21-2 THE SYSTEM OF FARM-YAED MANUEING. 



produce is intimately connected with tlie condition and 

 composition of the soil ; it varies, therefore, in different 

 fields, simply because the composition of the soil varies. 



To understand the action of farm-yard manure, it is 

 necessary to remember that the exhaustion of a field 

 arises from the loss of a certain amount of nutritive con- 

 stituents, at the end of a rotation, inflicted upon the soil 

 by preceding crops, which of course leave less available 

 food in the soil for the following crops. 



However, the loss of each individual constituent has 

 not the same effect upon the exhaustion of the soil. 



The loss of lime which a calcareous soil suffers by a 

 cereal or by clover, matters little to the growth of a suc- 

 ceeding plant that requires large quantities of lime to 

 thrive well. The same applies equally to the loss of potash, 

 magnesia, iron, phosphoric acid, nitrogen, on fields seve- 

 rally abounding in potash, magnesia, iron, phosphoric 

 acid, or ammonia. Where a soil is abundantly provided 

 with one of the mineral constituents, the amount of that 

 constituent removed by the crops is so small a fraction of 

 the whole mass, that the effect of the diminished store is 

 not appreciable from one rotation to another. 



But practical experience shows that the crops do 

 decrease from one rotation to another, and that the land 

 requires a fresh supply of certain ingredients by manur- 

 ing, if it is again to produce as large harvests as before. 



Now, as a supply of lime cannot be expected to 

 restore the fertility of an exhausted field where lime con- 

 stitutes the principal bulk of the soil, just as little as a 

 supply of potash or phosphoric acid to a field abounding 

 in potash or phosphoric acid, it is easy to understand that 

 where the productive power of an exhausted field is 

 restored, the fertilising effect is to be attributed simply to 

 the manure returning to the field those elements of food 



