EXPERIMENT STATION BULLETINS. 735 



evaporation was prevented, then all the diverse classes of soil were equally 

 warm during the spring and summer months. 



The knowledge, therefore, that a thin layer of sand at the surface is 

 capable of raising several degrees the temperature of clay and loam and 

 especially of peat, is of the greatest practical importance. 



It has already been stated that up to the end of the third year the 

 temperature of the different types of soil was measured at 6, 12, and 18- 

 inch depths. With the beginning of the fourth year, December 1, 1914 

 to November 30, 1915, the thennometers were removed and placed at the 

 depths of 2, 4, and 6 inches. This change was made with the desire to 

 obtain information on the soil temperature more close to the surface, 

 and also to ascertain whether the general trend of order of the magni- 

 tude of temperature at the upper depths would be similar to that at the 

 lower depths. The cover of sand was also removed from the top of these 

 soils so that their natural surface was exposed to the atmosphere during 

 this year, . The data obtained during the fourth year are contained in 

 Tables 25 to 36 inclusive. 



An examination of these tables shows that (1) all the different types 

 of soil began to freeze almost simultaneously at the respective depths 

 with peat at a slightly slower rate. By the end of December they had 

 all frozen practically down to the 6-inch depth. From this period on 

 they remained at the freezing point throughout the winter until in the 

 spring when they commenced to thaw. During the winter months their 

 temperature was not only almost exactly the same at any corresponding 

 depth but also of practically equal magnitude at all depths. In other 

 words the upper 6-inch layer of soil had the same magnitude of tem- 

 perature. The lowest average temperature was reached on January 30. 

 At this date the average temperature of the sand went down a few de- 

 grees lower than that of the other soils. 



(2) Thawing began to occur by March 12 but it did not actually com- 

 mence until April 1. The gravel, sand, loam, and clay thawed almost 

 simultaneously, with peat about four days later. The temperature of 

 the mineral soils rose quite rapidly to about the same degree, followed 

 by peat with a few days later. 



(3) The average temperature of all the soils then tended to be equal, 

 as it ascended, but it did not reach equality until about September. Up 

 to this time the magnitude varied in the different types, being greatest in 

 sand and gravel, followei by clay, loam, and peat, respectively. The 

 temperature of peat, however, was only about 2°F lower than that of the 

 sand or gravel at any corresponding depth, throughout the months of 

 May, June, July, and August. 



(4) By the beginning of September this variation in average tem- 

 perature in the different soils began to disappear and the magnitude be- 

 came practically identical in all of them at all three depths, and con- 

 tinued to be equal until November 30, which is the end of the year. 



The results of the fourth year, show, therefore, that the average tem- 

 perature of the upper depths of the various types of soil behaved almost 

 in the same manner as that of the lower depths of the second year or 

 corresponding year, when the surface of the soils was not covered with 

 the sand. 



