52 BACTERIA AND SOIL FERTILITY 



temperature is raised to the kindling point with the result that 

 spontaneous combustion may occur in hay and grain stacks. Bac- 

 teria generate considerable of the heat, but other chemical pro- 

 cesses are also active. This is made use of in the building of cold 

 frames. A heavy dressing of fresh horse manure is placed under 

 the top soil. This decomposes and furnishes sufficient heat to 

 force the plants. 



