OBGANISMS m THE SOIL 435 



357. Temperature. — Soil bacteria, like other plants, 

 continue life and growth under a considerable range of 

 temperature. Freezing, while rendering bacteria dor- 

 mant, does not kill them, and growth begins slightly 

 above that point. It has been shown that nitrification 

 goes on at temperatures as low as from 37° to 39*^ F. It 

 is not, however, until the temperature is considerably 

 higher that their functions are pronounced. From 7(f 

 to 110° F. their activity is greatest, and it diminishes 

 perceptibly below or above those points. The thermal 

 death point of most forms of bacteria is found at some 

 point between 110° and 160° F., but the spore forms even 

 resist boiling. Only in some desert soils does the natural 

 temperature reach a point suflSciently high to actually 

 destroy bacteria, and there only near the surface. In fact, 

 it is seldom that soil temperatures become sufficiently high 

 to curtail bacterial activity. 



358. Organic matter. — The presence of a certain 

 amount of organic matter is essential to the growth of 

 most, but not all, forms of soil bacteria. The organic 

 matter of the soil, consisting as it does of the remains of 

 a large variety of substances, furnishes a suitable food 

 supply for a very great number of forms of organisms. 

 The action of one set of bacteria on the cellular matter of 

 plants embodied in the soil produces compounds suited to 

 other forms, and so from one stage of decomposition to 

 another this constantly changing material affords sus- 

 tenance to a bacterial flora the extent and variety of which 

 it is difficult to conceive. Not only do bacteria affect the 

 organic matter of the soil, but, in the case of certain 

 forms, their activities produce changes in the inorganic 

 matter that cause it to become more soluble and more 

 easily available to the plant. 



