78 THE SOIL. 



greater or less porosity ; a dense, heavy clay soil and a 

 loose sandy soil possess the absorptive power in the 

 smallest degree. 



There can be no doubt that all the component parts 

 of arable soil have a share in these properties, but only 

 when they possess a certain mechanical condition, like 

 wood or animal charcoal ; and that this power of 

 absorption depends, as in charcoal, upon a surface 

 attraction, which is termed a physical attraction, be- 

 cause the attracted particles enter into no chemical 

 combination, but retain their chemical properties.* 



The arable soil owes its formation to the disintegra- 

 tion of minerals and rocks, brought about by the action 

 of mighty mechanical and chemical agencies. Though 

 the comparison may not be altogether apt, the rock 

 may be said to stand in about the same relation to the 

 arable soil resulting from its disintegration as the wood 

 or the vegetable fibre to the humus resulting from its 

 decay. 



The same causes which in the course of a few years 

 convert wood into humus act also upon rocks, with this 

 difference, however, that it requires the combined action 

 of water, oxygen, and carbonic acid, for probably a 

 thousand years, to produce from basalt, trachyte, fel- 

 spar, or porphyry, the thinnest layer of arable soil (such 

 as is found in the plains of river valleys and low lands) 

 with all the chemical and physical properties suited for 

 the nutrition of plants. Sawdust possesses the proper- 

 ties of humus no more than powdered rocks have the 

 properties of arable soil. No doubt sawdust may pass 

 into humus and powdered stones into arable soil, but 

 the two states are essentially distinct ; and no human 

 art can imitate the operations which were necessary, 

 during immense ages, to convert the divers kinds of 

 rocks into arable soil. , 



Arable soil, resulting from the disintegration of 

 various kinds of rocks, bears the same relation, in 



* The term, ' physical attraction,' as used here, does not signify a 

 peculiar attractive force, but merely designates the ordinary chemical 

 affinity, which shows differences of degree in its manifestation. 



