92 DECOMPOSITION OF ORGANIC SUBSTANCES IN SOIL 



constituents. Some plant residues largely disappear in a few weeks, 

 while others still retain their original appearance after several 

 months in the soil. Generally, however, certain portions of prac- 

 tically all organic materials decompose somewhat rapidly within a 

 few days subsequent to their addition to the soil. This period is 

 followed by a uniform but rather abrupt retardation in the speed 

 of decomposition which is followed by a slow but continuous decom- 

 position of the remaining material. Even a year or more after the 

 addition of the organic matter, the soil is still in a more active 

 biological condition than it was before the treatment. 



Although differences exist between the rates with which soils of 

 different fertility are able to decompose the same organic materials, 

 these differences are not very pronounced after the first few days. 

 As a rule, however, conditions favorable to development of culti- 

 vated plants, such as relatively high temperature and water con- 

 tent, presence of sufficient available inorganic substances, and 

 reaction close to neutrality, also exert favorable effects on decom- 

 position. Consequently, the more fertile the soil, the more rapid 

 the mineralization of the added organic matter. 



Organic materials are applied in farming practice with the object 

 of producing beneficial effects on plant growth. Whether or not 

 such effects are produced will depend principally upon the reac- 

 tions which occur during decomposition of the organic substances. 

 As has been stated previously, the application of organic materials 

 results within a very short period in an enhanced development of 

 the microorganisms in the soil. This is not confined to any 

 one group, but generally applies to all soil microbes; the extent of 

 stimulation differs with the various species of bacteria, fungi, 

 actinomyces, and protozoa, and depends upon the composition and 

 the amount of the organic material added. The increase in abun- 

 dance of cells is also accompanied by increases in their activities, 

 as measured by the formation of carbon dioxide and other end 

 products. 



Coincident with the decomposition of organic matter con- 

 taining small amounts of nitrogen, there is an immediate and appre- 

 ciable decrease in the amounts of ammonia and nitrate nitrogen 

 originally contained in the soil, as is shown in Table 22. In this 

 experiment, the substances were added in such amounts as 

 to introduce the same quantity of nitrogen in each case. As a 

 result of the application of appreciable amounts of organic matter 



