RELATIONS OF SOILS TO HEAT. 



305 



result is concerned. For it is found that, other things being 

 equal, a loose or cloddy surface disperses in many directions 

 the heat it receives, and does not permit it to penetrate by 

 conduction to so great extent as would a more compact soil, 

 whose smooth surface would waste less of the heat received 

 by radiation. 



King has called special attention to the difference of temperature 

 existing between soils smoothed and compacted by a roller, and the 

 unrolled soil having a loose surface. He found that the former at a 

 depth of one and a half inches was as much as 5.5C. (ioF.) warmer 

 than the loose soil, and that even at a depth of three inches a difference 

 as high as 3.5C. (6.5F.) existed between the two. He observed at 

 the same time that the temperature of the air over the unrolled ground 

 was considerably warmer than above the rolled, thus corroborating the 

 differences observed in the soil itself. But at night the heat is given 

 out more rapidly from the rolled than from the unrolled surface, the 

 latter acting as a non-conductor and keeping the soil warmer than that 

 of the more compact rolled land. King gives as the average difference 

 observed between rolled and unrolled land on eight Wisconsin farms, 

 i.6C. (3F.) in favor of the rolled land between i and 4 p.m. 



It will thus be seen that the loose tilled layer, while im- 

 peding the penetration of the sun's heat into the deeper por- 

 tions of the soil during the day, on the other hand serves to 

 retain it at night better than a more compact soil. This ob- 

 viously places it within the power of the farmer to exert con- 

 siderable control over the soil-temperature at critical times; 

 restraining or favoring the access of the sun's heat in accord- 

 ance with the requirements of the climate or season, as the 

 case may be. 



Influence of a Covering of Vegetation, and of Mulches. A 

 cover of either living or dead vegetation depresses the tem- 

 perature of the soil as compared with the bare land, as elabor- 

 ately shown by Wollny and Ebermeyer. In the monthly aver- 

 ages these differences rarely exceed .8 C. (1.5 degrees F.), 

 and are mostly below .50 C. (i F.), but during different 

 parts of the day they may rise to 2.2 to 2.5 C. (4 to 4.5 F.), 

 at 4 inches depth. In summer they are greater than at other 

 seasons. Of course the density of the vegetation or the thick- 

 ness of the mulch influences them materially. Forests exerf 

 20 



