SUMMARY 73 



Consequently, results of determinations of the amounts of carbon 

 dioxide produced give information which indicates, more accurately 

 than determinations of the numbers of any single group of organ- 

 isms, the level of biological activity, irrespective of the organisms 

 that are concerned. 



The method does not discriminate between those organisms 

 which can be counted by plate methods and those failing to grow 

 on artificial media. Although one cannot distinguish what organ- 

 isms are responsible for producing the carbon dioxide, one has in 

 this method a device for determining to what extent microbial 

 activity as a whole becomes depressed or favored by particular 

 soil conditions. 



In general, those factors, considered in the preceding pages, 

 which favor development of aerobic bacteria, actinomyces, and 

 fungi, also favor rapid formation of carbon dioxide. High carbon 

 dioxide evolution is associated with productive soils, and factors 

 which increase crop production also increase formation of carbon 

 dioxide. Such factors as relatively high temperature and moisture 

 content, thorough aeration, presence of an abundance of organic 

 matter, inorganic plant nutrients, and lime, all exert favorable 

 effects on biological activity as measured by the rate of formation 

 of carbon dioxide. 



The resultants of some of these effects are indicated in the sea- 

 sonal variation of the gas formation which is shown in Fig. 37. 



The rate of formation of nitrates, oxygen absorption by soil, 

 and heat evolution, as a result of the activities of microorganisms, 

 can also be used as measures of the activity of the soil population, 

 but none of these is as simple as the determination of evolution of 

 carbon dioxide. 



Summary. — The soil is inherently a very complex system, 

 where chemical, physical, and biological systems are active. All 

 of these agencies tend to become adjusted to the environmental 

 conditions existing in the soil at any one time. If a soil is kept 

 under uniform conditions, the speeds of the numerous processes 

 taking place in the soil also tend to become uniform. Any influ- 

 ence which tends to disturb this adjustment in the soil conditions 

 alters these processes, which will now tend to become adjusted to 

 the new conditions. Thus, a change in soil reaction causes a series 

 of changes in the biological condition of the soil; after a lapse of 

 time, the new population becomes adjusted to the new reaction. 



