5 



Humus: Nature and Formation 



What Is Humus? 



In the past, various meanings attached to the term "humus." Some 

 used this term to designate a certain fraction of the organic matter 

 in soils and in composts; others used it to indicate all the organic 

 matter of the soil; still others recognized as humus the organic ma- 

 terials of natural origin in advanced stages of decomposition, whether 

 in soils, in composts, or in peat bogs, and whether plant or animal 

 in nature. Fresh plant roots and stubble, fresh stable manures and 

 green manures, fresh kitchen wastes and garbage, undecomposed 

 bodies of worms and insects, fresh tankage and fish, and numerous 

 other products of plant and animal life, when their origins are still 

 recognizable, are not in a humified or in a humus state. All these 

 serve as sources of humus; upon their decomposition by microorgan- 

 isms, humus is produced. 



When various plant and animal residues are plowed into the soil 

 or are made up into composts, they are immediately attacked by 

 numerous microorganisms, including bacteria, actinomycetes, fungi, 

 protozoa, and worms. As a result of their decomposition, some of 

 the constituents of the fresh materials are volatilized, others are used 

 by the microorganisms for the building of microbial cell substance, 

 and still others are gradually transformed into a uniform, dark- 

 colored, amorphous mass, which is designated as "humus." The 

 rate of formation of humus and the amounts produced depend upon 

 the physical and chemical nature of the residues, the nature of the 

 soil or of the compost in which decomposition is taking place, the 

 nature of the microorganisms concerned, and the environmental 

 conditions, notably temperature, moisture supply, aeration, and 

 reaction. In humus, the products of decomposition can no longer 

 be distinguished from the original plant and animal materials from 

 which they have been formed. 



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