88 THE SOIL. 



Of nitric acid, it is generally assumed that it may, 

 like ammonia, serve to sustain the body of the plant. 

 Thus, chloride of sodium and the nitrates act in two 

 distinct ways : one direct, by serving as food for the 

 plant ; one indirect, by rendering the phosphates avail- 

 able for the purposes of nutrition. 



The salts of ammonia act upon earthy phosphates 

 in the same way as the salts just mentioned, but with 

 this distinction, that their power of dissolving phos- 

 phates is far greater ; a solution of sulphate of ammo- 

 nia will dissolve twice as much bone-earth as a solution 

 of an equal quantity of chloride of sodium. 



However, as regards the phosphates in the soil, the 

 action of the salts of ammonia can hardly be more 

 powerful than that of chloride of sodium or nitrate of 

 soda, since the salts of ammonia are decomposed by the 

 soil much more speedily, and often even immediately ; 

 so that, as a general rule, no solution of such a salt can 

 be said to be actually moving about in the soil. But 

 as a certain volume of earth, however small, is required 

 to decompose a given quantity of salts of ammonia, the 

 action of those salts upon this small volume of earth 

 must be all the more powerful. "While, then, the 

 action of salts of ammonia is barely perceptible in 

 the somewhat deeper layers of the arable surface soil, 

 that which they exercise on the uppermost layers is so 

 much the stronger. Feichtinger observed that solu- 

 tions of salts of ammonia decompose many silicates, 

 even felspar, and take up potash from the latter. Thus, 

 by their contact with the arable soil, they not only 

 enrich it with ammonia, but they effect, even in its 

 minutest particles, a thorough transposition of the nu- 

 tritive substances required by plants. 



The vegetable and animal remains in a soil seem to 

 exercise a remarkable influence upon the diffusion of 

 silicates. The experiments made on this point show 

 that the absorptive power of an arable soil for silicic 

 acid is in an inverse ratio to the amount of organic re- 

 mains in it ; so that a soil rich in such remains will, 

 when brought into contact with a solution of silicate of 



