THE BIOLOGICAL CONDITIONS IN THE SOIL 229 



carrying Sanai {Crotalaria junced) and other crops during the 

 monsoon the CO2 rose to 16 to 20 per cent, and the oxygen 

 fell to 2 to 4 per cent, only.^ 



The Temperature of the Soil. 



The temperature of the surface layer of soil, which in turn 

 determines the temperature of the lower layers, is the resultant 

 of several different effects. The actual amount of heat reach- 

 ing the surface is that portion of the sun's rays that passes 

 unabsorbed through the atmosphere, and is therefore dependent 

 on the climate. The intensity of distribution of the heat over 

 the surface depends on the slope of the land, and is greater 

 the more nearly the land lies at right angles to the mid-day 

 rays: thus, in our latitudes a south slope is warmer than a 

 north slope, so much as often to produce marked vegetation 

 differences. Many of the rays may be intercepted by vegeta- 

 tion, consequently land densely covered by plants is cooler 

 and moister than bare land ; advantage is often taken of this 

 fact in tropical countries to protect soil from intense evapora- 

 tion by the growth of "shade" crops. Of the rays that do 

 finally reach the surface not all are absorbed, an unknown 

 fraction being reflected back again into space : although no 

 actual measurements have been made, the loss from this cause 

 is probably greater on a white chalky soil than on a black 

 humus soil. 



The extent to which a given quantity of absorbed heat 

 raises the temperature of a soil depends on its specific heat 

 and this again on its water content. Dry soil has a specific 

 heat of 02, while wet soil has a specific heat approximating 

 more closely to i , hence under equal sunshine conditions a dry 

 soil will attain a higher temperature than a moist one. Not 

 infrequently the surface layer of the soil is hotter than the air, 

 especially on a sunny day.'*^ 



ij. W. Leather, Pusa Chem. Memoirs, 1914, 4, 85-134. The effect on 

 germination is discussed by A, Howard in Ag. yourn., India., 1915, 10, 106. 



*This is especially the case in hot climates. Leather (Pusa Chem. Memoirs, 

 1914, 4, 1-49) states that the maximum temperature at Pusa (India) is 20° C. 

 above the maximum air temperature in the shade. 



