4 MANURES FOR FRUIT TREES 



chemistry," but his theory is curious, for he states 

 that plants " are composed of gases with a small 

 proportion of calcareous matter." 



Einhof and Thaer's Theory. — This theory 

 sta,ted that the humus of the soil was the source 

 of plant-food, and fertility depended upon humus. ^ 

 This was an impeding theory which had to 

 be got rid of, but it was cut short by the work 

 of Liebig. According to the humus theory, it was 

 believed that trees and other plants feed upon 

 prepared organic matter, or humus,^ in the soil, 

 and this was regarded as a, source of both carbon 

 and nitrogen. Liebig showed, however, that (fungi 

 apart) plants derive from the soil only water, 

 ammonia, and inorganic salts, and corroborated the 

 already established conclusion that all the carbon 

 supplies are in the carbon dioxide of the air. As 

 plants die they necessarily enrich the soil with 

 humus, but this humus as such forms no part of 

 the food supply. 



De Saussure's Theory. — In 1804, De Saus- 

 sure published his famous work : Recherches 

 Chimiques sur la Vegetation, and in it he laid 

 special stress on the ash constituents of plants 

 being essential for life, and that they were absorbed 



1 The cultivators of Homer's time spoke of xa/i"' (humus). 



2 This theory made aXl plants saprophytes (!). 



