EDAPHIC OR SOIL FACTORS: PHYSICAL 



219 



extreme temperatures are recorded or when records of long-continued 

 observations are available. 



Soil temperature is dependent upon the exposure and the physical 

 nature of the habitat. Both dead and living ground cover are impor- 

 tant. Radiation has a depressing effect on soil temperature. The 

 greater the angle of incidence of sunlight and the longer its duration, 

 the greater the intensity of insolation the higher the soil temperature. 

 The contrasts are most marked in mountains, and this is one of the 

 prime factors which cause the great differences in alpine vegetation in 

 sunny and shady exposures. The scenery changes with surprising 

 abruptness in passing from shady to sunny slopes. Maurer (1916) has 

 made measurements of the relation between soil and air temperatures 

 at different altitudes. They refer to soil temperatures at a depth of 120 

 cm., where the root layer is developed in good soil. 



Table 24. — Mean Soil Temperatures with Increasing Altitude in the 



Swiss Alps 

 (After Maurer) 



The relative soil temperature thus shows a regular increase with an 

 increase in altitude. While the annual average of the air temperature 

 in the Central Alps reaches 0°C. at a little above 2,000 m., the soil 

 temperature averages 0°C. only at 2,700 m. 



An interesting treatise by Henne (1894) deals with the dependence 

 of soil temperature upon the physical condition of the soil. He studied 

 the temperature of the uppermost layers of differently colored soils. 

 His observations extended over a period of seven months (April to 

 October) and were carried out in the experimental forest garden of 

 Adlisberg near Zurich, at depths of 3 to 5 cm. 



