192 PUTREFACTION, FERMENTATION, AND DECAY 



Humus is mainly valuable because of its physical effect upon the 

 soil and because of its content of nitrogen, potassium, and phos- 

 phorus which are slowly liberated by bacteria. The beneficial effect 

 of organic matter upon the bacterial flora of the soil and soil 

 fertility, however, is mainly exerted before it reaches the stage of 

 humus. 



Chemistry of the Processes. The primary and secondary products 

 resulting from the decay of organic matter in the soil are classed as 

 humus. They are not, as was once believed, a few comparatively 

 simple organic compounds but are a heterogeneous mixture of 

 colloidal and crystalline organic compounds resulting from the 

 action of bacteria upon plant residues. 



The chemical composition of the end products being in many 

 cases unknown, the chemistry of the process is still to be explained, 

 but we have some very suggestive information due to the fact that 

 acids and alkalies when they act upon carbohydrates yield brown 

 humus-like substances very similar to, if not identical with, the 

 substances found in the soil and resulting from bacterial activity. 



It is known that the aldehyd group of a carbohydrate easily 

 opens its double bonds between carbon and oxygen and adds water 

 to form a polyhydric alcohol, as follows : 



OH 

 / 



R C=O + H 2 O = R C\ 



| OH 

 H H 



This reacts with sodium hydroxid with the formation of the 

 following salt: 



H H ^ H H 



+ NaOH = R C & + H 2 O 



OH 



This salt is unstable and the molecule forms enols: 



HHHHHH HHOH OHH 



I I I I I I III II 



C C C C C C ONa = HO C C = C C = C C ONa + 2H 2 O 



I I I I I I I I I 



OH OH OH OH HO HO H H OH 



These break apart at the double bonds: 



H H OH H OH H 



II II 



HO C C = ; = C C = ; = 

 I 



H 

 H H OH H OH 



I I I I I 



HO C C = C G - ; = C C 



I I 



H H 



