SOIL TEMPERATURE 2i 



greater than that of the air, especially in summer ; the 

 maximum is also usually greater in winter, although it is 

 sometimes below in summer. In winter time, however, 

 the curve is often flat all the twenty-four hours, and some- 

 times shows no variation for two or three days together." 

 At that depth the soil tends to have a higher and more 

 uniform temperature than the air ; at greater depths — 

 which may be more important for root growth — the 

 temperature is still more uniform but with a lower mean. 

 Cannon (1915) has recorded very high temperatures in 

 desert soils at depths to 15 to 30 cms. : an average maximum 

 of over 30"^ C. may be maintained for four months of the 

 year. 



Kraus (191 1) has studied the temperature of the surface 

 regions of the soil on the Wellenkalk, at Gambach on the 

 Main. He shows that in bare soil, with scanty vegetation, 

 the temperature of the surface layers (2-5 cms.) habitually 

 rises very high in summer in sunny weather ; it is frequently 

 10*^ C. higher than the air temperature. In dull weather 

 and at night, air and soil temperatures are about the same. 

 The amount of soil moisture has a very marked effect on 

 the temperature ; as the specific heat of water is very much 

 higher than that of dry soil, dry soil heats up much more. 

 Kraus compared the temperature of moist soil near a spring 

 with that of the dry ground a short distance away ; on 

 sunny days the dry soil was frequently 5° C. higher than 

 the wet. Here the cooling effect of evaporation also 

 comes into play. Similarly bare soil shows a temperature 

 exceeding that of soil with a vegetable covering (Grass, 

 Thymus, Hieracium Pilosella, etc.) by 2°-8° C. The soil 

 temperature belov/ grass is, through the day, often as much 

 below the air temperature as the temperature of bare 

 soil is above it. Calcareous and sandy soils which drain 

 rapidly reach a higher temperature than clay and humus 

 soils which retain more water. 



Summary. — The soil, then, is a shallow layer of loose 

 material covering the fertile surface of the land. It is 

 derived from the weathered debris of the rocks, which is 



