126 THE SOIL. 



adopting a proper system of rotation, and in using all the 

 means at his command to make the nutritive elements in 

 chemical combination available for plants. 



The achievements of practical agriculture in these re- 

 spects are wonderful, and they demonstrate that the 

 triumphs of art far exceed those of science, and that the 

 farmer, by aiduig the agencies which improve the chemical 

 and physical condition of his land, can obtain much more 

 abundant crops than by supplying nutritive matters. Be- 

 cause, what he can supply in the shape of manure, with 

 due regard to a proper return, is so small in comparison 

 Avith the store of nutritive matter contained in a fruitful 

 soil, that a perceptible 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 actuaUy 

 increase, this is less attributable to the addition made to 

 the store of mineral constituents than to their distribution, 

 and to the fact that certain quantities of inoperative sub- 

 stances have been rendered available. 



If we wished, by increasing the phosphoric acid re- 

 quii^ed for the formation of seed, to enable a wheat field 

 yielding an average produce of six grains to give two 

 additional grains, it would be necessary to mcrease by Jrd 

 the whole amoimt of the phosphoric acid present in the 

 field, and servmg for the formation of seed. For it is 

 always but a small fraction of the total quantity supplied 

 that comes into contact with the roots of the plants ; and 

 that they may be able to absorb this ird more, it is indis- 

 pensably necessary to increase the phosphoric acid by ^rd 

 in all portions of the soil. This reflection explains the 

 rule found true in experience, that to produce a marked 

 effect upon crops by manuring, a mass of manure must 



