96 Decomposition of Plant and Animal Residues 



2. Plant residues plowed into the soil; these include plant stubble 

 and special crops which are grown specifically for this purpose, as 

 cover crops or green manures. 



3. Stable manures; these consist of the solid and liquid animal 

 excreta and bedding. 



4. Artificial manures and composts. 



5. Organic commercial fertilizers; these include a \ariety of ani- 

 mal and plant products, such as bone meal, dried blood, tankage, 

 cottonseed meal, linseed meal, peat. 



6. Microorganisms and their dead bodies. 



The plant residues are made up of tliree groups of constituents: 

 water, organic materials, and inorganic compounds. The water con- 

 tent of plant residues varies from 50 to 95 per cent, depending on 

 the nature and degree of maturity of the plant, usually about 80 per 

 cent for young and 60 per cent for mature plants. The water-free 

 plant material consists of 88-99 per cent organic matter, and 1-12 

 per cent mineral or inorganic matter. The organic constituents com- 

 prise a large number of chemical compounds containing the ele- 

 ments carbon, hydrogen, oxygen, and nitrogen and, in lesser amounts, 

 sulfur, phosphorus, potassium, and a variety of others, some of which 

 are usually present in mere traces. 



When the organic matter synthesized by the plants undergoes 

 digestion by herbivorous animals, many of the constituents are de- 

 stroyed and the elements changed back into simple gases or inorganic 

 compounds such as COo, H^O, NH3, phosphates, sulfates, potassium 

 salts. Out of the plant materials, directly or after they have been 

 transformed into simpler compounds, the animals synthesize their 

 own tissues. These animal bodies may now be used, in their turn, 

 as food by other animals. The bodies of these omnivorous or carniv- 

 orous animals also undergo a series of transformations; part of the 

 elements and compounds which they consumed as nutrients are lib- 

 erated as waste products, in the form of gases ( CO;., NH3 ) , as simple 

 organic compounds (urea, organic acids), or as complex organic 

 materials comprising the residual and partly digested plant and 

 animal residues found in the feces. 



Animals depend upon plants and some of them upon other ani- 

 mals for their necessary energy, for the organic nitrogenous com- 

 pounds, for some of the fatty substances, and for the vitamins. On 

 the other hand, animals are able to synthesize, out of the complex 

 materials supplied to them by the plants and other forms of life, 

 new organic compounds, largely of a protein and fatty nature. Some 



