334 The Physiology of Plants BOOK in 



nitrogen is generally returned to the soil in the form of 

 ammonia. It is clear, therefore, that if Boussingault's 

 views are well-founded, we have to seek in the soil the 

 processes by which nitrates are constructed from such 

 ammonium compounds. 



The researches which proceeded on these lines and 

 extended over nearly the whole of the period under 

 review brought to light a certain circulation of nitro- 

 gen in nature. In many cases decay of organized matter 

 gives ammonia and in some free nitrogen. From these we 

 have the construction of nitric acid and its compounds, 

 and their absorption by plants with subsequent reconstruc- 

 tion of proteins and their incorporation into living sub- 

 stance. Again we find the round completed by the death 

 and decay of the latter. 



The story thus tersely presented was, however, the 

 outcome of many laborious researches, undertaken in a 

 somewhat piecemeal fashion, and only properly collated 

 near the end of the century. 



The first researches were those having a bearing on the 

 formation of nitrates in the soil, the search for their 

 antecedents and the agencies by which the transformation 

 is effected. 



Among the workers who contributed to the solu- 

 tion of these problems there are two who claim special 

 mention. These are Warington in England and Wino- 

 gradsky in Russia ; their researches were prolonged, and 

 contributions appeared from time to time from both. 

 Though working quite independently, their results in the 

 main agreed and were thus confirmatory of one another. It 

 is owing chiefly to them that an accurate knowledge of the 

 subject was obtained. 



In his first series of experiments at the Agricultural 

 Station at Rothamstead, the results of which were published 

 in 1879, Warington found that a solution of ammonium 



