524 KEPOKT— 1880, 



The most striking points brought out by the foregoing illustrations are the 

 following: — 



First. Without nitrogenous manure, the gramineous crops annually yielded, for 

 many years in succession, much more nitrogen over a given area than is accounted 

 for by the amount of combined nitrogen annually coming down in the measured 

 aqueous deposits from the atmosphere. 



Second. The root crops yielded more nitrogen than the cereal crops, and the 

 leguminous crops very much more still. 



Third. In all cases — whether of cereal crops, root crops, leguminous crops, or a 

 rotation of crops — the decline in the annual yield of nitroffen, ivhen none loas supplied^ 

 ivas vei-xj yreat. 



How are these results to be explained ? Whence comes the nitrogen ? and 

 especially whence comes the much larger amount taken up by plants of the 

 leguminous and some other families, than by the gramineiE ? And, lastly, what i.'i 

 the significance of the great decline in the yield of nitrogen in all the crops when 

 none is supplied in the manure ? 



Many explanations have been offered. It has been assumed that the combined 

 nitrogen annually coming down from the atmosphere is ■Nery much larger than we 

 have estimated it, and that it is sufficient for all the requirements of annual growth. 

 It has been supposed that ' broad-leaved plants ' have the power of taking up nitro- 

 gen in some form from the atmosphere, in a degree, or in a manner, not possessed 

 by the narrow-leaved gramineie. It has been argued that, in the last stages of 

 the decomposition of organic matter in the soil, hydrogen is evolved, and that this 

 nascent hydrogen combines with the free nitrogen of the atmosphere, and so forms 

 ammonia. It has been suggested that ozone maj' be evolved in the oxidation of 

 organic matter in the soil, and that, uniting with free nitrogen, nitric acid woidd be 

 produced. Lastly, it has by some been concluded that plants assimilate tbe free 

 nitrogen of the atmosphere, and that some descriptions are able to do this in a 

 greater degree than others. 



We have discussed these various points on more than one occasion ; and we have 

 given our reasons for conchiding that none of the explanations enumerated can be 

 taken as accounting for the facts of growth. 



Confining attention here to the question of the assimilation of free nitrogen by 

 plants, it is obvious that, if this were established, most of our difficulties would 

 vanish. This question has been the subject of a great deal of experimental inquiry, 

 from the time that Boussingault entered upon it, about tlio year 1837, nearly up to the 

 present time. About twenty years ago it was elaborately investigated at Rotham- 

 sted. In publishing the results of that inquiry, those of others relating to it were 

 fully discussed ; and although the recorded e\'idence is admittedly very conflicting, 

 we then came to the conclusion, and still adhere to it, that the balance of the direct 

 experimental evidence on the point is decidedly against the supposition of the assi- 

 milation of free nitrogen by plants. Indeed, the strongest argument we know of in 

 its favour, is, that some such explanation is wanted. 



Not only is the balance of direct experimental evidence against the assumption 

 that plants assimilate free or uncombined nitrogen, Ijut it seems to us that the 

 balance of existing indirect evidence is also in favour of another explanation of our 

 difficulties. 



I have asked what is the significance of the gradual decline of produce of all 

 the difl'erent crops when continuously grown without nitrogenous manure ? It 

 cannot be that, in growing the same crop year after year on the same land, there is 

 any residue left in the soil that is injurious to the suljsequent growth of the same 

 description of crop ; for (excepting the beans) uku'c of each description of crop has 

 been grown year after year on the same land than the a\erage yield of the country 

 at large under ordinary rotation, and ordinary treatment — provided only, that suit- 

 able soil-conditions were supplied. Nor can the diminishing produce, and the 

 diminishing yield of nitrogen, be accounted for on the supposition that there was a 

 deficient supply of available mineral constituents in the soil. For, it has teen 

 shown that the cereals yielded little more, and declined nearly as much as without 

 manure, when a complex mineral manure was used, such as was proved to be ade- 



