CHEMICAL CHANGES 23! 



Temperature. Relative amount 



Degrees C. of carbon dioxide 



10 2.8 



20 ; 15.5 



30 - 3 6 - 2 



40 42.6 



50 76.3 



(8) Moisture. The oxidation goes on most rapidly at a cer- 

 tain moisture content, which depends on the material. Increase 

 or decrease of moisture causes decreased oxidation. 



(9) Character of Soil. The character of the soil has a great 

 effect upon the changes of organic matter. The various factors 

 which affect the decomposition may support or counteract one 

 another, and the effect is due to the predominating quantitative 

 relations. The easy permeability and more rapid warming up of 

 quartz sand are favorable to the decomposition of organic matter, 

 while its low water capacity is a retarding factor. Hence the 

 supply of moisture is the controlling factor in organic decomposi- 

 tion in a sandy soil. In humid regions, the decomposition may 

 proceed so rapidly in such a soil that organic matter does not 

 accumulate to any extent. In a dry climate, decomposition pro- 

 ceeds more slowly than where sufficient moisture is present, but 

 more rapidly in a sand than in other kinds of soil. 



Clay soils retain plenty of water, but there is a deficiency of air, 

 and these soils are essentially cold. The decomposition of organic 

 matter in clays is thus determined by the temperature and per- 

 meability to air, and proceeds slowly under ordinary conditions. 

 In humid regions, compact clays may exclude the air to such an 

 extent that putrefaction predominates. Heavy rains may discon- 

 tinue oxidation in clay soils, and bring about deoxidation pro- 

 cesses, among them denitrification. 



(10) Vegetation. Oxidation of organic matter appears to go 

 on much more rapidly in a bare soil than in a soil covered with 

 vegetation. A straw mulch decreases oxidation, but not as much 

 as a covering of vegetation. The following results of Wollny 

 illustrate this point : 



