EDAPHIC OR SOIL FACTORS: PHYSICAL 



223 



Measurement of Soil Temperature. — Numerous soil thermometers 

 are in use for the measurement of soil temperature. However, they 

 all have the disadvantage that they are introduced into the earth from 

 above and therefore, at least in soils which are not very loose, call for 

 some sort of excavation. This, of course, destroys the natural soil 

 arrangement somewhat, and the reading may be erroneous. Toumey 

 and Stickel (1925) propose an excavation from which the thermometer 



c 



50% 



A.M. 6 12 6 P.M. 12 6 



FiQ. 114. — Diurnal variation of soil temperatures at Giza, Cairo: (a) at the soil 

 surface; and at depths of (6) 5 cm.; (c) 10 cm.; (d) 16 cm.; and (e) 20 cm. 

 McKenzie-Taylor and Williams.) 



(After 



can be introduced horizontally at various depths. Superficial, indi- 

 vidual measurements are usually made with the so-called punch 

 thermometer in a brass case, but these readings can claim no great 

 accuracy. 



4. SOIL ATMOSPHERE 



Carbonic acid is continuously released in the soil by the action of 

 soil organisms and by the respiration of roots. Moreover, the living 

 organs and organisms demand oxygen, which they can obtain only from 

 the air by gaseous exchange. For the great "majority of the higher 

 plants an active exchange of gases in the soil is a necessity of life. 

 Livingston and Free (1917) have shown that the labiate, Coleus blumei, 

 ceases to absorb water through the roots within a few hours after the 

 withdrawal of oxygen. Contrariwise, Salix nigra was not affected at 



