MEANS FOE CAUSING THE DIFFUSION OF FOOD. 87 



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 potash from a solution will, in a solution of 

 chloride of sodium or nitrate of soda of the same alka- 

 line strength, leave undecomposed three-fourths of the 

 dissolved chloride of sodium and half of the nitrate of 

 soda. 



If, therefore, a field exhausted by culture, which 

 contains 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 will remain undecomposed 

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

 fluence, 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 

 phosphates 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 saturated with them, they are thereby withdrawn from, 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 already fixed in 

 the soil by physical attraction, 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 

 in the blood of all 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 nature has endowed fodder-plants, tuber- 

 ous and root-plants, which serve more particularly as 

 food for cattle, with a greater power of taking up 

 chloride of sodium from the soil than is possessed by 

 other plants ; and agricultural experience shows that 

 the presence of a small amount of common salt is 

 favourable to the luxuriant growth of these plants. 



