78 THE SOIL. 



K we compare the action of arable soil upon salts of 

 potash and salts of soda, we find that the soil has far less 

 attraction for soda than for potash ; so that the same 

 volume of earth which will suffice to remove all the pot- 

 ash from a solution will, in a solution of chloride of 

 sodium or nitrate of soda of the same alkahne strength, 

 leave undecomposed three-fourths of the dissolved clilo- 

 ride of sodium and half of the nitrate of soda. 



If, therefore, a field exhausted by culture, which con- 

 tains earthy phosphate scattered here and there, is 

 manured with nitrate of soda or chloride of sodium, and 

 by the action of rain a dilute solution of these salts is 

 formed, a portion of them w^ill remain undecomposed in 

 the ground, and must in the moist soil exert an influence, 

 weak in itself, but sure to tell in the long run. 



Like carbonic acid generated by the putrefaction of 

 vegetable and animal substances, and dissolving in water, 

 these salt solutions become charged with earthy phos- 

 phates in all places where these occur. Now when these 

 phosphates diffused through the fluid come into contact 

 with particles of the arable soil not already saturated with 

 them, they are thereby withdi'a^\ai from the solution, and 

 the nitrate of soda or chloride of sodium remaining in 

 solution again acquires the power of repeatedly exerting 

 the same dissolving and diffusing action upon phosphates 

 which are not abeady fixed in the soil by physical attrac- 

 tion, until these salts are finally carried down by rain- 

 water to the deeper layers of the soil, or are totally 

 decomposed. 



It is well-known that chloride of sodium is present m 

 the blood of aU animals, and that it plays a part in the 

 processes of absorption and secretion ; hence it may be 

 regarded as indispensable for these functions. We find 

 also that natm-e has endowed fodder-plants, tuberous and 



