DECOMPOSITION OF ORGANIC SUBSTANCES IN SOIL 



LiGNiN AND ITS DECOMPOSITION. — The lignin in plants increases 

 in abundance as the plants become older; this complex makes up a 

 considerable portion of the mature plants. Since Ugnin is very- 

 resistant to decomposition by the common soil fungi and bacteria, 

 it tends to accumulate with the advance in decomposition of the 

 plant materials, while the sugars, proteins, hemicelluloses, and 



Mgm. 

 180 



160- 



cul40 

 1 120 



-iioo 



O 



80 - 



o 



1 60 

 o 



^ 40 

 20 







10 20 30 40 50 10 20 30 40 50 

 Time (Days) 



Fig. 45. — Comparative growth of cellulose-decomposing bacteria, fungi, and 

 actinomyces in a neutral soil. Left curves at 30 per cent of the moisture- 

 holding capacity of the soil; curves at right at 50 per cent of the moisture- 

 holding capacity (after Dubos). 



cellulose are quite readily decomposed. Only certain groups of 

 Basidiomycetes (species of Polyporus, Merulius) and certain 

 Actinomyces are capable of attacking the lignin; however, even 

 the action of these organisms is much slower than in the decom- 

 position of other plant constituents. Under anaerobic conditions, 

 as in peat soils, the lignin remains undecomposed and rapidly 

 accumulates. 



Fig. 46 shows the relative resistance to decomposition of the 



