Fig. 35. Plants give out water through their leaves
Another thing you will notice is that fields lying at the side of a river and liable to be flooded, and fields high up in wet hill districts, are covered with grass. In a clay country there is also a great deal of grass land and not much ploughed land; if you live where there is much clay you can easily discover the reason. Clay becomes very wet and sticky when rain falls, and very hard in dry weather: it is, therefore, difficult to cultivate. Farmers cannot afford to spend too much money on cultivation, and so they prefer grass, because once it is established it goes on indefinitely and does not want ploughing up and re-sowing. And besides, farmers have learned by experience that grass can tolerate more water and less warmth than most other English crops. There is much more grass land in those parts of England where the rainfall is high and the temperature rather low—e.g. the northern parts of England—than in the eastern counties where the rainfall is low.
Fig. 36. Stephen Hales's Experiment (from Vegetable Staticks, Vol. I. 1727)
The difference in water supply, therefore, leads us to expect the following differences between sandy soils and clays or loams:—
On sandy soils (the water content being small) the wild plants and trees usually have small leaves. Cultivated plants do not give very heavy crops, but they ripen early.
On clay soils (the water content being good) wild plants and trees usually have larger leaves. Cultivated plants give good crops, but they ripen rather late. If the water content is too good or the clay is too sticky the land is generally put into grass.