Decomposition of Hiinuis 



137 



8.0 



7.5 



5.0 



4.5 



7.0 6.5 6.0 5.5 



Reaction of soil, pH 



Fk;. fiO. InHuc'iKc of reaction upon the nitrate and aiuiuoiiia nitrogen lil)erati()n 

 in forest soil (from Aaltonen). 



Decomposition of Humus 



When compared with fresh plant residues, humus is rather re- 

 sistant to decomposition; otherwise it would not accumulate at all 

 in the soil, and it would certainly not persist there for long periods. 

 Under favorable conditions, however, humus can decompose further. 

 Were this not the case, the soil long ago would have become covered 

 by a surface layer of organic debris of varying degrees of thickness, 

 similar to that formed in peat bogs, in certain forests, and especially 

 in coal. If humus in soil were as resistant to decomposition as humus 

 in coal and in peats, the surface of the earth would have become 

 organic and would not have remained predominantly inorganic in 

 nature; the limited supply of available carbon would soon have be- 

 come exhausted, making all further life impossible. In fact, under 

 certain conditions, the decomposition of humus in the soil may be so 

 rapid that the farmer experiences considerable difficulty in keeping 

 up the supply, especially when he cultivates his soil year after year 

 and does not return to it sufficient plant residues and organic manures, 

 as in the growing of intertilled crops. 



When the temperature, moisture, reaction, and aeration of the soil 

 are faxorable, the soil humus undergoes constant decomposition, as 

 evidenced by the continuous stream of carbon dioxide given off into 

 the air and by the nitrate that accumulates in the soil; the nitrogen 

 is first liberated as ammonia, which is changed rapidly to nitrate by 

 certain bacteria. The gradual disappearance of the humus becomes 



