Soil Temperainres 177 



Plot A maintained the highest maximum temperature during the 

 spring and summer and until quite cold weather in the fall, when plot C 

 registered the highest maximum temperature. This lasted for a month 

 or so during the coldest weather, and as soon as it began to moderate 

 in late winter plot A warmed up rapidly, with D next. 



The greatest variation between plots occurred during the summer 

 months. In the spring and fall there is a transition period in which the 

 temperature differences are less. During the summer of 191 3 plot A 

 registered a maximum temperature of 80.5° the week of July 7, when 

 plot D was 78.5°, and plot C v^as 71° F. However, plot C later, the week 

 of August 18, registered a maximum temperature of 72° F, During the 

 week of July 13, the following summer, plot A registered a maximum 

 of 80°; plot D, 77°, the week of July 24; and plot C, 70° F., the week of 

 August 3. 



Figures i to 4 are reproductions of typical seasonal charts of soil tem- 

 peratures prevailing under the three cultural systems. These give an idea 

 of the diurnal variations. During the winter the temperatures are quite 

 constant from day to day, with very little variation between plots 

 (fig. i). In the spring the diurnal range is considerable in the plot under 

 tillage with cover crop and the grass land, but varies little under the straw 

 mulch, which exhibits a very gradual warming up (fig. 2). During the 

 summer season, fluctuations become quite pronounced under tillage and 

 grass, but the straw mulch still maintains its uniformity (fig. 3, C). 

 During the season of greatest daily range the maximum and minimum 

 temperatures occur about 10 p. m. and 10 a. m., respectively (fig. 3, 

 A and D). In the fall the temperatures and ranges are not radically 

 different from those of spring, except that the general trend of tempera- 

 atures is reversed (fig. 4) . 



In conclusion, it may be said that a system of clean cultivation with 

 a winter cover crop is characterized by extreme diurnal and annual 

 fluctuations in soil temperature; that a straw mulch equalized these 

 fluctuations to a marked extent, as does also a grass crop, though in 

 less degree. 



