THE PHYSICAL PROPERTIES OF THE SOIL. 959 



INFLUKNCE OF THE POSITION OF THE SOIL. 



Tlie temperature of the soil varies with a change in exposure. A 

 southerly exposure is warmest; then easterly, westerly, and northerly 

 exposures follow iu the order luxmed. In the course of the seasons 

 the maximum temperature shows a fairly regularly recurring varia- 

 tion in a southerly exposure. In the winter months (November to 

 April) the maximum is in the southwest, in summer (May to August) 

 in the southeast, in autumn in the south, after which it returns toward 

 the southwest. A southerly exposure is warmer and a northerly 

 exposure colder the greater the inclin.ition to the horizontal. The 

 inclination of an easterly and a westerly exposure has comparatively 

 less influence on the warmth of the soil, but the greater the inclination 

 the warmer is the easterly and the colder is the westerly exi^osure. 

 The difference in temperature between a northerly and a southerly 

 exposure is considerably greater than between an easterly and a 

 westerly. The difference in temperature between a northerly and 

 a southerly exposure is wider the greater the inclination toward 

 the horizontal. The angle of inclination has a much smaller influ- 

 ence on the difi:erences iu temperatures between an easterly and 

 a westerly exposure. The west side is, when only slightly inclined 

 (up to 15°), generally rather warmer than the east side, but when 

 strongly inclined is colder.^ 



INFLUENCE OP THE COVERING OF THE SOIL. 



A soil covered with living vegetation or with dead plants (leaves, 

 straw, stable manure, wood, etc.) is warmer during the cold season and 

 c )oler during the warm season than an uncovered soil, other conditions 

 being the same. This difference is greatest in summer and winter and 

 least in spring and autumn. When warm periods occur in winter and 

 cold ones in summer, the influence of the covering on the soil tempera- 

 ture is the reverse of that mentioned, at least in the uppermost layers. 

 A bare soil warms up more quickly in spring and cools down more rapidly 

 in autumn than one covered with vegetation or dead plants. Frost 

 penetrates more slowly and to a less depth into the latter than into the 

 former, but, on the other hand, a bare soil becomes free from frost earlier 

 in the spring. The influence of the covering in the warm seasons 

 is more marked as the exposure to the sun's rays is more complete, 

 and vice versa. The characteristic action of vegetation is greater the 

 denser and the further developed it is, and of other covers the thicker 

 and more compact the layer. As regards the influence of different 

 crops on the soil temperature, the development of the leafy parts, the 



'F. Kerner Ton Marilann, Ztschr. osterr. Ges. MeteoroL, 6, p. 65; Sitzungs- 

 bericlite cler kaiserlicheu Akademie der Wisseuschafteu ia Wien, 1891, C Abtlieil- 

 uug 2a. C. Eser, Forscli. Geb. agr. Pliys., 7, p. 100. E. Wollny, Ibid,, 1, p. 263; 

 10, pp. 8, 345. 



