646 STATE BOARD OF AGRICULTURE. 



times more than 3° F. higlier, than in llie uneultivateil, so that the hitter 

 soil had to gain almost twice the amount of heat in order that its aver- 

 age or even its maximum temperature might equal that of the former. 

 What brought about this gradual gain in heat in the cultivated soil 

 will be explained subseiiuenlly. During all this time, however, that the 

 temperature of the upper depth of the cultivated soil was higher than 

 that of the uncultivated, the temperature of the lower depth or 20 

 inches of both soils was practically the same. As soon, however, as the 

 temperature of the lower depth of the undisturbed soil began to rise 

 higher than that of the disturbed, the temperature of the upper depth 

 of the former also began to rise higher than that of the latter, and con- 

 tinued to be higher until about October. This is a strong indication 

 that the temperature of the lower depths influences somewhat that of 

 the upper depth, and it is probably this factor which causes the unculti- 

 vated soil, at the upper depth, to have a higher temperature than the 

 cultivated soil, at the same depth, during the warmest part of the year. 

 The foregoing description of the behavior of the temperature of the 

 cultivated and uncultivated soils, shows, therefore, that the uncultivated 

 or compact soil is not always the warmer of the two, in the warm part 

 of the year, as is commonly believed, but only during the warmest part 

 of the year, and perhaps under certain conditions during the other 

 warm seasons. The question now is what factor or factors bring about 

 or cause this ditference in temperature. The possible explanations may 

 be found in the following general facts: When a soil is cultivated the 

 area of its surface exposed is by far greater than that of a compacted 

 or undisturbed soil, and of necessity the amount of evaporation in the 

 former is greater than in the latter. As a result of this difference in 

 evaporation the temperature of the cultivated soil does not rise, at the 

 beginning, as high as that in the uncultivated. As soon, however, as a 

 dry mulch is formed on the disturbed soil its loss of moisture by evap- 

 oration is reduced considerably, while the loss in the undisturbed soil is 

 still large and consequently its rise of temperature is small. On the 

 other hand, the heat that is not expended in the evaporation of water 

 is rapidly conducted downward in the case of the uncultivated soil, 

 while in the case of the cultivated soil only part of the heat is conducted 

 downAvard the other part is radiated back to the atmosphere by the dry 

 mulch at the top. This dry mulch forms a very poor and imperfect con- 

 tact with the moist soil below and as a result acts as a blanket, analo- 

 gous to a growing crop or a cover of straw, and does not impart all its 

 heat to the soil below. When this mulch becomes completely dry it 

 gets very hot during the sun insolation, in fact hotter than the air above, 

 and a large amount of this excessive heat is radiated back to the atmos- 

 phere. This fact was proved by the following series of experiments : The 

 bulbs of mercury thermometers were placed one inch above the surface 

 of both the cultivated and uncultivated soils and it was found that the 

 thcrniomeler over the cultivated soil registered at certain days from 9 to 

 18° F. higher than that placed over the uncultivated soil. In another ex- 

 periment the bulbs of the thermometers were placed horizontally over the 

 surface of both soils and it was found also in this case that the surface 

 of the cultivated soil was about lO"^ F. warmer than that of the uncul- 

 tivated soil. In still another experiment the bulbs were simply covered 

 with the very top surface material from both plots and the results ob- 



