260 HUMIC ACID. 



favorable to decay, inasmuch as they remain long 

 moist, and a certain degree of moisture is essential to 

 decay. 



678. It is necessary to acquire a clear conception 

 of the nature of what is called humus, and the office 

 which it performs in the nutrition of plants. It was 

 formerly supposed that plants derived their carbon 

 from the organic matter of the soil, that the brown 

 decaying substances called humus were directly ab- 

 sorbed by plants, and their carbon appropriated : it 

 is now known that this is not the case. The real use 

 of humus is, that it constantly generates carbonic 

 acid. 



679. When water is poured on humus or decaying 

 vegetable matter, very little of it is dissolved, and the 

 water acquires a pale yellow color: if, however, the 

 water contains potash or soda, either caustic or in 

 the state of carbonate, it dissolves far more humus, 

 and acquires a deep brown color ; the cause of this 

 is, that the alkalies facilitate the decay of that sub- 

 stance. Under the influence of the alkalies, a pecu- 

 liar acid is formed, which has accordingly been 

 termed the humic. The brown solution obtained 

 under these circumstances contains humic acid in 

 combination with potash or soda. 



680. It has been supposed that this acid might be 

 formed by the action of alkalies on the humus or de- 

 caying matters which the soil contains, and that the 

 humates, or compounds of humic acid, thus formed, 

 being absorbed by plants, might supply them with 



