32 FIELD AND LABORATORY STUDIES OF SOILS 



Place a thermometer in each with the bulb covered with 

 soil. Take readings every two hours during the day. 



List the three surfaces in the order of their temperature. 

 What location would you seek for an early garden plot? 



Discussion: The temperature of the soil is affected by 

 the slope, because in one case a given amount of the sun's 

 heat must warm a larger area of soil than in another. 

 When the slope is towards the sun the soil stands more 

 nearly at right angles to the sun's rays, but when the sur- 

 face is level or slopes away from the sun, it is inclined away 

 from the direct rays. The same amount of heat must then 

 warm a larger area. This increase in area may be shown 

 by sawing off a board at right angles and then at an obtuse 

 angle, and measuring the cross-section each time. 



