COMMERCIAL FERTILIZERS 491 



researches on plants, in which he, for the first time, 

 called attention to the significance of the ash ingredients 

 of plants, and pointed out that without them plant life 

 is impossible and, further, that only the ash of the plant 

 tissue is derived from the soil. 



Justus von Liebig, 1 in his writings published about 

 the middle of the nineteenth century, emphasized still 

 more strongly the importance of mineral matter in the 

 plant and the extraction of this matter from the soil. 

 He refuted the theory, at that time popular, that plants 

 absorb their carbon from humus, but he made the mis- 

 take of attaching little importance to the presence of 

 humus in the soil. He showed the importance of potas- 

 sium and phosphorus in manures, but in his later expres- 

 sions he failed to appreciate the value of nitrogenous 

 manures, holding that a sufficient amount is washed 

 from the atmosphere in the form of ammonia. 



A true conception of the necessity for a supply of 

 combined nitrogen in the soil was even at that time enter- 

 tained by Boussingault and by Sir John Lawes, although 

 the elaborate experiments conducted by Lawes, Gilbert, 

 and Pugh 2 in 1857 were required to fully demonstrate 

 the fact. Their care in conducting the experiments 

 resulted in their sterilizing the soil with which they ex- 

 perimented, and hence their failure to discover the utiliza- 

 tion of free atmospheric nitrogen by legumes. 



1 Liebig, J. Justus von. Principles of Agricultural Chemistry 

 with Special Reference to the Late Researches Made in England. 

 London. 1855. Also, Chemistry in its Applications to Agri- 

 culture and Physiology. New York. 1556. 



2 Lawes, J. B., Gilbert, J. IT., and Pugh, E. On the Sources 

 of the Nitrogen of Vegetation, with Special Reference to the 

 Question whether Plants Assimilate Free or Uncombined Nitro- 

 gen. Rothamsted Memoirs, Vol. 1, No. 1. 1862. 



