804 STATE BOARD OF AGRICULTURE. 



cooling and warming effect upon tlie soil in opposite direction to the 

 air temperature of the season. 



The slightly higher temperature of the uncultivated soil over the 

 cultivated during the summer and possibly in the spring, is due chiefly 

 to the dry mulch formed on the cultivated soil. This dry mulch forms 

 rather poor and imperfect connections with the lower layers of the soil 

 and the heat energy which it receives from the sun is not all conducted 

 downward rapidly but a large amount of it is allowed to accumulate on 

 the surface of this mulch and then some of it is radiated back to the 

 atmosphere, while on account of the compacted condition and conse- 

 quently better heat conducting power a greater amount of heat is propo- 

 gated to the lower depths in the case of the uncultivated soil and not so 

 much accumulates at the surface and then radiated into space. These 

 facts were experimentally proved by the following series of experiments. 

 The bulbs of mercury thermometers were placed one inch above the sur- 

 face of both the cultivated and uncultivated soils. It was found that the 

 thermometer over the cultivated soil would register at certain days about 

 10°F higher temperature than that placed over the uncultivated soil. In 

 another experiment the bulbs of the thermometers were placed hori- 

 zontally over the surface of both soils. It was discovered also in this 

 case that the surface of the cultivated soil would be 7 or 8°F warmer in 

 certain days than that of the uncultivated. 



This same mulch, however, for the same reason that it reduces some- 

 what the amount of heat penetrating the lower depths during the insola- 

 tion, diminishes equally well the quantity of heat that is lost during the 

 night. 



The dry mulch then tends to act as a blanket analogous to that of 

 vegetation cover, but to a less degree. It is perhaps due to this mulch 

 and also to the slightly higher moisture content that the temperature 

 of the cultivated soil rises or falls somewhat less rapidly than that of the 

 uncultivated. 



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

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

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

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

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

 rise is dependent upon the wind and convection currents and conduction, 

 and not upon any direct heat absorption. (2) The air temperature 

 around the plants and at the surface of the soil is low on account of the 

 great amount of transpiration and some evaporation that take place. 

 Both of these processes have, of course, a tremendous influence upon the 

 soil temperature. 



In the cold part of the year the dead or partially decayed vegetation 

 acts as a blanket over the surface soil. This cover performs several 

 functions chief which are: (1) The prevention of the cold currents of 

 air from coming in contact with the surface soil and thereby cooling it. 

 and (2) the layer of vegetation is porous and the spaces are filled with 

 air. This air being a very poor conductor of heat does not transmit 

 very rapidly heat from the soil to the air above and vice versa and as 

 a result the soil loses its heat less rapidly than if it were unprotected. 



It is through the vegetation cover that is afforded probably one of the 

 most efficient and expedient means of preventing extreme low soil tem- 



