126 THE TEMPERATURE OF THE SOIL [chap. 



bacteria are active within about the same limits of 

 temperature as have been indicated above for the 

 higher plants ; the nitrification bacteria, for example, 

 cease their work below 41 ° F. and above 130 F., their 

 optimum temperature being about 99 F. 



The way a low temperature will check the production 

 of nitrates until they are inadequate for the needs of 

 the crop is often seen in spring, and may be connected 

 with the yellow colour of the young corn during a spell 

 of cold and drying east wind. 



Radiation. 



The main source of the soil warmth consists in the 

 heat received from the sun by radiation ; this, according 

 to Langley, amounts to about 1,000,000 calories per 

 hour per square metre of surface from a vertical sun in 

 a clear sky. Supposing this energy were wholly 

 absorbed by a layer of dry soil 10 cm. thick, its 

 temperature would rise by as much as 90 F. in an hour. 

 Of course in nature many other factors are at work to 

 reduce this temperature; the sun is rarely vertical, 

 the soil material does not completely absorb but reflects 

 some of the sun's rays unchanged ; at the same time it 

 is always radiating in its turn rays of lower pitch than 

 the majority of those received. The latter rays are 

 easily caught by many substances, glass and water 

 vapour in particular, which are transparent to the rays 

 of higher refrangibility proceeding from the sun. A 

 greenhouse, for example, is practically a radiant heat 

 trap ; the temperature inside runs up because the sun's 

 rays of light and heat can penetrate the glass, whereas 

 the obscure heat rays radiated back again from the 

 warmed-up surfaces inside the house are not able to 

 pass through the glass again. Just in the same way the 

 temperature rises and the sun's heat becomes oppressive 



