of Science to Agriculture. 149 



be presented to a crop which, like the turnip, contains a large per 

 centage of phosphoric acid, the growth of the latter may be 

 checked, from its inability to obtain from the ground alone the 

 requisite supply of this ingredient, whereas the grasses, which take 

 up so much less of it, may find enough for their purpose in the soil, 

 and therefore continue unarrested in their growth. 



And this I take to afford the true explanation of the fact ob- 

 served, that certain manures act more efficiently when first applied 

 than they are found to do subsequently, a result which matter- 

 of-fact persons are contented to express by the phrase that the 

 ground is tired of a certain kind of dressing ; and which those 

 amongst them who indulge in theory account for by saying, that, 

 like other stimulants, they lose their influence by repetition. 



Now I am ready to admit that these manures, besides supply- 

 ing the plant v/ith nutriment, may exert an influence upon its 

 development, of a description which it is, perhaps, necessary to 

 denominate a stimulating one, until a more appropriate term shall 

 have been proposed. 



This has been clearly explained by Professor Johnston of Dur- 

 ham, in one of his recently published lectures,* where he shows, 

 for example, by an appeal to actual experiment, that if two equal 

 portions of the same grass or corn field in early spring be mea- 

 sured off, and one of them be top-dressed with nitrate of soda, or 

 v/ith saltpetre, the v/eight of nitrogen contained in the crop of hay 

 or corn reaped from the latter v/ill generally be found to exceed 

 that contained in the crop from the former, by a quantity much 

 greater than that which was present in the nitrate with which 

 the field had been dressed. It seems to me, therefore, probable, 

 that the fuller and more rapid development of the organs of the 

 plant, which was occasioned by the exuberant supply of nitrogen 

 derived from the manure, had enabled the living tissue to draw, 

 from the atmosphere, and from the rain-water in contact with its 

 roots, a larger amount of nitrogen than it v/ould otherwise be ca- 

 pable of deriving from the same sources : just as an animal, which, 

 during its infancy, has thriven on a liberal supply of Avholesome 

 and generous food, if it should afterwards be confined for a time 

 to meagre diet, will extract more support from such materials 

 than another would do, whose organs of digestion had been already 

 enfeebled by a course of abstinence or depletion. 



But in whatever sense we choose to interpret the so-called 

 stimulating influence of certain manures, it is obviously pursuing 

 a mistaken analogy to consider the falling off of their efficacy 

 after repetition as a case at all parallel to that insensibility to 



* See Johnston's Lectures on Agricultural Chemistry and Geology. 



