The roots of plants are living and require air. The soil must not be trodden too hard round them or air cannot get in, nor can it if too much water is present.
Grass can put up with more water and less warmth than most cultivated crops.
Fig. 39. Rough grass pasture near the river, above that is arable land and still higher is woodland
Instances of these facts may be found in going down any hill 500 ft. or more in height: the top is usually wood or waste, being too cold for crops, below this may come grass land, lower still arable land. It is both warmer and moister in the valley (since water runs down hill), and so we can account for the proverbial fertility of valleys. But just near the river, if there is one, the ground may be too wet for crops, and therefore grass is grown. Clay land that is rather too wet to plough is usually left in grass.