REASON OF THE EFFECTIVE ACTION OF GUANO. 263 



the same proportion as the quantity of these other food 

 elements decreases in the ground. Tlie fertiHsing action 

 of all compound manures is rarely dependent upon one 

 constituent alone ; and as guano contains, in its ammonia 

 and phosphoric acid, two food elements, which require 

 the presence of each other to be available, manuring with 

 guano insures the action of the phosphoric acid, because 

 the particles of the latter are in immediate contact with 

 ammonia particles, that are at the same time also avail- 

 able to the roots ; and in the same way the phosphoric 

 acid insures and increases the action of the ammonia. 



In a soil abounding in ammonia, manuring with phos- 

 pliates alone possessing the same degree of solubility, 

 will produce the same ejQfect as guano. 



\yiien ammonia salts fail to produce any effect on a 

 field whilst guano is found to act favourably, there is 

 reason to attribute the beneficial effect of the guano prin- 

 cipally to the phosphoric acid in it ; but in the reverse 

 case the conclusion would not hold equally good, because 

 the salts of ammonia produce two different kinds of 

 effects ; they may, under certain circumstances, consider- 

 ably increase the amount of produce, and yet the favour- 

 able effect may not be positively attributed to the action 

 of ammonia as such (see page 77). 



The presence in the soil of a sufficient quantity of 

 potash and silicic acid is always presupposed when guano 

 increases the produce of corn ; and on a soil rich in 

 potash and magnesia, the application of guano alone 

 insures a succession of crops of such plants, which, like 

 potatoes, require for their growth chiefly potash and 

 magnesia. 



Meadows and corn fields which gave at first large crops 

 with guano, become at last, by tlie continued use of this 

 agent, frequently so drained of silicic acid and potash, as 



