142 ABSORBENT POWER OF THE SOIL. 



country as broad pine-barrens, on which the beautiful 

 white pine is the predominating and characteristic tree ; 

 wliile the pure sands, which form the uppermost layer, 

 are covered by the yellow pine [Pinus mitis.) Of these 

 latter, when brought into cultivation, experience has 

 shown that the loamy sands are less affected by the 

 heat than the stiff clays, but that the apparently purer 

 sands bear the drought better than either. The mecha- 

 nical porosity of a soil, in fact, has almost as much to 

 do with its power of resisting scorching heats in sum- 

 mer, as any other of its properties, either physical or 

 chemical. 



This will appear very plain by a reference to one 

 consideration only. The leaves of plants exhale watery 

 vapour in large quantity, and this quantity increases 

 with the intensity of the summer heat. No experiments 

 have been made on this subject in America, I believe ; 

 but in the cooler climate of England, an acre of our 

 cultivated crops has been estimated to exhale from its 

 leaves, during the four months of summer, about three 

 million pounds of water 5* while in the same time there 

 falls, of available rain, not more than eight hundred 

 thousand pounds — little more than a fourth of the 

 exhalation. The soil and the plant, therefore, must 

 inhale from the air during the cool night far more water 

 than they derive from the rain that falls, however grate- 

 ful and refreshing to plants this may be. 



Now, the more porous a given soil is made, the more 

 absorbent it is of moisture from the air, and hence one 

 reason why light soils suffer less from drought than 

 heavy soils — why heavy clay soils are so much improved 



* Mr Lawes, in some recently published experiments, found the 

 quantity of water exhaled to be less than it was estimated to be from 

 the experiments referred to in the text. But his results also show that 

 plants must derive moisture from some other source than the rain-water 

 that sinks into the soil. 



