r,10 STATE BOARD OF AGRICULTURE. 



tivity plavs iu the control of temperature of the different soils under 

 natural field conditions. This is indeed a very difficult question to 

 answer definitely and satisfactorily for the reason that these soils have 

 different water holding capacities, consequently the great influence that 

 this inequality of water content has upon the travel as well as upon 

 the rise and fall of temperature. 



It is commonly believed, however, that heat conductivity has a very 

 important and beneficial influence because the soils with the greater heat 

 conducting power are generally warmer both at the upi)er surface and 

 at lower de])ths due to the fact that llie heat travels to greater depths 

 during the sun insolation and during the night or cold periods, this 

 heat returns to the surface and thus keeps it warmer. In the case of 

 soils of ])oor thermal transmitting power only their upjier surface will 

 be heated during the sun insolation and during the night this heat will 

 be radiated into space and the soils are left cooler. 



Theoretically this argument is sound, but in nature, as shown by the 

 results of the different types of soil, later to be presented, it does not 

 seem to be borne out. It will be found that in the summer, fall, and 

 winter months, the average temperature of all the different types of 

 soil is about the same with a small difference in favor of the loam, clay, 

 and peat respectively. In the spring months, however, the temperature 

 of the sand and gi'avel began to rise sooner than that of the latter 

 soils. This, however, may be attributed largely to the different amount 

 of heat that is required to warm a cubic foot of the various types of 

 soil to the same degree on account of their different water content. 

 The sameness in temperature of these soils in the fall is also largely 

 due to their different moisture content or specific heats as will be ex- 

 plained. 



Everything considered it does not appear that heat conductivity alone 

 ])lays a very important part in the warming and cooling of the soils in 

 the spring and fall respectively, which seasons from the agricultural 

 or practical standpoint are the most important. The force of this state- 

 ment will be evident when we come to consider the results of the field 

 experiments. Even the heat conductivity between cultivated and un- 

 cultivated soil does not appear to be of such a great practical impor- 

 tance. The difference in temperature that is observed in these differently 

 managed soils is mostly due to the dry mulch at the surface of the culti- 

 vated soil. 



Where heat conductivity may be of great importance and of practical 

 benefit is in the spring daylight hourly march of soil temperature. If 

 the temperature of a sandy soil at a depth of 6 inches begins to rise two 

 or three hours earlier every day than that of a clay soil, the plants 

 grown in the former soil will have that number of hours of more favor- 

 able daylight growth, at which time the carbon assimilation and other 

 activities are at their maximum. 



