Nature of Plant and Animal Residues 97 



ot the plant eonstituents, sueh as the sugars, starches, fats, and pro- 

 teins, are utilized by animals for their own metabolism and for 

 suppUing their energy needs. Other plant eonstituents, like the 

 hemieelluloses, cellulose, lignins, and waxes, are used not at all 

 in the animal system or only to a very limited extent. The cow and 

 other ruminant animals are able to digest a large part of the cellu- 

 lose, with the help of bacteria lixing in their digestive tracts. The 

 undigested residues are excreted by the animals and sooner or later 

 find their way into the soil. 



The bacteria and certain protozoa may play an important part in 

 the digesti\e mechanism in the animal body: (a) by digesting the 

 cellulose and certain other carbohydrates to organic acids, they make 

 these constituents axailable to the animal for its nutrition; (b) by 

 synthesizing certain vitamins and other complex substances in the 

 animal, they supply nutrients which the animal is unable to synthe- 

 size; (c) they may also form certain products that are undesirable 

 or even toxic to the animal body. 



The plant and animal residues find their way into the soil either 

 directly or after preliminary decomposition in composts or on the 

 surface of the ground. These residues comprise either the whole 

 plant, stems, lea\es, and seed, or only certain parts of the plant, 

 needles and lea\es, surface stubble, and subsurface roots. The sur- 

 face portion of certain crops, such as grasses used for pasture, cereals, 

 and corn, may be largely removed for cattle food or for other pur- 

 poses. In some cases the straw may be returned to the soil, either 

 as such or as a constituent part of the stable manures. In crops like 

 peas and beans, only the seed may be removed from the land, whereas 

 the rest of the plant may find its way into the soil. In still other 

 crops, used for soil cover or as green manures, the whole plant may 

 be returned to the soil. In pastures and in forests, where the soil 

 is not plowed at all, the plant residues are attacked by microorgan- 

 isms either in the soil itself or on the surface of the ground; the 

 products of decomposition gradually find their way into the soil 

 through leaching or by land cultivation. 



Thus the cycle of life is completed, from the soil back to the soil. 

 In this broad cycle, numerous secondiuy cycles occur, in which one 

 or more elements are concerned. In the transformation of each one 

 of these, microorganisms play a highly important part. Without 

 them, life would soon come to a standstill; upon their activities, the 

 continuation of life on the planet depends. 



