74 AGRICULTURAL BACTERIOLOGY 



ter is slow and often incomplete. Here the efforts of the 

 farmer must be directed toward more perfect aeration. All 

 processes of cultivation aid, as does drainage. 



Temperature. The temperature zone in which bacterial 

 growth is possible is a wide one, extending from below 

 to 70 C. (32-158 F.). Representatives of both psy- 

 chrophilic and thermophilic bacteria are found in the soil. 

 The great mass of bacteria in the soil are mesophilic, and 

 are favored by relatively high temperatures, from 16 to 

 40 C. (61-104 F.). It is customary to speak of cold 

 or late soils, and of warm or early soils. The former are 

 those of close texture, which in the spring become dry 

 enough to cultivate only after a considerable period. 

 Water has a high specific heat. A soil filled with water will 

 therefore increase in temperature slowly as compared with 

 one containing a smaller amount of moisture. The time 

 required for the temperature of the soil to reach a point at 

 which bacterial growth will be rapid will be prolonged in a 

 close soil. The supply of available plant food is largely 

 removed from the soil by the percolating water of fall, win- 

 ter, and spring. Before the crop can make any marked 

 growth, opportunity must be afforded for the soil organisms 

 to form available plant food. This process will be retarded 

 in a wet soil, hence the expression "late soil." The loss of 

 water and decomposition of organic matter take place 

 quickly in open-textured soils. The expression "early 

 soils" is therefore a fitting one to apply to them. The 

 gardener needs a loose, sandy soil for his early crops. 



Reaction of the soil. The bacteria are favored by a neu- 

 tral or slightly alkaline rather than by an acid reaction. 

 An acid reaction in the food medium is not injurious to 

 yeasts and molds. The greater part of the elaboration of 

 plant food from organic matter in the soil is occasioned by 



