THE ELEMENTS OF FOOD IX FARM-YARD MANURE. 229 



of Straw and potatoes, and nothing else, manuring a field 

 with it could merely restore the productive power of tlie 

 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 pro- 

 ductiveness 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 contained 

 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 actuall}'" 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 phos- 

 phoric acid and nitrogen in the arable soil, it increases, 

 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-consti- 

 tuents removed from the field' are brought back to it in 

 that manure, higher crops of com, straw, and potatoes 

 are the natural result. 



This increase of the produce of all cultivated plants 

 drawing their principal food from the ai^able surface soil, 

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

 certain appointed limit. 



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



