EXPERIMENT STATION BULLETINS. 675 



SUMMARY. 



In this bulletin there is presented the results of an investigation upon 

 the general subject of soil temperature. It was shown at the outset 

 that the problem is very complex — it involves a variety of factors — and 

 in order to arrive at proper and definite conclusions it must be studied 

 from as broad a standpoint as possible. The attempt was made and the 

 results obtained may be summarized as follows: 



The specific heat of dififerent types of soil, gravel, sand, loam, clay 

 and peat, in dry condition did not differ very materially; this was true 

 both by equal weights as well as by equal volumes. The specific heat of 

 peat was about half as great as that given to it by other investigators. 

 The moisture content of these different soils in their natural condition 

 varied very greatly and since water has such high specific heat, it made 

 a tremendous difference in their final specific heat. 



The heat transference in these different soils was measured (1) in 

 their dry condition; (2) in their natural state under laboratory condi- 

 tions; and (3) under field conditions. It was found in all these three 

 states that the order of heat conductivity was the same: gravel pos- 

 sessed the highest heat transmitting power, followed in order by sand, 

 clay, loam and peat, respectively. Convectional currents, molecular 

 diffusion and distillation influenced very greatly the rate of flow of heat, 

 and consequently the values obtained do not represent the true heat 

 conducting power of these different types of soil. 



In field conditions, the solar radiation tended to travel with greater 

 rapidity and facility vertically than horizontally. 



The study on radiation showed that color had no effect upon radia- 

 tion, which is contrary to the common belief, but it had upon absorp- 

 tion; that the different types of soil tended to radiate differently when 

 dry, about the same and more when well moistened and in their natural 

 condition, and that a dry surface or mulch reduced the radiation. In 

 the dry state, sand exhibited the highest radiating power followed by 

 gravel, clay, loam, and peat respectively. The water, however, had by 

 far the highest radiation capacity of any soil either in the dry or moist 

 state. 



When the temperature of these different types of soil (all covered 

 with a thin layer of the same kind of soil in order to eliminate the 

 factor of color, and other factors) was studied under field conditions, 

 it was found that they all cooled and froze about the same time in the 

 upper 6 inches, but in the spring they thawed and warmed up at differ- 

 ent rates. This was attributed to their different specific heats and to 

 the downward and upward trend of air temperature in the fall and 

 spring respectively. The gravel and sand thawed first, followed by clay 

 1 day later, loam 2 days later, and peat 10 days later. The temperature 

 of the first two soils rose very rapidly after tha-v\ang. while that of the 

 others rose very slowly. When the lower depths of the latter soils had 

 thawed, however, their temperature rose also quite rapidly and finally 

 reached the same degree of magnitude as in the former or lighter soils, 



