HISTORICAL INTRODUCTION 5 



from the soil ; the carbon of plants was derived 

 from the carbon dioxide of the air, and that the 

 hydrogen and oxygen were derived from water. 

 But his theory was erroneous, for he believed that 

 lime and potash, found in the ashes of trees and 

 other plants, were produced by the action of 

 nitrogen during their combustion. De Saussure's 

 theory was interwoven with the doctrine of 

 " organic mould " — the humus of older writers ; 

 and that the most perfect plant-food was dung, 

 and the ash or mineral constituents of plants were 

 formed by dung (a nitrogenous substance). Later 

 in life the famous Frenchman modified his theory 

 concerning plant nutrition — and he fully recognised 

 the great value the mineral constituents of the soil 

 played in the growth of plants. 



Davy's Theory. — In 1831, Sir Humphry 

 Davy published a work on Agricultural Chemistry, 

 and in it he based a theory for the growth of 

 plants on the water-and-heat absorbing powers 

 of a soil. In fact, Davy believed that fertility 

 depended entirely upon the mechanical properties 

 of a soil. 



Liebig's Theory. — In 1 840, the great work ^ 

 was published, and in it he laid down the all- 



' "Die organische Chemie in ihrer Anwendung auf Agricultur 

 und Physiologie." 



