The Soil. 49 



where 40 tons of manure were incorporated with the soil to a 

 its maximum. They then pass through a minimum thickness of 

 the atmosphere. At sunrise they are weakened by diffusion over 

 a wide area and in addition are diminished in intensity by ex- 

 cessive atmospheric absorption. The difference in the angle of 

 incidence of the sun's rays is the principal cause of the difference 

 between a tropical climate and that of "Wisconsin. The slopes on 

 our own fields often offer examples of such effects. It is on a 

 slope facing south that the soil will reach its highest temperature 

 during sunshine. 



A dark colored soil becomes warmer in the sun's rays than a 

 light colored one, a larger proportion of the sun's energy being 

 absorbed and converted into heat. No difference will be observed 

 on cloudy days. At night all soils will cool to the same point. 



When a soil is freely exposed to the sky the temperature at the 

 surface will reach a higher maximum and fall to a lower min- 

 imum than the air above it. Schuebler found that the freely ex- 

 posed soil in his garden at Tuebingen, Germany, averaged at 

 one-twelfth inch below the surface, shortly after noon and in per- 

 fectly clear weather, about 120 Fahr. for every month from 

 April to September inclusive, and in July reached 146 ; this 

 latter temperature w r as 65 above that of the air taken at the 

 same time. 



With dry soils, including only hygroscopic water, about 3 cubic 

 feet would be heated by the sun to the same degree as one cubic 

 foot of water. In this condition there is little difference between 

 different soils; a dry peat will consume the least heat and a dry 

 clay the most. When, however, soils become wet great differ- 

 ences appear. In a freshly drained condition, a coarse gravel or 

 sand will warm to a greater depth, while soils retaining more 

 water will warm to a less depth. The specific heat of wet peat 

 does not differ greatly from that of its own bulk of water. 



The depth to which a soil will be heated depends, however, 

 partly on the conductive power of its constituents. Sand has the 

 greatest power of conducting heat of any soil constituent. Air, 



