452 



THE PRINCIPLES OF SOIL MANAGEMENT 



(2) Indirectly through the radiation which is im- 

 parted to the atmosphere, from which it is radiated to 

 the soil or is given up by direct contact of the atmosphere 

 with the soil. Clouds in the atmosphere reflect back to 

 the soil some heat which has been received by the 



soil and is again 

 given off. They may 

 serve as a cover or 

 blanket. 



(3) In the spring, 

 rain-water carries a 

 large amount of 

 heat into the soil. 

 The percolation of 

 warm spring rain is 

 a means of rapidly 

 warming up the soil, 

 and its strong in- 

 fluence is shown by the large quickening of growth 

 which follows such rainfall. 



322. Conduction. Conduction of heat from the 

 interior of the earth is negligible as an appreciable 

 source of soil heat. 



323. Organic decay. Organic decay liberates heat, 

 and may be so rapid as to greatly change the tempera- 

 ture of the soil. This is exemplified by the heating of 

 manure heaps and in the use of the hotbed. The same 

 amount of heat is set free bjr decomposition as would 

 result from ignition of the material, but its liberation 

 is distributed over a much longer period of time accord- 

 ing to the conditions for decay. 



Fig. 121. Mean annual sunshine of Canada 

 and the United States. The figures indicate the 

 number of hours of bright sunshine in a year. 

 (From Bartholomew's Atlas of Meteorology.) 



