296 SOIL PHYSICS AND MANAGEMENT 



The results seem to indicate that radiation is slightly better from 

 white sand, but the differences are so small that they come within 

 the experimental error, and so the conclusion is reached by the 

 experimenter that color does not affect radiation. A large amount 

 of the heat radiated from the soil is brought to the surface by con- 

 duction. It is absorbed during the day and is conducted downward 

 to a depth of from one to twelve inches. As the temperature of the 

 air becomes lower at night the heat in part is conducted back to the 

 surface and is radiated into the air. From February to August 

 mere heat is received by the soil than is radiated from it, but during 

 the rest of the year radiation is greater than absorption, and as a 

 result the temperature of the soil is becoming lower. (See the 

 table, page 307.) 



2. By Conduction Downward Into the Soil. The process of 

 conduction is a very slow one, so slow that the soil at a depth of 

 36 inches has an average annual range of only 28.7 degrees F. for a 

 ten-year average, while at a depth of one inch the average range was 

 45.8 degrees. Some of the heat is conducted to such a depth that it 

 cannot influence the growth of plants in any way and may be con- 

 sidered lost. 



3. By Evaporation of Water. When water is evaporated large 

 amounts of heat are carried away as latent heat in the vapor. 



4. By Convection Currents of Air. The heated soil warms 

 the adjacent air, causing it to expand and rise. These currents of 

 warm air are constantly carrying large amounts of heat upward. 

 The effect of this in comparison to radiation may be seen by placing 

 thermometers at equal distances above and on the side of a heated 

 object. 



Soil Temperature for Vital Functions of Plants. 1. Tem- 

 perature for Germination. The temperature at which' ger- 

 mination takes place varies with different classes of plants. 

 Slow germination in a cold soil brings about favorable con- 

 ditions for the action of fungi and bacteria upon the seed which may 

 cause decay. Some of our cultivated crops, as corn and beans, are 

 especially susceptible to injury in this way. This may bring about 

 a low percentage of germination and a poor stand results. Uloth * 

 found that certain seeds, one of which was wheat, would germinate 

 in a dark cellar on a cake of ice, the rootlets descending into the ice 

 to a slight depth by melting cylindrical cavities. The rootlets of 

 Norway maple descended into the ice to a depth of 7.5 centimeters. 

 The next table gives the minimum, optimum, and maximum tem- 

 perature at which germination takes place. 



