HISTORICAL INTRODUCTION 3 



Tull's Theory. — According to Jethro Tull, 

 the mineral particles of the soil are the true food 

 of trees, etc. ; and this being the case he insisted ' 

 upon the importance of tillage operations in order 

 to render the mineral particles in such a state 

 of division that they may readily be absorbed 

 by the rootlets. " The more thoroughly a soil 

 is tilled, the more luxuriant the crops would be." 

 He was right as far as his tillage operations went, 

 but he was wrong in asserting that the rotation 

 of crops and manures might be dispensed with 

 altogether.^ 



Bonnet's Discovery. — Charles Bonnet (1720- 

 1793) discovered the source of carbon and the 

 function of the leaves ; but it was Percival who 

 first pointed out that carbon dioxide was a plant- 

 food; and S^n^bier (1742-1809) proved that leaves 

 decomposed the carbon dioxide of the air, assimilat- 

 ing the carbon, and giving out the oxygen. 



Dundonald's Theory.— In 1795, Earl 

 Dundonald published the first book in the English 

 language on agricultural chemistry ; and in it he 

 asserts that " there is no operation or process not 

 merely mechanical that does not depend on 



' In the Japanese system of soil-improvement, stable manure 

 and rotation of crops play only a subordinate rdle. The productive 

 capacity of arable land is maintained by sub-soil working, tillage, 

 watering, and artificial manuring. 



B 2 



