“But it would be much against one’s interest to exact from the soil another crop of wheat the third year. Exhausted by the grain it has just produced, the soil would yield but a scanty harvest unless it were freshly manured, a process that would make of the whole operation, not a piece of farming, but an example of gardening, and would also entail too great expense. For that reason the third year is devoted to the raising of an enriching crop, such as clover. After furnishing a supply of fodder, what is left of the clover is turned under, and all its remnants of roots, stems, and leaves are reduced to mold, which renders the soil fit for another wheat harvest the fourth year. A third enriching crop to be turned under after the final mowing, is likewise needed for the fifth year; and this crop may be sainfoin. At the end of the series comes another cereal, oats, for example. The rotation is now complete, and the program begins all over again. “Crop-rotation is capable of innumerable variations, and the series may be longer or shorter, but there should be the slightest possible departure from the rule that a cereal crop ought always to be preceded by some crop that enriches the soil.” CHAPTER XLII LAND-DRAINAGE “In the bottom of a flower-pot you will find a small round hole. Over this hole it is customary to lay a bit of broken tile, and on this, if the plant to occupy the pot is delicate, a few small pebbles. This done, the pot is filled with vegetable mold. Why this hole, this bit of tile, these pebbles? That is what we are now about to consider. “Water is absolutely indispensable to plants, since it is the medium that dissolves the various nutritive ingredients of the soil and thus renders them capable of assimilation by the roots. Accordingly the soil penetrated by these roots must be constantly supplied with sufficient moisture either by rainfall or by artificial irrigation. But air is not less indispensable. It disinfects the soil and by causing slow combustion of the humus gives rise to a slight but uninterrupted liberation of carbonic acid gas, one of the nutritive substances required by vegetation. Should the roots be cut off from this life-giving agency, they would languish and finally decay. Thus it is that if vegetation is to thrive the soil in which it grows must have at the same time both air and water. But if the bottom of the flower-pot has no opening, or if its opening is stopped up, the water from the watering-can will not flow through, nor will there be any air admitted from below, and for lack of this the roots will decay. On the other hand, if the water, after saturating the earth, runs out freely by the hole in the bottom of the pot, the damp soil will become a sort of sponge to which the air will have access from all sides, and the plant will thrive.