RELATIONS EXISTING AMONG FOOD ELEMENTS. 127 



be laid on, utterly disproportionate to the expected in- 

 crease. 



A manure will exercise its beneficial action upon a field 

 in the most marked manner, when it estabUshes a more 

 suitable relative proportion between the several mineral 

 constituents in the soil ; because upon this proportion the 

 crops are dependent. No special argument is needed to 

 demonstrate, that where a wheat soil contains 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 (accordingly for every part by weight 

 of phosphoric acid two parts by weight of potash), an ad- 

 ditional 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 re- 

 quu'es for its fuU developement a certaui relative propor- 

 tion of both nutritive substances, and any increase of one 

 beyond this proportion makes the other not a whit more 

 effective, because the additional supply exercises by itself 

 no action. 



An increase of phosphoric acid alone has just as little 

 influence in making the returns greater, as an increase of 

 potash alone : this law apphes equally to eveiy nutritive 

 substance, potash, magnesia, and silicic acid ; no supply of 

 these substances beyond the requirement of the wheat- 

 plant, or its capacity of absoi-ption, will have any effect 

 upon its growth. The relative proportions of the mineral 

 substances, which the plants draw from the soil, are easily 

 determined by analysing the ashes of the produce. It is 

 found by analysis that wheat, potatoes, oats, and clover 

 receive the following proportions of phosphoric acid, 

 potash, lime, magnesia, and silicic acid : — 



