PHYSICAL INTER-RELATIONS IN SOILS 43 



63. Influence of soil structure on temperature. In 

 comparing a soil of good physical structure with one of 

 poor structure, the following facts are offered in favor of 

 the former : it is not subject to such extremes of tem- 

 perature; it does not cool so rapidly by radiation; it 

 receives more heat from the sun than does a lumpy soil 

 (see Fig. 16) ; in weather of average rainfall less water 

 evaporates from the soil of good structure, and therefore it 

 is able to utilize more of the sun's heat ; it is the warmer soil. 



INFLUENCE OF GRAVITATIONAL WATER ON OTHER PHYSICAL 

 CONDITIONS 



Mention has already been made of gravitational water, 

 its nature and movements. Its relations to other factors 

 in agriculture and its own individual functions now call 

 for specific attention. In three respects, gravitational 

 water plays a very important part in crop production. 

 In several particulars its extended presence is undesirable, 

 for reasons set forth in this chapter. 



64. A replenisher of capillary water. The water- 

 table is the surface of the water fully occupying the pore 

 space in the soil. It is sometimes spoken of as the surface 

 of the standing water in the soil. It determines the sur- 

 face of the water in the well. While water will rise in a 

 soil to a considerable height above the water-table by 

 capillarity, the rate of rise and the height to which it will 

 rise are not sufficient to supply the ordinary crops in an 

 average soil. The capillary supply, therefore, must be 

 replenished in another way. It is accomplished by the 

 passage into, or through the soils, of gravitational water. 

 When a rain occurs, the water therefrom passes down into 

 the soil as gravitational water. If the quantity is rela- 



