EEASON OF THE EFFECTIVE ACTION OF GUANO. 251 



extent, when the soil contains a large store of other as- 

 similable food elements, which require only the presence 

 of the guano constituents to make them available for 

 nutrition. In the increased produce thus obtained, 

 there is, of course, carried off, together with the guano 

 constituents, also a part of the store of the other food 

 elements ; and upon repeated manurings with guaho 

 the fertilising effect of that agent must therefore neces- 

 sarily become feebler in the same proportion as the 

 quantity of these other food elements decreases in the 

 ground. The fertilising action of all compound ma- 

 nures 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 available 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, manrfring with 

 phosphates alone possessing the same degree of solubil- 

 ity, will produce the same effect as guano. 



When 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 

 principally 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 dif- 

 ferent kinds of effects ; they may, under certain circum- 

 stances, considerably increase the amount of produce, 

 and yet the favourable effect may not be positively at- 

 tributed to the action of ammonia as such (see page 86). 



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 largp 



