38 THE SOIL. 



materials which, when appHed to our fields, increase the 

 crops ; but the same effect is produced by the plough. 

 It is evident that the mere fact of a favourable influence 

 exerted by chloride of sodium, nitrate of soda, salts of 

 ammonia, lime, and organic matter, affords no conclusive 

 proof that these have acted as nutritive substances. The 

 work performed by the plough may be compared to the 

 mastication of food by those special organs with which 

 nature has endowed animals ; and nothing can be more 

 certain than that the mechanical operations of agriculture 

 do not add to tlie store of nutritive substances in a field, 

 but that they act beneficially by preparing the existing 

 nutriment for the support of a future crop. With equal 

 certainty we know that chloride of sodium, nitrate of 

 soda, salts of ammonia, humus, and lime, beside the 

 action peculiar to their elements, perform also a kind of 

 digestive function comparable to that of the stomach in 

 animals, and in which they may partly replace each 

 other. These substances, therefore, act beneficially upon 

 those kinds of soil only in which there is a defect, not in 

 the quantity, but in the form and condition of the nutri- 

 tive elements ; and .they may accordingly in their perma- 

 nent action be replaced by a mechanical comminution, or 

 exceedingly fine pulverisation of the soil. 



The true art of the practical farmer consists in rightly 

 discriminating the means w^hich must be applied to make 

 the nutritive elements in his field effective, and in dis- 

 tinguishing these means from others which serve to keep 

 up the durable fertihty of the land. He must take the 

 greatest care that the physical condition of his ground be 

 such as to permit the smallest roots to reach those places 

 where nutriment is found. The ground must not be so 

 cohesive as to prevent the spreading of the roots. 



In a stiff, heavy soil, plants with fine, slender roots will 



