DRAINAGE AND SOIL TEMPERATURE 243 



raises the average temperature of the soil. The specific 

 heat of water is much higher than that of soil, and there- 

 fore the larger proportion of water a soil contains the 

 more heat is required to increase its temperature. 

 (See page 461.) Further, in a wet soil the surface evapo- 

 ration is large, and since the evaporation requires several 

 hundred times as many units of heat as is necessary 

 to raise the same volume of water from the normal 

 temperature to the boiling point, it is clear that the 

 process must consume a large amount of heat. But the 

 heat supplied to any given area of soil is fairly uniform, 

 and consequently, if it is used up in evaporating water, 

 it is not effective to raise the temperature of the soil 

 mass. If the soil contains water which must be removed 

 by evaporation, its temperature will be kept correspond- 

 ingly low; or, what is the same result, the time required 

 to warm the soil will be correspondingly extended. 

 For this reason a wet soil is a "late soil," while a well- 

 drained soil is much "earlier" in attaining the tempera- 

 ture necessary for the germination and growth of plants. 

 The practical result of this rapid warming of a well- 

 drained soil is to lengthen the growing season by per- 

 mitting its earlier seeding in the spring, and the later 

 growth of crops in the fall. In some sections of the world, 

 this margin in the length of the growing season deter- 

 mines the growth of certain crops, and materially affects 

 all crops. All of the activities of the soil, both chemical 

 and biological, are favorably affected by the higher 

 temperature. In the peat bogs of England, Parkes found 

 that at a depth of seven inches the drained soil was 15 

 warmer than the undrained soil, and at thirtv-one 



