96 BIOGKAPHIES OF SCIENTIFIC MEN 



pletely the various tissues of plants. He taught that the 

 carbonic acid of the atmosphere was the principal source 

 of the carbon in plants ; that their hydrogen came from 

 water, their nitrogen from ammonia present in the air 

 and soil. The sulphur, which is an ingredient in the 

 protoplasm ("the basis of life") of living plants, comes 

 solely from the sulphates and sulphur compounds con- 

 tained in the soil ; and, finally, that the mineral ingredients 

 found in the ashes of plants come from the soil in which 

 they grow. 1 Liebig further demonstrated that plants 

 cannot live without these mineral ingredients, and that 

 the humus of the soil is incapable of forming the sole 

 nutrient material for plants. 



Liebig says in his book (p. 174) : 



Although the quantity of humus in a soil may be increased to a 

 certain degree by an artificial cultivation, still, in spite of this, there 

 cannot be the smallest doubt that a soil must gradually lose those of its 

 constituents which are removed in the seeds, roots, and leaves of the 

 plants raised upon it. The fertility of a soil cannot remain unimpaired, 

 unless we replace in it all those substances of which it has been 

 deprived. 



Liebig's theory was a revolutionary movement on the 

 accepted ideas of scientific agriculture which dawned at 

 the commencement of the nineteenth century. 



He supported his theory by well-conducted experi- 

 ments, showing that fields containing an excess of humus 



1 See A. B. Griffiths' Treatise on Manures, and the Spanish transla- 

 tion of the same work by Eugenio Guallart, under the title of Abonos 

 (1908). 



