Plants in Relation to Man 473 



conditions must be considered, as they are vital to agricultural 

 practice. 



Exercise. The temperature of a soil may be found by a ther- 

 mometer left for a while in a hole made by driving a sharp-pointed 

 stick into the ground. The reading shows very closely the tempera- 

 ture at given depths. Select different kinds of soil, and soils in differ- 

 ent positions with reference to the sun. Tabulate carefully the results 

 of your observations, and make use of them in consideration of the 

 conditions of moisture, topography, surface color, surface character, 

 and kind of soil, which determine the temperature. 



Soil temperature and moisture. Moisture and soil tempera- 

 ture are closely related, as may be inferred from the common 

 observation of gardeners that a soil is cold or warm. By this 

 it is meant that a cold soil has a large capacity for moisture 

 and holds it long ; such a soil is usually of fine texture, as clay 

 or clay loam. A warm soil has less moisture capacity, better 

 air-circulation and air-capacity, and is usually of a sandy 

 character. 



Water is an important factor in a soil's temperature for two 

 reasons. First, water has a high heat capacity (page 142). It 

 is warmed slowly, absorbing a large amount of heat without 

 becoming sensibly warmer; and it gives up the heat it may 

 contain just as slowly. Second, water is being constantly 

 evaporated, and evaporation utilizes heat (page 306), thus hold- 

 ing down or reducing the temperature of the body giving up 

 moisture. The heat utilized thus is taken away as insensible 

 or hidden heat (page 305). The body giving moisture is kept 

 cooler. 



Exercise. Apply these facts to any given soil temperature. 

 Suppose an area of soil receives directly the sun's rays, but gives up 

 freely a large amount of soil water through evaporation. What will 

 be its temperature character? Suppose there is a long, hot, dry spell. 

 What will be the result as regards the moisture retained? How will 

 this affect the soil temperature ? Does soil itself retain heat well or 

 long? Does the observed fact indicate that the uniformity of a soil's 

 temperature depends upon its moisture capacity and retention? 



