EXPERIMENT STATION BULLETINS. 811 



they all tend to have the same average temperature under the same 

 meteorological elements. 



In excessive degree, however, certain of the intrinsic factors predomi- 

 nate and cause a variation in the temperature of the diverse classes of 

 soil. The factors which are almost always responsible for differences in 

 temperature that may exist in the different kinds of soil are the follow- 

 ing: (1) latent heat of fusion of ice, (2) latent heat of evaporation of 

 water, (3) a covering of the surface soil, (4) color, and (5) topographic 

 position. All these factors are able to exert either an ephemeral or last- 

 ing predominant effect upon the temperature of the soil which possesses 

 them. 



The line of thought thus far developed is supported by the results ob- 

 tained from the various experiments conducted under natural conditions. 



The field studies on the temperature of different types of soil, namely, 

 gravel, sand, humus loam, clay, and peat at 6, 12 and 18-inch depths, 

 showed that when the surface of all these soils was covered with a thin 

 layer of sand they had almost exactly the same average temperature 

 throughout the year except during a short period in the spring when 

 thawing was taking place. 



Thawing would usually commence about the end of March or beginning 

 of April. The sand and gravel would thaw completely first, followed 

 by clay and loam, one or Uvo days later, and peat ten or fifteen days 

 later. Immediately upon complete thawing the average temperature of 

 the mineral soils would rise rapidly and approach the same magnitude in 

 about three or four days. Meantime the average temperature of the 

 peat would be far below, on account of the slow rate of thawing. As 

 soon, however, as it was also completely thawed its temperature would 

 rise very rapidly and would approach, in a few days, that of the mineral 

 soils. The average temperature then of all the soils would be remarkably 

 the same from this period on throughout the summer, fall and winter 

 months until the next thawing period. 



During the summer months the peat would have a monthly average 

 temperature of a few tenths of a degree higher than the mineral soils. In 

 the fall, however, the sand and gravel would possess a slightly higher 

 monthly average temperature than the peat. 



Whenever rapid and sudden changes of air temperature occurred the 

 sand and gravel would warm and cool the fastest, followed by clay, loam 

 and peat, respectiveh'. The equilibrium or equality, however, would 

 quickly be reestablished. 



The degree of amplitude was greatest in sand and gravel, somewhat 

 smaller in loam and claj', and least in peat. The highest fluctuation 

 occurred in summer and the lowest in winter. 



The maximum temperature was approximately the same for all types 

 of soil, after complete thawing had taken place, but the minimum varied 

 somewhat; it was lowest in sand and gravel, slightly higher in clay and 

 loam, and highest in peat. Even during the winter, the minimum varied 

 only slightly in the different soils. During a sudden severe cold weather, 

 however, the minimum of the sand and gravel would fall appreciably 

 lower than that of the peat, but the differences would not be very great. 



The equality of average temperature in the diverse classes of soil, 

 therefore, was not made up of extreme variation in the maximum and 

 minimum temperature between them, i. e., a very high maximum and 



