MANURES. 261 



*' 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 " soil not already saturated with them, they are thereby withdrawn 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 ex- 

 " erting 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." 



" Of nitric acid, it is generally assumed that it may, like am- 

 " monia, 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 available 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 phosphate is far greater ; a solution of 

 '' sulphate of ammonia 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 decom- 

 " pose 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 solutions of salts 

 " of ammonia decompose many silicates, even feldspar, 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 



