Knvir:9 



drying and wetting of the soil there might not actually be a higher 

 total activity than if the soil remains continually moist. In some 

 cases, I thinl<: this has actually been demonstrated. I will not try 

 to explain how this stimulation comes about. 



Table 7. Effect of Alternative Wetting and Drying on 

 Decomposition of Plant Material and Loss 

 of Soil Carbon 



Muscatine Silt Loam (20 Gram Samples) 



Rate 

 of com 



residue 

 addition 



Carbon Dioxide 



evolution 

 from 20 g. soil 

 during US days 

 Plant Soil 



Continuously moist 

 Continuously moist 



Alternately wet and dry^ 

 Alternately wet and dry 





 2.% 



261i.6 



239.^ 



165.3 

 173.6 



150.1 

 159.3 



■«• Wet to 150^ of moisture equivalent, left wet for 3 

 days, dried with dry air for 6 days, left dry for 3 

 days, re-wet at 12 day intervals. 



Table 7 compares samples that were kept continuously moist with samples 

 that were alternately wet and dry. In this particulat study there was 

 not a great deal of difference in the carbon dioxide evolution or, shall 

 we say, in the total microbiological activity in the two systems. Dessi- 

 cation is a peculiar phenomenon and a very interesting one. It is prob- 

 ably one that has quite a marked influence on not only the total microbial 

 activity but on specific members of populations. 



We have heard a lot from various so\irces about the influences of the 

 gaseous components in the soil on the overall activity of the micro- 

 organisms. A number of studies have been made. Figure 8 shows the 

 influence of three levels of oxygen content upon the microbiological 

 activity expressed in terms of carbon dioxide production for a period 

 of 52 days. It was found that going from 21^ oxygen in the air stream, 

 which is about the normal content of the atmosphere, down to the low 

 percentage had very little influence on the carbon dioxide evolution. 

 The oxygen content has to get to a rather low amount before a marked 

 effect is evirlent. Also, one would expect to still get carbon dioxide 

 production in an anaerobic condition, so this does not tell much about 

 the activj-ty except that It is not reduced a great deal by reducing the 

 atmospheric ojry-f^en. Anothor oonnideration is that from the normal 



