io SOIL CONDITIONS AND PLANT GROWTH 



to nutrition (270). Schlibler was working at soil physics (254), and a 

 good deal of other work was quietly being done. No particularly im- 

 portant discoveries were being made, no controversies were going on, 

 and no great amount of interest was taken in the subject. 



But all this was changed in 1840 when Liebig's famous report to 

 the British Association upon the state of organic chemistry, after- 

 wards published as Chemistry in its Application to Agriculture and 

 Physiology (173), came like a thunderbolt upon the world of science. 

 With polished invective and a fine sarcasm he holds up to scorn the 

 plant physiologists of his day for their continued adhesion, in spite of 

 accumulated evidence, to the view that plants derive their carbon from 

 the soil and not from the carbonic acid of the air. " All explanations 

 of chemists must remain without fruit, and useless, because, even to 

 the great leaders in physiology, carbonic acid, ammonia, acids, and 

 bases, are sounds without meaning, words without sense, terms of an 

 unknown language, which awake no thoughts and no associations." 

 The experiments quoted by the physiologists in support of their view 

 are all " valueless for the decision of any question ". " These experi- 

 ments are considered by them as convincing proofs, whilst they are 

 fitted only to awake pity." Liebig's ridicule did what neither de 

 Saussure's nor Boussingault's logic had done : it finally killed the 

 humus theory. Only the boldest would have ventured after this to 

 assert that plants derive their carbon from any source than carbon 

 dioxide, although it must be admitted that we have no proof that plants 

 really do obtain all their carbon in this way. Thirty years later, in 

 fact, Grandeau (in) adduced evidence that humus may, after all, 

 contribute something to the carbon supply, and his view still finds 

 acceptance in France ; * for this also, however, convincing proof is 

 lacking. But for the time carbon dioxide was considered to be the sole 

 source of the carbon of plants. Hydrogen and oxygen came from 

 water, and nitrogen from ammonia. Certain mineral substances were 

 essential : alkalies were needed for neutralization of the acids made 

 by plants in the course of their vital processes, phosphates were 

 necessary for seed formation, and potassium silicates for the develop- 

 ment of grasses and cereals. The evidence lay in the composition of 

 the ash : plants might absorb anything soluble from the soil, but they 

 excreted from their roots whatever was non-essential. The fact of a 

 substance being present was therefore sufficient proof of its necessity. 



Plants, Liebig argued, have an inexhaustible supply of carbonic acid 

 in the air. But time is saved in the early stages of plant growth if 



1 See e.g. L. Cailletet (65) and Jules Lefevre (169). 



