F.nvir:10 



oxygen content of the atmosphere of about 20/u, it has to fjo to a rather 

 low value before anaerobic conditions exist. In normal, well-drained 

 soils, except when fil].ed with water for short periods of time, it is 

 not lil-:ely that the oxygen content is sufficiently lov; to have any 

 marked effect on the overall microbiological population. Aeration, as 

 far as the microbes are concerned, is not very much of a problem. Fig- 

 ure 9 illustrates the same thing: reduction of the oxygen to a low 

 percentage had only a minor effect on overall activity. 



Figure 10 illustrates the Influence of oxygen in its gaseous phase on a 

 strictly aerobic process, that of oxidation of ammonia to nitratej. In 

 going from the normal oxygen content of the air down to the neighborhood 

 of less than 2%, the rate of oxidation is reduced by only a little more 

 than ^Q)%. Reducing the oxygen content down to about ^% only reduces the 

 oxidation rate about 30^. In other words, to inhibit the oxidation of 

 ammonia to nitrate, the oxygen has to get well down belo^f 1% in the 

 gaseous phase. Seldom do we find it so low in well-drained soils, 

 except for short periods of time. 



The last of the factors that I want to stress a little bit is tempera- 

 ture. In the soil, there is a lot of temperature fluctuation. If one 

 plots, for example, soil temperature against time, one finds that, in 

 general, the upper soil levels have minimum temperatures along in the 

 early morning hours. During the day, the temperatures climb and reach 

 maxima sometime in the afternoon and then begin to diminish. Layers 

 beneath the soil surface do not fluctuate in temperature this much. 

 They do not get temperatures as low in the morning or as high in the 

 afternoon. If the soil is covered, in general, the uppermost layer 

 does not get as high in the afternoon as does the air temperature, 

 whereas, in bare soil, the temperatures may exceed the air temperature. 

 Covered soil will also not get as cool as the air temperature during 

 the night. Likewise, there is a seasonal pattern to the soil tem.pera- 

 tures. All these pose a problem as to just how does one characterize 

 soil temperature to someway relate it to soil microbiological activity. 



There is no uniform system at all. There are a series of fluctuations 

 frora day to day, differing at varying depths and to varying extents, 

 depending on such things as soil cover, weather, and season. VJe tried 

 one time to come up with some particular temperature reading which would 

 characterize the temperature condition for the upper six inches of soil. 

 This vjas done by taking the fluctuation patterns for various depths in 

 the upper six: inches of soil. A mean was taken for the several horizons 

 at each hour during the day. Then taking the mean of all these means, 

 we came out i-rith a value that perhaps has some significance. At least, 

 it is a single figure that can be used, x^rhereas one cannot use a whole 

 group of mean values for anything. For xjhat it may be worth, we found 

 that this particular mean x^as very close to the soil temperature reading 

 at four inches depth at about noon. Thus, when we wanted to get some- 

 thing to characterize the soil temperatures for a full day, we merely 

 took a noon reading of the temperature at four inches depth. This does 

 a fairly good job, but, of course, it docs not tell anything about the 

 fluctuations which probably are very important. 



