182 FARM-YAKD MANUKE. 



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 cul- 

 tivation 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 nitro- 

 gen, than it originally possessed ; yet after a number 

 of years it becomes unproductive for clover, and no 

 longer gives remunerative returns of that crop. 



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

 ganic substances after harvest, than before. ,As a gen- 

 eral rule, cultivation increases the store of combustible 

 constituents in the ground, while its fertility, however, 

 steadily diminishes. After a consecutive series of re- 

 munerative crops of corn, turnips, and clover, these 

 plants will thrive no longer in the same field. 



Since, then, the presence of decaying organic re- 

 mains in the soil does not, in the slightest degree, pre- 

 vent or arrest its exhaustion by cultivation ; it is im- 

 possible that an increase of those substances can restore 

 the lost capacity of a field for production. In fact, 

 when a field is completely exhausted, neither boiled 

 saw-dust nor salts of ammonia, nor both combined, will 

 impart the power of yielding the same series of crops a 

 second and third time. When these substances im- 

 prove the physical condition of the ground, they exert 

 a favourable influence upon the produce ; but, after all, 

 their ultimate effect is to accelerate and complete the 

 exhaustion of the soil. 



But farm-yard manure thoroughly restores to the 



