FOOD OF PLANTS IN GENERAL. 489 



upon the nature of the cultivated plant. In a six years' rotation of crops, 

 was yielded in 



Nitrogen. 



1st year by Potatoes, on the acre 24*75 Ibs. 



2nd Wheat . . .18-92 



3rd Clover . . . 45-21 



4th Wheat and Turnips 29'93 



5th Pease . . . 52-63 



6th Rye . . . 17'33 



In the whole, 1 88*77 Ibs. nitrogen were produced, whilst the manure 

 spread at the beginning contained only 130-31 Ibs. of nitrogen. Again, 

 in three rotations of crops, two of five years, and one of six, an equal 

 quantity of manure was supplied to all ; namely, for the year and for the 

 acre 2 1-90 Ibs. ; but the annual superabundance of the nitrogen obtained 

 over that contained in the manure was, in 



1st course of 5 years 5 -06 Ibs. 

 2nd 5-45 



3rd 6 9-83 



This last fact is sufficient of itself to show the independence of the 

 production of nitrogen of the contents supplied in manure. 



In six rotations, embracing twenty-one years, the average produce of 

 nitrogen from all the harvests, as compared with the manure supplied, 

 was as 1 : 2-8. According to information afforded by Crud in the cul- 

 ture of Lucerne, Boussingault reckons it as 1 : 4*8. It is supposed that 

 a positive proof of this dependance of the production of nitrogen upon 

 the quantity conveyed to the plants in manure, is found in the fact, that 

 with the increase of the one there is an increase of the other. But it is 

 obviously an error to confound what may be a coincidence with cause 

 and effect. If the fact was as represented, why does a plant always sink to 

 the ground when watered with a solution of ammoniacal salts ? Manifestly 

 because the healthy and strong development of the plant, and therewith 

 the assimilation of the nitrogen, demand further conditions than simply the 

 presence of salts of ammonia. I would here also direct attention to the 

 close connection between the salts of phosphoric acid and the nitrogenous 

 substances of plants. Liebig has correctly stated that we must recognise 

 the fact that the latter are never formed without the presence of the 

 former ; but he has not made experiments with various manures con- 

 taining different proportions of nitrogenous substances and phosphates. 

 It is possible that the nitrogen of cultivated plants may come from this 

 source, and thus we may account for its quantity in them being con- 

 stantly the same. Such plants find abundance of nitrogen at their com- 

 mand without need of receiving any through the medium of manure, 

 for we see that with all applications of this kind we cannot produce so 

 much as is yielded by irrigated meadows which are entirely unmanured. 



We want much some exact experiments upon this subject. We can only 

 adduce those of Schattenmann and of Kuhlmann : the first give almost a 

 doubling of the produce of meadows after the application of carbonate 

 of ammonia (Boussingault*); whilst those of Kuhlmanf show the in- 



* The favourable action of the salts of ammonia may perhaps be explained, according 

 to the experiments of Schultze of F.ldena, through some change produced in the 

 mechanical condition of the soil. 



} See Appendix A. 



