EXPERIMENT STATION BULLETINS. 803 



Table 87 shows (1) that the average temperature of the sod soil at 

 7 inches depth was about the same or only slightly lower than that of 

 the cultivated and uncultivated, during the month of April. From this 

 month on, the sod plot began to have decidely lower temperature than 

 the other plots and continued to be colder until October. The highest 

 variation occurred in June and July when the average temperature of 

 the sod land was about 6 or 7°F lower than that of the bare land. These 

 differences, however, commenced to become smaller in August and by 

 September they had practically disappeared. In October the order of 

 magnitude was reversed, the sod plot became warmer than the other 

 two plots and continued to have a higher temperature until freezing 

 and even during the winter months. The data for the winter months 

 for this year are not included in the above table on account of incom- 

 pleteness but they are on file and show that as a monthly average the 

 temperature of the sod plot remained about 2°F higher than that of the 

 cultivated and uncultivated plots. 



(2) During the months of April, May and June the average tem- 

 perature of the uncultivated and cultivated soil was practically equal. 

 During July and August, however, the uncultivated plot was about 1°F 

 warmer than the cultivated, but during September, October, and Novem- 

 ber and also during the winter months they had again e(iual temperature. 



(3) The degree of amplitude was highest in the uncultivated plot, 

 slightly smaller in the cultivated, and very small in the sod. Thus, for 

 instance, in July it amounted to 10.87° in the uncultivated plot, 7.62° 

 in the cultivated, and only 2.87°F in the sod. 



(4) As a rule the uncultivated plot had a slightly higher maximum 

 temperature than the cultivated, but the latter kept a slightly higher 

 minimum than the former. The magnitude of the maximum and mini- 

 mum of the sod plot was practically equal to that of the other plots in 

 the early spring and fall; in the summer it was far below, but in the 

 winter somewhat higher. 



Table 87 contains also the monthly maximum, minimum, amplitude 

 and average temperature of the same plots during the year of 1915 but at 

 different depths — 3 and inches. The data in this table reveal exactly 

 the same principles as the preceding and consequently need no further 

 discussion. 



The daily march of temperature which is not included here on account 

 of lack of space, indicates (1) that in the spring all the three plots 

 thawed practically simultaneously, (2) The temperature of the cultivated 

 and uncultivated plots would rise quicker and higher whenever the air 

 temperature rose very rapidly and suddenly than that of the sod. The 

 converse of this was also true. (3) As between the cultivated and un- 

 cultivated the rate of rising and falling of temperature was somewhat 

 similar to that of the sod, but to less degree, of course. 



Tibs investigation, therefore, proves two very important facts. (1) A 

 soil with compacted or undisturbed surface is only slightly warmer, 

 about 1°F, than a cultivated soil, and that is only in the summer months 

 and possibly in the spring; in the fall and winter months both soils are 

 equally warm. (2) A soil covered with a growing or dead vegetation has 

 a slightly lower temperature in the spring, considerably lower in the 

 summer, but slightly higher in the fall and appreciably higher in the 

 winter than the bare soils. The vegetation cover, therefore, has both a 



