Conditions Influencing Soil Temperatures. 215 



winter wheat in 12 days, and barley and oats in 13 days, 

 while cucumbers did not germinate in 42 days. 



258. Effect of Soil Temperature on Root Pressure. The 



power which sends the soil moisture into the roots of plants 

 and up into the leaves is osmotic pressure, developed by 

 the warmth of the soil, and unless the soil temperature 

 is sufficiently high plants may wilt, as Sachs has shown, 

 where he demonstrated that pumpkin and tobacco plants 

 wilted badly, even at night with an abundance of moisture, 

 as soon as the soil temperature fell much below 55 F., the 

 moisture not rising fast enough to compensate for even 

 the slow evaporation during the night. 



29. Influence of Soil Temperature on the Formation of 

 Nitrates. The nitrates in the soil do not develop until the 

 temperature has risen above 41 F. ; the action of the 

 germs is extremely feeble at 54 and they do not attain 

 their maximum activity until a soil temperature of 98 has 

 been reached; but if the earth becomes as warm as 113 F. 

 then the action is nearly stopped, it being as weak as at 54. 



CONDITIONS INFLUENCING SOIL TEMPERATURE. 



260. Specific Heat of Dry Soil When the same number 

 of heat units are given to like weights of different kinds 

 of soil their temperatures are not raised through the same 

 number of degrees and this is because their specific heats 

 (40) are different. 



From the determination of Oemler it appears that the 

 number of heat units required to raise the temperature of 

 100 Ibs. of water and 100 Ibs. of soil of different kinds 

 from 32 to 33 F. is as stated in the table which follows: 



