Conditions Influencing Soil Temperatures. 217 



One thousand heat units would raise the differences in 

 temperatures to 4.2, 5.1 and 39, making it clear that 

 the differences in weight and in water' content greatly in- 

 fluence the degree of warmth. 



262. Influence of Color on Soil Temperature. The color 

 of a soil, especially when dry, so that the rate of evapora- 

 iton from its surface is small, has a marked influence on 

 the temperature, even at considerable depths. Wollny 

 made a series of experiments to note the effect of color, 

 using white marble dust and lampblack in different pro- 

 portions, to secure different shades from light grey to black, 

 in which he placed two thermometers, one with the bulb 

 just beneath the surface and the other 4 inches below. The 

 temperatures were taken every two hours of the 24 and 

 the results are given in the table below, together with those 

 of a similar trial using yellow ocher. 



Table showing the influence of color on the temperature of soil. 



From this table it appears that the darkest soil, whether 

 black or brown, was more than a degree warmer than the 

 light soil at four inches deep ; and that the black soil had 

 a daily variation in temperature at four inches more than 

 3 F. greater than the light soil, and the dark brown soil 

 one of 1.55 F. 



