64 CARNEGIE INSTITUTION OF WASHINGTON. 



determining soil temperature and to evaluate their importance with reference 

 to plants and animals. 



The influence of the color of the soil surface was investigated at Tucson in 

 the spring of 1923. Three soil thermographs were installed, 4 feet from each 

 other, at a depth of 3 inches in a brown clay of basaltic origin. A preliminary 

 run of two weeks showed a correspondence of the maxima of the three instru- 

 ments with 1° and of the minima within 2°. An area 4 feet square above one 

 of the bulbs was then blackened by application of drop black dissolved in 

 turpentine, this giving a uniform matt black to the soil and small rock 

 particles. Over the second bulb the soil was covered with a very thin layer 

 of crushed mortar, so as to secure a uniform white without any minute 

 reflecting surfaces. The soil over the third bulb was left in its natural light- 

 brown color. The four succeeding weeks were clear, with the exception of 

 two partly cloudy days, and there was no disturbance of the artificial 

 surfaces in this time. During the first four weeks the average maximum of the 

 black soil was 2.8° above that of the control, and the average maximum of 

 the white soil was 7.7° below that of the control, meaning a difference of 

 10.5° between the black and white soils. The average minima, as would be 

 expected, were nearly identical. While these results are what would be 

 expected from physical theory, they are of interest as showing that the 

 superficial layers of natural desert soils under constant insolation are only 

 about 3° cooler than under the conditions of surface color that give the 

 maximum absorption of heat rays. The difference between the superficial 

 temperature of natural soils that are nearly white and those that are very 

 dark would never be greater, at this latitude, than the difference of 10.5° 

 found in this experiment. 



At the Coastal Laboratory, in the summer of 1923, an investigation was 

 begun on the influence of soil moisture as affecting the temperature of the soil. 

 Three soil thermographs were installed in adjacent plots with the bulbs at a 

 depth of 6 inches. The soil was a fine sand with considerable organic matter 

 near the surface and a compact structure below, with a small admixture of 

 clay. The surface was kept bare. After a preliminary run to assure identity 

 of readings, one of the plots was given one weekly sprinkling equivalent to 

 0.5 inch of rain, the second plot the equivalent of 0.25 inch of rain, and the 

 third left as a control. Soil-moisture samples were taken weekly at 6 inches, 

 just before each wetting. The results show that the cooling due to the 

 increased soil moisture was within 1° to 2° of the same for the two amounts 

 of simulated rainfall. The lowering of 5° to 6° below the control lasted for 

 only 48 hours after wetting, affecting maxima and minima alike. Thereafter 

 for three days the moistened soils were only 2° to 4° below the dry soil. On 

 the last two days of each week the temperatures in the three plots were the 

 same, and the moisture of the irrigated plots had fallen to that of the control. 

 The small effect of increased moisture and its brief duration are probably 

 typical of all soils of light texture, from which evaporation is active and in 

 which percolation is great. It is planned to extend the work to other types 

 of soil, both at Carmel and Tucson. 



