FERTILITY NOT DUE TO ORGANIC MATTER. 185 



Tliat fann-yard manure Avill completely restore the fer- 

 tility of u field exhausted by cultivatiou is a fact fully 

 established by the experience of a thousand years. 



Farm-yard manure supplies to the field a certain quan- 

 tity of organic, i. e. combustible substances, together mtli 

 the ash-constituents of the food consumed. We must 

 now consider what part is taken, in the restoration of 

 fertihty, by the combustible and incombustible consti- 

 tuents of the manure. 



The most superficial examination of a cultivated field 

 shows that all the combustible constituents of the plants 

 grown upon it are derived from the air and not from the 

 soil. If the carbon even of a portion of the vegetable 

 matter in the crop were derived from the soil, it is quite 

 clear, that if the ground contained a certain amount of 

 carbon before the harvest, this amount must be smaller 

 after every harvest. A. soil deficient in organic matter 

 must necessarily be less productive than a soil abounding 

 in it. 



Now, experience proves that a field in constant culti- 

 vation does not, therefore, become poorer in organic or 

 combustible substances. The soil of a meadow which in 

 ten years has yielded a thousand cwt. of hay per hectare, 

 is found to be, at the end of those ten years, not poorer 

 in organic substances, but richer than before. A clover- 

 field after a crop retains in the roots left in the ground 

 more organic substances, more nitrogen, than it originally 

 possessed ; yet after a number of years it becomes 

 unproductive for clover, and no longer gives remu- 

 nerative retiurns of that crop. 



A field of wheat, or potatoes, is not poorer in organic 

 substances after harvest, than before. As a general rule, 

 cultivation increases the store of combustible constituents 

 in the ground, while its fertility, however, steadily dimi- 



