262 GUANO. 



At the beginning of the new rotation the arable soil 

 contained, accordingly, twice as much phosphates as at 

 the beginning of the preceding one. 



It will thus be seen that, under these circumstances, 

 where a field receives back, in the farm-yard manure, a 

 larger share of phosphate than it has lost in the crops, 

 the action of guano upon it will grow feebler from year 

 to year, until at last it ceases to be appreciable. 



But the case is very different as regards the application 

 of guano on fields to which a smaller quantity of phos- 

 phates is returned in the farm-yard manure than has been 

 lost in the crops, and that have, for instance, been culti- 

 vated for half a century upon the farm-yard manuring 

 system. It has already been explained, that on such fields 

 certain constituents of the fodder plants and of straw, 

 more particularly soluble sihcic acid and potash, are conti- 

 nually increasing in the arable soil, whilst by the export 

 of corn and flesh its store of mineral substances is re- 

 duced by the quantity contained in the exported matters. 

 The two sets of constituents had jointly produced the 

 crop. By taking away the seed-constituents a corre- 

 sponding amount of the straw and fodder constituents 

 was, accordingly, rendered ineffective. In fields of this 

 description, manuring with guano not only brings up the 

 amount of produce to the former standard, but frequently 

 even increases it to a surprising extent, Avhen the soil 

 contains a large store of other assimilable food elements, 

 whicli require only the presence of the guano consti- 

 tuents to make them available for nutrition. In the in- 

 creased produce thus obtained, there is, of course, car- 

 ried off, together ^vith the guano constituents, also a part 

 of the store of the other food elements ; and upon 

 repeated manurings with guano the fertihsing effect of 

 that agent must therefore necessarily become feebler in 



