EXPERIMENT STATION BULLETINS. 643 



plots next in order. The latter two soils had practically the same temper- 

 ature, the difference in favor of the uncultivated plot was very slight. 



The daily records, which are not shown in the above table, show that 

 the lowest fall in temperature occurred on January 7 when the average 

 temperature of the different soils at the 7 inch depth was as follows: 

 cultivated 24.55°, uncultivated 22.0°. and sod 27.85°, while the highest 

 average temperature took place on July Avith the following results: 

 7 inch cultivated 70.00°, uncullivated 77.80°, sod 69.45°; 20 inch culti- 

 vated 69.0°, uncultivated 69.31°, sod 65.25°. 



The first freezing occurred on December 28th. On this day the culti- 

 vated and uncultivated plots at the 7 inch depth froze at the same 

 time, while the sod plot froze 7 days later (January 5). The first thaw- 

 ing took place on April 5th when both the uncultivated and the sod 

 plots thawed at the same time. The other plot thawed about a day later. 



In the second winter (1912-13) the different plots froze at the upper 

 depth as follows: cultivated December 11th, uncultivated December 

 11th, sod December 13th. At the 20 inch depth the uncultivated froze 

 Jan. 20th, cultivated Jan. 22, and sod did not freeze at all. Throughout 

 this winter the sod plot maintained the highest temperature at both 

 depths, the temperature of the other two plots was about he same. The 

 results of the second winter, therefore, are in agreement with those of 

 the first winter, the only difference being that in the second winter the 

 effect of the vegetation in keeping the soil warm was more pronounced, 

 due undoubtedly to the greater abundance and greater thickness of the 

 layer of vegetation. 



The foregoing data show, then, that the conditions of cultivation, 

 non-cultivation, and sod have a very distinct influence upon soil temper- 

 ature. Especially interesting is the influence of vegetation. It was ob- 

 served in the first soring that the sod plot thawed at the same rate 

 as the bare and uncultivated plot, and that its temrterature rose immedi- 

 ately several degrees above that of the latter and remained so for the 

 next 10 days when the plants began to grow, and then its temperature 

 dropped back and stayed much lower than the temperature of both 

 bare plots until September 25th. From this date on the temperature of 

 the other two plots fell below that of the sod plot and remained so 

 throughout the second winter and up to the middle of the second spring. 



The cooling and warming effect of the cover crop in both the warmer 

 and colder seasons of the year is self evident. In summer time the 

 growing crop keps the soil temperature low for the following reasons : 

 (1) Practically all the sun rays are intercepted bv the growing vegeta- 

 tion so that the ground surface is nearly all shaded, and its temperature 

 rise is dependent upon the wind currents and convections; (2) the air 

 temperature around the plants and at the surface of the ground is low 

 on account of the great amount of transpiration and evaporation that 

 takes place. The process of evaporation has a most tremendous influ- 

 ence upon the diminution of temperature. The amount of transpiration 

 by plants is not inconsiderable. Mulham makes the statement that 

 areas covered with vegetation under the same conditions evaporate 

 about one-third more Avater than a free water surface. 



In the cold part of the year, the decayed and i>artially decayed vege- 

 tation acts as a blanket over the surface soil. This blanket performs 

 several functions, chief of which are : First, and most important, the 



