206 THE SYSTEM OF FARM-YARD MANURING. 



The crops produced by farm-yard manuring differ 

 not only in every country, but even in every locality ; 

 and, strictly speaking, every field dressed with farm- 

 yard manure yields an average produce of its own. 



The action of farm-yard manure upon the increase 

 of produce is intimately connected with the condition 

 and composition of the soil ; it varies, therefore, in dif- 

 ferent 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 avail- 

 able 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 

 succeeding plant that requires large quantities of lime 

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



Eotash, magnesia, iron, phosphoric acid, nitrogen, on 

 elds severally abounding in potash, magnesia, iron, 

 phosphoric acid, or ammonia. Where a soil is abun- 

 dantly 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 de- 

 crease 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. 

 JSTow, as a supply of lime cannot be expected to re- 

 store 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 



83 Ibs., and that of the English quarter 425 Ibs., 100 Ibs. (Zollv. weight 

 = 110-2 Ibs. avoir.). 



