742 STATE BOARD OF AGRICULTURE. 



The above figures, llierefore, show that the heat is propogated into 

 the soil in the form already described. 



They also show that the degree of gradient: of temperature is much 

 greater during the day when the maximum temperature is reached than 

 during the niglit when the minimum is attained, at the surface-. Thus, 

 for instance in the former case the difference is almost as high as .30° F, 

 while in the latter case it is only about 10°F. It is to be noted that in 

 both instances the amplitude is greatest between the surface and 2-ineh 

 (lc]>tli and decreases rapidly with the succeeding depths. 



Another extremely interesting fact that is revealed by the above tables 

 is that the rate at which the maximum and minimum waves travel at the 

 various depths of any particular soil tends to follow approximately a 

 mathematical law. This law may be stated as follows: the lag of the 

 maximum and minimum epochs tends to be approximately proportional 

 to the depth, in all the different types of soil. 



The larger gradient during the day over that at night is due, of course, 

 to the fact that during the insolation the soil receives its heat both 

 by absorption and by C(mtact with the warm air and since it is a poor 

 conductor of heat the temperature rises at the top much faster than it 

 is conducted downward. AVhile during the night it is this high tempera- 

 ture at the depths that is being lost, through radiation and contact with 

 the cold air, and since the rate of cooling is slower than that of warm- 

 ing, the various adjacent depths do not attain very large gradients of 

 temperature. 



The night or early morning gradient at the various adjacent depths 

 is greatest in the fall, and slightly smaller in the summer, and spring. In 

 the fall the magnitude between the surface and G-inch depth is usually 

 about 5°F while in the summer and fall it is about 3°F, in the mineral 

 soil, and slightly higher in the peat. 



The degree of gradient at the various depths both at night and day 

 is of great importance from the standpoint of aeration and thermal move- 

 ment of moisture in soils as shown by the author elsewhere*. 



EFFECT OF METEOROLOGICAL ELEMENTS ON THE SOIL TBiNirERATURE. 



It has already been stated that the external factors which comprise 

 the meteorological elements act upon the intrinsic factors and cause the 

 temperature of the soil to vary as they themselves vary. These meteoro- 

 logical elements, however, do not all influence the soil temperature in the 

 same direction, and are very complex as to their behavior and conse- 

 quently the influence of all of them is not readily apparent. In the pre- 

 vious report an attempt was made to establish a connection between the 

 soil temperature and the various chief meteorlogical elements but it was 

 not altogether successful. The only factors with which the soil tempera- 

 ture correlated very closely was the air temperature, the dew point 

 and the sunshine during the summer. 



'Technical Bui. 22 Mich. Expt. Sta., 1915. 



