MINERAL MATTERS RESTORED BY MAN. 183 



soil the power of producing the same succession of 

 crops a second, a third, and a hundredth time : where 

 it is applied in proper quantities it will fully cure the 

 state of exhaustion, and often make a field more fertile 

 than it ever was before. 



The restoration of fertility by farm-yard manure 

 cannot be attributed to the mixture of combustible 

 materials (salts of ammonia and the substance of decay- 

 ing saw-dust) : for if these had a favourable effect, it 

 must have been of a subordinate kind. The action of 

 farm-yard manure most undoubtedly depends upon the 

 incombustible ash-constituents of the plants which it 

 contains. 



In farm-yard manure the field actually receives a 

 certain quantity of all the mineral ingredients which 

 have been removed in the crops. The decline of fertil- 

 ity was in proportion to the removal of mineral con- 

 stituents ; the renewal of productiveness is in propor- 

 tion to their restoration. 



The incombustible elements of cultivated plants do 

 not of themselves return to the soil, as the combustible 

 elements return to the atmosphere from which they 

 spring. The hand of man alone restores to the ground 

 the conditions of the life of plants : in farm-yard ma- 

 nure wherein they are contained, the farmer, following 

 a natural law, restores the lost power of production. 



