SOIL TEMPERATURE 35 



or mulched land only a small part of the heat is con- 

 ducted downward. The other is radiated back to the 

 atmosphere by the dry ground on top. This is because 

 the mulch breaks up capillary attraction with the moist 

 soil below, acting somewhat as a blanket to hold the 

 moisture down and keep the heat out. When this mulch 

 becomes completely dry, as it often does, during the hot 

 summer, it radiates back large amounts of excessive 

 heat to the atmosphere, thus, we find the effect of this 

 mulch upon the soil as follows: 



It prevents the soil from reaching a high temperature 

 during the day and a low temperature during the night; 

 it greatly warms the soil in the spring; it tends to con- 

 serve moisture in the lower strata and consequently 

 reduces the rate of cooling in the summer. The im- 

 portance of this is very manifest when one considers the 

 temperature necessary for the development of nitric acid. 



Soluble nitrates do not form at a temperature below 

 41 Fahrenheit. The most favorable temperature is 

 between 60 and 85 Fahrenheit. They form very 

 slowly at 115 and at 130 will not form. It has been 

 determined that wheat germinating at a temperature of 

 40 Fahrenheit is more resistant to cold than wheat 

 which germinates at 64. Obviously, the temperature 

 for the greatest formation of nitrates, which are neces- 

 sary for the development of plant food, is too high for 

 the successful growing of wheat to withstand the winter's 

 freezing. This makes it necessary, if the most favorable 

 condition for wheat is to be brought about, to plow the 

 ground and keep the seed bed at a temperature of from 

 60 to 85 Fahrenheit in order that plant food may be 

 developed for the growing of plants which must be 

 grown at a cooler temperature. 



