372 PRINCIPLES OF SOIL MICROBIOLOGY 



Reaction velocity. Reactions between ions take place in homogeneous 

 media with great velocity. In colloidal media, such as the living cell 

 or the soil, the time factor is of great importance. The reactions be- 

 tween molecules is much slower than between ions even in homogeneous 

 media, but more so in such an heterogeneous medium as the soil. The 

 kinetics of chemical reactions, which includes the decomposition of 

 fats, carbohydrates, glucosides, proteins, as well as the various oxida- 

 tion and reduction phenomena, is of prime importance in soil micro- 

 biology. The course of change is determined by measuring at definite 

 intervals of time the amount of change that has taken place and the 

 results are calculated on a definite basis, such as moles per liter. The 

 law of Wilhelmy, 6 that the reaction velocity is at any given time pro- 

 portional to the amount of unchanged substrate, applies to mono- 

 molecular reactions, or where only one substance changes in concen- 

 tration. The hydrolysis of sugars and glucosides belongs to this type 

 of reaction, while saponification of esters is already bimolecular. 



The fundamental equation of the monomolecular reaction is 



dt 

 It becomes, on integration, 



dx „ . . 

 = K (a — x), 



K = \ln 



t a — x 



where a is amount of undecomposed substrate, x the amount decom- 

 posed in t minutes. An understanding of the temperature coefficient 

 is also important. In most reactions, with every increase in 10°C. tem- 

 perature, the velocity is increased two to three times. 7 This usually 

 holds true at temperatures ranging from to 35°. Above that tem- 

 perature, the destructive effect of heat is greater than the stimulating 

 action. The thermophilic bacteria will exist even at 55° to 65°C. and 

 carry on their activities at such temperatures. 



Growth, life and death of microorganisms. The numbers of bacteria 

 and other microorganisms in the soil vary greatly not only from day 

 to day, but even within brief periods of time, as a result of the changes 

 in the environmental conditions. The age of the bacteria may be only 

 a few days, while they reproduce within twenty to thirty minutes. 



6 Wilhelmy, L. Uber das Gesetz, nach welchem die Einwirkung der Siiuren 

 auf Rohrzucker stattfindet. Pogg. Ann., 81: 413^99. 1850. 



7 van't Hoff, J. H. Chemische Dynamik. 1898, p. 224. 



