130 THE SOIL. 



tained in a fruitful soil, that a perceptible p increase of 

 produce can hardly be expected to result from it. 



But what the farmer may achieve by manuring is 

 at best the result unquestionably a most important 

 one that his crops suffer no diminution. "Where they 

 actually increase, this is less attributable to the addi- 

 tion made to the store of mineral constituents than to 

 their distribution, and to the fact that certain quantities 

 of inoperative substances have been rendered available. 



If we wished, by increasing the phosphoric acid re- 

 quired for the formation of seed, to enable a wheat-field 

 yielding an average produce of sk grains to give two 

 additional grains,, it would be necessary to increase by 

 ^rd the whole amount of the phosphoric acid present in 

 the field, and serving for the formation of seed. For it 

 is always but a small fraction of the total quantity sup- 

 plied that comes into contact with the roots of the 

 plants ; and that they may be able to absorb this -Jrd 

 more, it is indispensably necessary to increase the phos- 

 phoric acid by -Jrd in all portions of the soil. This re- 

 flection explains the rule found true in experience, that 

 to produce a marked effect upon crops by manuring, a 

 mass of manure must be laid on, utterly disproportion- 

 ate to the expected increase. 



A manure will exercise its beneficial action upon a 

 field in the most marked manner, when it establishes a 

 more suitable relative proportion between the several 

 mineral constituents in the soil ; because upon this pro- 

 portion the crops are dependent. ~No special argument 

 is needed to demonstrate, that where a wheat soil con- 

 tains just so much phosphoric acid and potash as will 

 suffice to afford the quantity of these two substances 

 required for a full wheat crop, and no more (accord- 

 ingly for every part by weight of phosphoric acid two 

 parts by weight of potash), an additional supply of one- 

 half more, or even of double the quantity of potash, 

 cannot exercise the slightest possible influence upon 

 the crop of corn. The wheat-plant requires for its full 

 developement a certain relative proportion of both nu- 

 tritive substances, and any increase of one beyond this 



