PHYSICAL, CHEMICAL, BIOLOGICAL 43 



Bacteria Abundant Near Surface. The soil bacteria are most abun- 

 dant in the plowed portion of the soil. Their numbers greatly diminish 

 as the depth increases, and disappear entirely at a depth of a few feet. 

 It is generally believed that direct sunshine is destructive to practically 

 all forms of bacteria. Consequently, we find few living bacteria immedi- 

 ately at the surface of a dry soil. In the practice of inoculating soils, 

 therefore, it is recommended that the bacteria be distributed on a cloudy 

 day or in the morning or evening when there is little sunshine, and that 

 the inoculation be at once thoroughly mixed with the soil, by disking or 

 harrowing. 



Barnyard manures are always teeming with myriads of bacteria, and 

 the practice of applying such manure adds many bacteria to the soil. 

 Bacteria are most active during the warmer portions of the year, and most 

 of them are dormant when the temperature of the soil falls below the 

 freezing point. Those instrumental in nitrification are very inactive 

 when the soil is cold and wet and become exceedingly active in mid-sum- 

 mer when the temperature of the soil is comparatively high, when plant 

 growth in general is most active and when nitrogen is most needed by 

 gro wing crops. This is a fortunate coincidence, since it enables the higher 

 plants to utilize the nitrates made available at that particular season by 

 bacteria. If nitrification through the bacteria were equally rapid during 

 periods when farm crops made little growth, a great loss of nitrogen would 

 occur through leaching of the soil. The freezing of the soil does not destroy 

 bacteria, as a rule, but simply causes them to be temporarily dormant. 



REFERENCE 

 "The Soil." HaU. 



