“There is, however, one way to obtain an uninterrupted succession of crops from the same land unless the soil is very poor. All plants derive their nourishment from the soil and the atmosphere; but some take more from the atmosphere, others from the soil. The plants that get their sustenance chiefly from the air are those that have luxuriant foliage. The potato is one of these. You know that it is through their leaves that plants obtain the carbonic acid gas diffused in the air. The greater the spread of foliage, the more abundant will this absorption be. The plants that depend almost wholly on the soil are those with only a few small, slender leaves, thus taking but little carbonic acid gas from the air. Such is wheat. “Moreover, from the potato plant we take only the tubers, which form but a small part of the whole, and we turn under the stalks and leaves, which are thereupon converted into humus. Thus the potato has the property of enriching the soil at the expense of the atmosphere, and it gives back more than it takes. It is, then, one of the enriching rather than impoverishing plants in respect to its action on the soil. Cereals, on the contrary, are utilized by the harvester both as to seed and haulm, nothing but the meager roots being left in the ground; and as, on account of their very scanty foliage, cereals derive almost their entire sustenance from the soil, they take from it much more than they give back to it. They accordingly belong to the class of plants that impoverish rather than enrich the soil in which they grow. “It is impossible, thus, except by a ruinous expenditure of fertilizer, to raise a crop of grain every year on the same land. But if we should let potatoes succeed wheat, and wheat succeed potatoes, what would be the result? The latter crop, deriving a large part of its nourishment from the air, would flourish in soil comparatively exhausted by wheat; and on having its leaves and stalks turned under it would give back to the soil a part of its former fertility. Wheat could then be successfully raised again on the same land.