THE SOIL. 31 



organic matter, and from the sun That received 

 from the interior of the earth is so small that it 

 may not be considered here. The greatest 

 amount comes from the sun. This can be easily 

 noticed by observing the difference in temperature 

 between winter and summer, and day and night 

 Both light and heat reach the earth from the sun, 

 but light rays have little to do with soil conditions; 

 on the other hand, heat rays play a very important 

 part. 



When light or heat falls upon an object it is 

 either reflected away or absorbed, according to the 

 color and other physical conditions of the object 

 upon which it falls. Light-colored objects reflect 

 away the most and dark- colored objects absorb 

 the most. A dark-colored object that absorbs 

 heat is, of course, made w r armer thereby. So the 

 darker the color of soil, other things not being 

 considered, the more heat rays will it absorb and 

 consequently the warmer it will be. 



Some things are heated very quickly but give up 

 their heat just as quickly; and those which are slow 

 in being heated, give up their heat slowly in return. 

 Water is a substance wlv.ch absorbs heat slowly 

 and retains it a long time. The boiling kettle of 

 water remains hot a long time after the fire goes 

 out and the stove becomes cold. So it is with 

 soils; some become very hot during the middle of 

 the day, and cold at night. Those which are 

 heated only moderately during the day, keep a 



