150 On the Apjylicatlon 



stimuli which animals acquire, from their continued use : for to 

 imagine the soil, an inert mass, susceptible of such an influence, 

 is obviously absurd ; and inasmuch as every successive crop grown 

 upon the land consists of an assemblage of new individuals, it 

 seems no less so to imagine that a given stimulus can in any 

 respect operate less energetically upon it, in consequence merely 

 of its having been applied to the crop of the year antecedent. 



In many of the cases in which, in the language of husband- 

 men, the soil is said to tire of a particular species of manure, the 

 phenomenon may, I think, be readily explained, from the ground 

 being already surcharged with the material which the latter was 

 calculated to afford, as under such circumstances no benefit can 

 be expected to accrue from a more liberal supply of it. 



Thus, bone-dust, after producing great returns on its first ap- 

 plication, seems frequently to lose all effect afterwards : in which 

 case, however, it may be said that the ground remains benefited 

 from the effect of the first year's dressing, and is only not further 

 improved by the subsequent application of the same manure. 



But, in other instances, it has been asserted, that the land, after 

 appearing to reap great immediate benefit from a certain kind of 

 fertiliser — as, for example, from the nitrates — is left by it after- 

 wards in even a worse condition than it was in previously. Now 

 this effect may, I think, become intelligible, if we only recollect, 

 that the different ingredients which each plant contains must be 

 secreted in relative proportions one to the other, so that an in- 

 creased absorption of any one will produce a larger demand for 

 all the rest. 



Hence we may suppose, that the exuberant supply of nitrogen 

 furnished by these salts had caused the organs of the plant to 

 secrete so much larger an amount of the earthy phosphates, of the 

 alkalies, &c., of which there is only a limited proportion present, 

 that the crop of the succeeding year became stunted by reason of 

 the soil being no longer able to afford to it, in adequate quan- 

 tities, these essential elements of its constitution. And if this be 

 the true account of the matter, the practical inference would seem 

 to be that, instead of abandoning the use of the nitrates altoge- 

 ther, as the theory I have been combatting would suggest, it 

 should be our endeavour to supply those other ingredients which 

 are deficient, by the addition, at intermediate periods, of bone- 

 earth or of wood-ashes, in proportions adequate to the demand 

 for them, occasioned by the expected increase of crop. 



FortunateU^ we are provided, in the dung of animals, with a 

 species of manure of which the land never can be said to tire ; and 

 for this snnple reason, that it contains within itself not one alone, 

 but all the ingredients which plants require for their nutrition. 



