15nvir:)i 



thin mointi;re film which will limit movement and other activities of the 

 microorganiGms. That is the moisture envirorLnent the microbes find iJi 

 tbo noil. Only vrhen the soil is quite wet is there enourh pore space 

 filled v-rith x-ratcr to provide for free movefoent through the soil. 



figure h illustrates the quantity of water held in the soil at particti- 

 lar tensions in two soils. One is a sandy soilj and the other, silt 

 loam. They do not represent extremes in ran^;e, but they will ['.Ive sor.io 

 idea of the ranges we deal with. You T-ri-ll note from the previous figure 

 that the ranjr;e of soil moisture where plants grow is from about log soil 

 moisture tension of 2.2-2.5 to U.O. From the graph you can obtain an 

 idea of the range of the m.oisture content between the two soils over the 

 growing range of tensions for plants. 



The illustrations in Figure 5 relate to water and air movement and mois- 

 ture content of the soil. As water moves into or out of the pore space 

 of the soil it influences the movement of gas in the pores. VJater moves 

 more freely in the large pores, and so does the air, anytime there is 

 mass movement. The relationship between the Xirater and air content is 

 not a linear one, because movement of air is by mass movement, X'jhen larger 

 pore spaces are present, and by diffusion, which is not a function of the 

 large pores. It makes little difference to the diffusion rate whether 

 the pores are large or small, because diffusion is a function of the 

 total pore space per unit area. 



We are interested here in some of these environmental factors as they 

 influence microbial activity. Before we get into that, I would like to 

 have you look at Table 2 '.:hich gives data concerning composition of the 

 air in the soil. 



Table 2. Comoosition of Soil Air-/ 



Per cent CO^ and Op by volume in soil air 



Nov. lU, 1937 - 1.2 19. I| 2.h 11.6 9.6 0.7 15.5 0.2 



Mar. 23, 1933 - 0.15 20.15 2.1 0.l5 ?.6 0.3 — 



May 2U, 1933 - 3.7 l6.2 3.95 13-95 5.6 I3.U — 



July 25, 1938 - 2.0 19.3 3.1 19.1 5.2 17.5 9.1 lU.5 



Soot. 22, 1930 - 3.0 15.3 U.8 11.0 5.3 10.0 8.5 9.0 



1/ From Baver, Soil Physics, 2nd Ed., I9U8. 



Supposedly, tlie air in the soil is in equilibrium xcith the air of the 

 atmosphere above. Up near the surface of the soil, the composition of 



