302 



SOIL PHYSICS AND MANAGEMENT 



It is very interesting to note the effect of drainage in the above 

 experiment upon the germination of seeds and early growth of 

 plants in the drained and undrained soil (Fig. 95). 



4. Presence of Water. Aside from the lowering of tempera- 

 ture by evaporation of water from soils, the presence of water keeps 

 the temperature down because of the slowness with which it changes 

 or because of its high specific heat. This is partly the cause of 

 peats, clays, and undrained land being cold and late. If a cubic foot 

 of dry soil having a specific heat of 0.2, weighing 100 pounds, should 

 have 100 heat units applied to it, its temperature would be increased 

 five degrees Fahrenheit. If a cubic foot should contain 20 pounds 

 of water, its temperature would be increased two and one-half 

 degrees, or the specific heat of the soil would be doubled. Sand 

 soils are " early " because of the small amount of moisture which 

 they contain and their low specific heat. 



5. Absorption and Radiation of Heat. The absorption of 

 heat by soils and consequently their temperature depends largely 

 upon their color. The dark colors absorb more heat than light ones. 

 Black, blue, brown, and red absorb heat in the order given, while 

 green, yellow, gray, and white absorb less, white being the slowest 

 absorber of all. Bouyoucos colored white sand with dyes and deter- 

 mined the comparative absorbing power as measured by the tempera- 

 ture obtained. This table gives the results : 



Effect of Color on Temperature of Sands M 



A very interesting demonstration is to fill a tray three by six 

 feet with soil, plant an equal number of seeds in each half of the 

 tray, and cover one-half with very dark soil and the other half with 

 white soil and place in the sunshine (Fig. 134). For best results 

 this should be carried on in spring or fall. Plants come up from 24 



