102 THE SOIL. 



phosphoric acid is of itself sufficient to prove that these 

 materials in the soil cannot change their place. The 

 ground must contain a certain amount of moisture to 

 be able to furnish food to plants ; but it is^not necessary 

 for their growth that the water should be free to move 

 about. It is well known that stagnant water in the 

 soil is injurious to most of the cultivated plants ; and 

 the favourable effect upon their growth produced by 

 draining just depends on this, that an out let -is opened 

 to the water moving by the force of its own gravity, 

 and the earth is moistened by that water only which is 

 retained by capillary attraction. 



If we regard the porous earth as a system of capil- 

 lary tubes, the condition which must render them best 

 suited for the growth of plants is unquestionably this, 

 that the narrow capillary spaces should be filled with 

 water, the wide spaces with air, and that all of them 

 should be accessible to the atmosphere. In a moist 

 soil of the kind, affording free access to atmospheric 

 air, the absorbent root-fibres are in most intimate con- 

 tact with the earthy particles ; the outer surface of the 

 root-fibres may here be supposed to form the one, the 

 porous earthy particles the other wall of a capillary 

 vessel, the connection between them being effected by 

 an exceedingly thin layer of water. This condition is 

 equally favourable for the absorption of fixed and of 

 gaseous elements of food. If, on a dry day, a wheat or 

 barley-plant is cautiously pulled up from a loose soil, a 

 cylinder of earthy particles is seen to adhere like a 

 sheath round every root-fibre. It is from these earthy 

 particles that the plant derives the phosphoric acid, pot- 

 ash, silicic acid, &c., as well as the ammonia. These 

 substances are introduced into the plant by means of 

 the thin layer of water, the molecules of which are in 

 motion only in so far as the roots exercise an attractive 

 power upon them. 



From the composition of spring-water, and the 

 water of brooks and rivers, every single drop of which 

 has been in contact with rocks, or with the soil of for- 

 ests and fields, we see what exceedingly minute quanti- 



