320 SOIL PHYSICS AND MANAGEMENT 



(c) Temperature. The optimum temperature for bacterial 

 activity lies between 65 and 95 degrees F. (18 and 35 degrees C.). 

 It diminishes as the temperature increases, and at 130 to 140 degrees 

 F. action ceases and many are killed. Below 65 degrees F. the bac- 

 teria become less active and cease at 32 degrees F., although they 

 are not killed. Early tillage, drainage and a dark color raise tem- 

 perature and encourage bacterial action. 



(d) Aeration. Bacteria are divided into two general classes, 

 aerobic, those requiring oxygen for their growth and activity or 

 work, and the anaerobic, which require no oxygen. Aeration is very 

 essential to the first group. Since nitrification is the most im- 

 portant work of bacteria in soils the amount of nitrates produced 

 may be taken as a measure of their activity. Experiments show 

 that in the absence of oxygen not only were no nitrates formed, but 

 the nitrates present were reduced with evolution of free nitrogen. 

 When six per cent of oxygen was present the amount of nitrates 

 formed was double what it was with 1.5 per cent. 2 



(e) Reaction. Soils giving acid reactions are not very favor- 

 able to the work of bacteria. They are more active in soils that 

 are neutral or slightly alkaline. The nitrifying bacteria produce 

 nitrous and nitric acids, which tend to inhibit their action. If 

 bases are present in the soil these will unite with the acids pro- 

 duced, thus keeping the soil neutral or alkaline, and in good con- 

 dition for their work. Limestone should be applied to the soil to 

 neutralize the acidity. 



Crops growing on water-logged soils are usually yellow. This 

 is due to a lack of available nitrates. The water excludes, the air 

 and the bacteria cannot do their work. The same conditions exist 

 when a soil in poor tilth runs together and bakes, forming a crust 

 impervious to air. When aeration is produced by cultivation 

 nitrates are formed and a crop such as corn resumes its normal dark 

 green color. 



Another important function of aeration is to remove the carbon 

 dioxide of the soil air. This is necessary because it excludes oxygen. 

 In the process of nitrification carbon dioxide is formed. Tillage is 

 the best means of bringing about aeration. Deherain 3 conducted an 

 experiment which shows the effect of tillage on aeration and con- 

 sequently upon the action of nitrifying bacteria. A quantity of 

 soil was thrown upon the floor and worked daily for six weeks. At 

 the end of this time the stirred soil contained 23.7 times as much 

 nitric nitrogen as the soil not disturbed. " Nitrate' farming " as 



