half to twenty per cent. of _inorganic_ ash. If we heat a portion of soil to redness in the open air, the organic matter will burn away, and, in general, the soil, if previously _dry_, will not be materially diminished in bulk. But if a handful of wheat, or of wheat straw, or of hay, be burned in the same manner, the proportion that disappears is so great, that in most cases a comparatively minute quantity only remains behind. Every one is familiar with this fact who has seen the small bulk of ash that is left when weeds, or thorns, or trees, are burned in the field, or when a hay or corn-stack is accidentally consumed. Yet the ash thus left is a very appreciable quantity, and the study of its true nature throws much light, as we shall hereafter see, on the practical management of the land on which any given crop is to be made to grow. Thus the quantity of ash left by a ton of wheat straw is sometimes as much as 360 lbs.; by a ton of oat straw as much as 200 lbs.; while a ton of the grain of wheat leaves only about 40 lbs.; of the grain of oats about 90 lbs.; and of oak wood only 4 or 5 lbs. The quantities of _inorganic_ matter, therefore, though comparatively small, yet, in some cases, amount to a considerable weight in an entire crop. The nature, source and uses of this earthy matter will be explained in a subsequent chapter. SECTION II.—CONSTITUTION OF THE ORGANIC PART OF PLANTS AND ANIMALS.