EXPERIMENT STATION BULLETINS. 815 



colder than the bare soils. This difference, however, would become 

 smaller and smaller so that by September it would entirely disappear 

 and by O^ctober the order would be reversed; the bare plots would be- 

 come colder and the sod warmer. The latter would continue to have a 

 higher temperature throughout the fall and winter months. 



A cover of vegetation, therefore, had a very large controlling in- 

 fluence upon the soil temperature; during the warmest part of the year 

 it kept the soil cold and during the coldest part, warm. Indeed, a cover 

 of vegetation is one of the most efficient and expedient means of pro- 

 tecting the soil against extreme low temperature during the winter. 



Like the vegetation cover, a ):hick layer of snow is also a most efficient 

 agent for keeping the soil warm in the winter and preventing it from 

 attaining extreme low temperature during severe cold weather. Obser- 

 vations made for four years upon this subject showed that in excep- 

 tional cold weather the soil covered with a vegetation cover and a layer 

 of snow w^ould have 25° F higher temperature than a bare soil at 3-inch 

 depth. 



Certain topographic positions have a marked controlling influence 

 upon the soil temperature. A southern exposure had about 2°F higher 

 average temperature than a northern during the spring and summer 

 months, but during the fall and winter months both locations would be 

 equally warm. The soil temperature at a river bank was far below that 

 of the south and north slope during the spring and summer months but 

 slighthly higher during the fall and winter months. 



The marked influence of the slant of the surface with respect to the 

 position of the sun on the soil temperature is even manifested on a culti- 

 vated soil with very lumpy and uneven surface. The sides of the lumps 

 or dead furrows facing the sun in the morning had a higher temperature 

 than those shaded. 



The heat escaping from a well-iusiulated steam pipe buried about a 

 foot into the ground influenced the temperature of the surrounding soil 

 to a considerable extent. The temperature of the soil was highest im- 

 mediately above the pipe and diminished with increase in distance. The 

 influence was felt and the soil was prevented from freezing to a distance 

 of about 10 feet on each side, making a total lateral distance of about 

 20 feet. 



Speaking broadly, the ability of man to control or influence the soil 

 temperature under field conditions, one way or the other, is greatly 

 limited. Since the temperature of the soil is finally dependent upon 

 the meteorological elements and especially upon the air temperature, 

 which are beyond his control, all he can do is to vary some of the in- 

 trinsic factors and thereby modify somewhat the degree of soil tempera- 

 ture, either temporarily or permanently. The factors which are in his 

 power to modify in order to hasten the rise of temperature in the spring, 

 which is the most important time, are: (1) draining the soil from an 

 excess of w^ater, (2) creating a dark color at the surface, (3) covering the 

 surface of these soils prone to low temperature with a thin layer of 

 sand, and (4) compacting the surface of the cultivated soil. 



A fulfillment of the above factors except (1) would probably be of 

 some benefit only if the planting is to be done in the spring. If it is 

 already accomplished in the fall so that the plants have already begun 



