960 EXPERIMENT STATION RECORD. 



closeness of planting, and tlie length of the period of growth are of 

 most importance, ^ 



Soil coverings of mineral matter influence the temperature of the soil 

 in different ways, according to their own thermal properties. Stones, 

 gravel, sand, etc., when in a more or less thick and uniform layer over 

 the soil, as a rule raise its temperature during the warmer seasons, but 

 lower it during the cold season, when the exposure to the sun is slight. 

 Tlie influence of such a covering is generally more marked the darker 

 the color and the more powerful the radiation. Soils having a high 

 water capacity and a light color (clay, marl, chalk) have an efl'ect the 

 reverse of that of stone, gravel, sand, etc., when used as a covering, 

 owing to the much greater loss of heat due to evaporation and the 

 lower absorptive power for the sun's rays. ^ 



Snow, as a bad conductor of heat, prevents rapid falls of temperature 

 during the cold season, diminishes variations in temperature, and re- 

 tards the penetration of frost. When the snow melts these conditions 

 are usually reversed. The soil free from snow warms up more quickly, 

 since it is generally dryer than that covered with snow. The latter 

 also takes longer to become free from frost. ^ 



INFLUENCE OF CLIMATE AND WEATHER ON THE TEMPERATURE OF THE SOIL. 



The constitution of the soil being the same its temperature depends 

 on the distribution of heat over the surface and also on certain local 

 causes affecting the influence of the sun's radiation. The intensity of 

 solar radiation varies with the altitude of the sun, and therefore 

 decreases with the geographical latitude. For this reason the tem- 

 perature of the soil decreases from the equator toward the poles, the 

 decrease being more rapid as the 45th parallel is approached, after 

 passing which it is slower. This decrease in soil temperature is, how- 

 ever, not uniform and proportional to the latitude, but exhibits many 

 variations due to difference in length of exposure to the sun's rays, 

 proportion of land and water, and local influences. The longer time of 

 radiation in northern countries during summer causes a relative rise in 

 temperature of the soil, without which the cultivation of the soil there 

 would be impossible. On islands and near the coast the maximum 



'E. Ebermayer, Die physikalisclien Einwirkungen ties Waldes, etc., 1873; Forsch. 

 Geb. agr. Pliys., 14, p. 379. E. Wolluy, Der Einfluss der Pflanzeudecke, etc., Ber- 

 lin, 1877, p. 15; Forsch. Geb. agr. Pbys., 6, p. 197; Ibid., 10, pp. 30, 345; 3, p. 143. 

 A. Muttrich, Beobaclitaugeu der Erdteniperatur an den forstlich-meteorologischen 

 Stationeu, 1880. Th. Nordlinger, Der Eiuflnss des Waldes auf die Luft und Boden- 

 wiirrae, 1885. E. Ramann, Forstlicbe Bodenkunde, 1893, p. 96. A. Petit, Forsch. 

 Geb. agr. Phys. 16, p. 285. 



*E.Wollny, Der Einfluss der Pflanzendecke, etc., 1877, p. 36; Forsch. Geb. agr. 

 Phys., 1, p. 44. 



*E. Wollny, Der Einfluss der Pflanzendecke, etc., 1877, 11, p. 24. E. and H. Bec- 

 querel, Compt. Rend., 92, p. 1253. A. Wosikof, Der Einfluss einer Schneedecke auf 

 Boden, Klima und Wetter. A. Penck, Geograpliis( he Abhandlungen, 3 (1889), 

 No. 3. A. Petit, Forsch. Geb. agr. Phys,, 16, p. 285. 



