106 THE SOIL. 



The injury done to wood-lands by raking away the 

 leaves cannot be explained merely upon the assumption 

 that the soil is deprived of its ash-constituents, which 

 are taken away with the foliage ; for, in themselves, 

 the fallen leaves and twigs are poor in nutritive sub- 

 stances, especially potash and phosphoric acid ; and 

 besides, these elements do not reach the deeper layers 

 of the soil, where they might be again absorbed by the 

 roots. The injury is, perhaps, rather attributable to 

 the fact, that the remains of leaves and plants consti- 

 tute a lasting source of carbonic acid, which, carried by 

 rain to the deeper layers, must powerfully contribute 

 to disintegrate and decompose the earthy particles. In 

 a dense wood, where the air is more rarely renewed 

 than in the open plain, this supply' of carbonic acid is 

 important ; moreover, the thick carpet of leaves pro- 

 tects the ground from being dried by the air, and main- 

 tains it in a permanent state of moisture, particularly 

 useful to foliaceous trees, which exhale from their 

 leaves larger 'quantities of water than the coniferous 

 plants. 



To understand the operations of agriculture, it is 

 indispensably necessary that the farmer should have 

 the clearest knowledge of the manner in which plants 

 derive their nutriment from the soil. 



The opinion that the roots of plants extract their 

 food immediately from those portions of the soil which 

 are in direct contact with their absorbent surfaces, does 

 not imply that potash, lime, or phosphate of lime, in 

 the solid, undissolved state can penetrate the membrane 

 of the cells ;* nor does it imply that the nutritive sub- 



* If a glass vessel is filled to the brim with water, in which are a few 

 drops of hydrochloric acid, and covered closely with a piece of bladder, so 

 that the water moistens the bladder and no air is left between them, and 

 the outside of the bladder is carefully dried, it may then be seen how a 

 solid body, without the cooperation of a fluid from the outside, can make 

 its way through the bladder to the water in the glass. For if a little chalk 

 or finely-pulverised phosphate of lime is strewed upon the dried outer sur- 

 face of the bladder, the powder will disappear in the course of a few hours, 

 and the usual reactions will show the presence of lime and phosphate of 

 lime in the fluid. 



Of course the passage of the carbonate and phosphate of lime in the 



