250 GUANO. 



ed, altogether 135 kilogrammes of phosphates ; the pro- 

 ductive power of this 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 the com- 

 mencement of a fresh rotation, 400 Ibs. of guano (with 

 34 per cent, of phosphates in it). 



Kilogrammes. 



The farm-yard manure supplied to the land . .272 of phosphates. 

 In the produce exported the field lost . . .135 u 



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 Ibs. of guano . . .135 



Altogether . . ' . . . . . 544 " 



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 h'eld 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 applica- 

 tion of guano on fields to which a smaller quantity of 

 phosphates is returned in the farm-yard manure than 

 has been lost in the crops, and that have, for instance, 

 been cultivated 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 silicic acid and 

 potash, are continually increasing in the arable soil, 

 whilst by the export of corn and flesh its store of min- 

 eral substances is reduced 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 corresponding amount of the straw and 

 fodder constituents was, accordingly, rendered ineffec- 

 tive. 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 



