I.] THEORIES OF PLANT NUTRITION 7 



much to pave the way for the adoption of a sound 

 theory. 



Thaer, about the same period, was still attributing the 

 chief share in the nutrition of the plant to the organic 

 juices, or, as we should say, to the humus in the soil, 

 which substance he showed to contain hydrogen, nitrogen, 

 sulphur, and phosphorus. Though knowledge of the 

 composition of plants, soils, and manures continued to 

 accumulate, as seen in the work of Sprengel and 

 Schubler, the next step forward was due to Boussingault, 

 who was the first man to undertake field experiments on 

 a practical scale. Farming his own land at Bechelbronne, 

 Alsace, from 1834 onwards, he systematically weighed 

 crops and manure and analysed both so as to obtain a 

 balance-sheet showing the quantities of carbon and nitro- 

 gen added in manure and removed in the crops. He 

 thus in 1838 demonstrated on a working scale the 

 enormous amounts of carbon w^hich are assimilated by 

 the plant from the atmosphere — far greater quantities 

 than the humus in the soil could continue to supply. 

 Boussingault's experiments led him to conclude that the 

 plant derives its nitrogen from the soil, though he also 

 showed that in certain rotations more nitrogen is 

 removed in the crop than is supplied in the manure. 



But it is to the great Liebig that we must attribute 

 the chief impulse which agricultural chemistry has 

 received ; though he made little original contribution 

 himself to the theory, adopting in the main the con- 

 clusions that arose from the work of Priestley, Ingen- 

 housz, Sennebier, and de Saussure, he was the man who 

 drove home to the minds, both of scientific men and of 

 farmers, the true theory of plant nutrition. 



In his report to the British Association, published 

 in 1840, and his Chemical Letters , he laid down the 

 general principle that the carbon compounds, which 



