28 FERTILISERS CONTAINING NITROGEN [chap. 



And if objection be made that such plants were 

 enfeebled by the unnatural conditions, so that they 

 had lost their power to bring nitrogen into combina- 

 tion — to " fix " it, in current language — there are many 

 other types of experiment which render such criticism 

 invalid. For example, Hellriegel performed a long 

 series of experiments with different plants, which 



Table V. — Barley (Hellriegel and Wilfarth). 



showed, up to a point, that the amount of growth was 

 very closely proportional to the amount of nitrogen 

 supplied in a combined form, when there was a suffici- 

 ency of the other elements of plant food present. 

 This would not be the case were the plant able to get 

 any nitrogen for itself from the atmosphere. Again, to 

 meet an early objection of Liebig and his followers that 

 the Rothamsted crops, which seemed unable to draw 

 upon the nitrogen of the air though freely supplied 

 with phosphoric acid and potash, failed to do so because 

 they had not got the necessary initial development of 

 leaf, to one plot there was supplied a very small amount 

 of active nitrogenous manure, just to give the young 

 plant a good start, whereupon it might be able to 

 continue to feed upon the atmospheric nitrogen. But, 

 as Table VI. shows, the small addition of nitrogen only 

 produced a small increase of crop, very fairly propor- 

 tional to the much larger increase produced by a normal 

 application of the same fertiliser. If, then, the yield of 



