THE ELEMENTS OF FOOD IN FABM-YAKD MANURE. 221 



field with it could merely restore the productive power 

 of the arable soil for straw and potatoes, but not for 

 corn. Under these circumstances it would remain as 

 rich as before in food elements for straw and potatoes, 

 but would be poorer for corn to the extent of the whole 

 quantity of corn-constituents taken away in the crops. 



If farm-yard manure is to restore the former produc- 

 tiveness of a field for corn, it must necessarily contain 

 an amount of corn-constituents corresponding to the 

 loss sustained, that is to say, as much or even more 

 than has been removed. 



The amount of the elements of food for corn con- 

 tained in the farm-yard manure naturally depends upon 

 the sum of these elements which have passed over into 

 manure, from the cattle feeding upon clover or turnips. 



Where this supply exceeds the loss sustained, the 

 arable soil is actually made richer in corn-constituents ; 

 but in that case it is enriched also in the conditions for 

 an increased produce of straw and tuberous plants. 

 Where, therefore, the farm-yard manure (by the clover 

 or turnip constituents in it) increases the amount of 

 phosphoric acid and nitrogen in the arable soil, it in- 

 creases, in a much greater proportion, the quantity of 

 potash and lime, and to some extent also that of silicic 

 acid ; and since, as already stated, the whole of the 

 straw-constituents removed from the field are brought 

 back to it in that manure, higher crops of corn, straw, 

 and potatoes are the natural result. 



This increase of the produce of all cultivated plants 

 drawing their principal food from the arable surface 

 soil, may go on for a very long time, but in all fields 

 it has a certain appointed limit. 



The time comes, sooner or later, for every field, 

 when the subsoil (which is to the clover or turnips 

 what the arable surface soil is to the cereals), suffering 

 a continued drain upon its stores of phosphoric acid, 

 potash, lime, magnesia, &c., begins to lose its produc- 

 tive power for clover or turnips ; and thus the nutritive 

 substances, taken away from the arable surface soil in 

 the corn crops, are no longer replaced from the store 



