172 RELATIONS OF SOIL TO HEAT 



will penetrate to a comparatively small depth. On the other 

 hand, a consolidated, stony soil, especially when moist, will 

 prove the best conductor of heat. In such a soil the heat 

 gained from the sun will be most evenly distributed, and will 

 penetrate to the greatest depth. Practical experience has 

 shown that a gravelly soil, having a good aspect, is one 

 especially suited for the production of early garden crops ; 

 this is probably due to the rapidity with which such a soil 

 is warmed in spring time. The favourable properties of soils 

 of this character would probably be improved by a top 

 dressing of soot. Such soils provide also some protection 

 from early morning frosts, their good conduction of heat 

 serving then to warm the surface, as during the day it had 

 proved effective in warming the subsoil. 



The operations of tillage have a very sensible effect on the 

 propagation of heat through the soil. If the surface soil is 

 brought into a loose pulverulent condition, the passage of 

 heat downwards is retarded. If, on the other hand, the 

 surface is rolled after cultivation, the temperature of the soil 

 beneath the surface is distinctly increased. These results 

 might have been predicted from the facts shown in the ex- 

 periments described above ; they have, however, been confirmed 

 by actual observations in the field made by King in Wis- 

 consin. 



In experiments made in six counties, and on soils of various 

 character, he found (Wisconsin Jth Report, 120) that the 

 temperature of rolled land at a depth of i i inch was i-9 F., 

 and at a depth of 3 inches i-6F., higher than that of 

 similar unrolled soil. The average excess of temperature 

 by rolling was 3 F. The temperatures were all taken 

 between i and 4 p.m. 



