Envir:[^ 



the air in the soil pores v;ill be largely that of the atmosphere above. 

 Deeper in the soil, higher concentrations of Carbon dioxide and loircr 

 concentrations of oxygen are found, and probably there are other gaseous 

 products forraed by the biological processes. Seldom, even at f^roat 

 depths, does one find all of the oxygen being used or very high concen- 

 trations of carbon dioxide. Diffusion seems to be rather rapid. Uhen 

 the soil is filled with water, the oxygen can be used up rather rapidly 

 x^ith resulting anaerobic conditions and high concentrations of carbon 

 dioxide. As soon as the soil pores drain, the gas diffusion is rapid and 

 an equilibrium system becomes set up quickly. V7e shall v;ant to refere 

 to some of the o:cygen and carbon dioxide values a little later. 



Now, as to the inflvience of these factors on the overall microbiological 

 activity. The measurements that have been made have generally dealt 

 with the overall acitvity, not xiith the activity of specific groups of 

 organisms. Little information is available on specific groups. It may 

 be presumed, and I am quite sure it would be all right, that all organ- 

 isms would not act alike. Some organisms would be more sensitive to 

 these changes in the environment than would others. I wish we had more 

 information on this particular topic. 



In Figure 6 is plotted microbiological activity, measured by carbon 

 dioxide production, as a function of moisture content of the plant 

 material. This is not in the soil but in plant materials. You xrill 

 note that moisture is given in a number of ways. Maximum activity 

 occurs at high moisture content, unless anaerobic conditions develope. 

 Then the microflora vrould be changed and, in so doing, alter the overall 

 activity. As the moisture content is decreased, there is less and less 

 activity until a threshold point is reached at which the organisras are 

 just barely active, at least, activity can be just barely measured. 



You xri.ll also note from the graph that microbes are quite active at 

 moisture levels below that at which plants \nll grow. I have indicated 

 on the chart the wilting point and the field capacity values. You will 

 note on the relative huirLidity scale of Figure 3 that in the range in 

 which plants will grow, the relative hvmiidity is arovind 99 to 100^ and 

 the air is well saturated. There are reports of microbial activity in 

 beet tops when the relative humidity has been down as low as 7$%. Thub, 

 microbes can grow under stringent moistiu*e conditions. 



Figure 7 gives an idea again of microbiological activity in the soil 

 under the influence of varying soil moisture. The information is 

 plotted in two ways: one as percentage moisture and the other as 

 logarithm of centimeters of water tension. You will note that, in 

 general, there is a wide range of moisture conditions in which the 

 overall population of microorganisms is rather active. Only at the 

 low moisture contents does the activity go doim. Although there might 

 be specific members of the soil population that iirould be affected a 

 good deal' differently, it is likely that the overall population activity 

 represents the mean. Thus, one does not reduce microbial activity 

 markedly until reaching moisture contents beloij that at which plants 

 xd-ll gi'ow. 



