MEANS OF MODIFYING THE SOIL TEMPERATURE 463 



The large effect of drainage on the soil temperature 

 is due to these heat relations of the soil moisture. If 

 the excess of water is removed by evaporation, it keeps 

 the soil unduly cold. King observed differences in tem- 

 perature of from 2.5 to 12.5 Fahr. between drained and 

 undrained soil on different days in April. These results 

 are abundantly borne out by practical experience. The 

 removal of the excess water by drainage conserves heat. 



IV. MEANS OF MODIFYING THE SOIL TEMPERATURE 



The means of modifying the soil temperature are 

 obvious from the above principles. The practices which 

 may be used for this purpose are: 



(1) Modification of the texture and structure of the 

 soil by appropriate tillage. 



(2) Modification of the color 

 of the soil, chiefly through the 

 addition of organic matter. 



Fig. 126. The spading disc. Adapted to much more stony, 

 hard soil than the solid disc. 



