ADDITIOX OF GUANO TO FARM-YARD MANURE. 261 



easy to understand wliy guano surpasses, in tliese respects, 

 many otlier manures. 



As regards certainty of action, guano will not bear 

 comparison with farm-yard manure, Avhich, from its 

 nature, is effective in all cases ; for farm-yard manure 

 restores to the land all the soil constituents of the pre- 

 ceding rotations, though not in the same proportions, 

 whereas guano restores only some of them, and cannot, 

 therefore, replace farm-yard manure. As guano, however, 

 contains, with the exception of a certain quantity of 

 potash, the chief constituents (phosplioric acid and 

 ammonia) of the exported corn and flesh, the addition of 

 a certain proportion of guano to farm-yard manure may 

 serve to restore the proper composition of the latter, and, 

 with it, also that of the soil. 



Let us suppose, for the pm-pose of illustration, that a 

 hectare of land has been manured with 800 cwt. of farm- 

 yard manure, containmg, according to Voelker's analysis, 

 272 kilogrammes of phosphate, and that the field has, at 

 the end of the rotation, returned the same quantity of 

 farm-yard manure of the same composition, and has lost 

 by the corn and the animal produce exported, altogether 

 135 kilogrammes of phosphates ; the productive power 

 of tliis field, in so far as it depends upon the phosphates, 

 would not only remain unaltered, but would even be 

 considerably increased, by adding to the 800 cwt. of 

 farm-yard manure supplied to it at tlie commencement 

 of a fresh rotation, 400 lbs. of guano (with 34 per cent, 

 of phosphates in it). 



Kilogrammes 



The farmyard manure supplied to the land , . 272 of phosphates 



In the produce exported the field lost . . .13.5 „ 



There remained in the arable soil . . . .137 „ 

 In the new rotation was added by the fresh supply 



of 800 cwt. of farm-yard manure . . . 272 ,, 



By the addition of the 400 lbs. of guano . 135 ,, 



Altogether 544 „ 



