52 LAND DRAINAGE 



Such attempts at lump reduction are usually only indif- 

 ferently successful, and the degree of failure is often 

 indicated by the number of lumps lying upon the surface 

 of the field. The writer has found that at 10 : 00 A.M. 

 on a sunshiny morning in late June, the temperature of a 

 lump-covered soil was 6 F. lower than that of a similar 

 near-by soil in proper tilth, at a depth two inches below 

 the surface in each case. Fig. 16 illustrates how the sun's 

 rays are intercepted as they approach the lump-covered 

 surface and, in considerable degree, reflected and radiated 

 back into the air. 



75. The relations of capillary water summarized. 

 Capillary water is helpful in agriculture. It performs a 

 variety of functions : 



A solvent of foods ; 

 A carrier of foods, 



To the plant, 



Through the plant ; 

 A food for the plant ; 

 A factor in soil structure ; 

 Assists in the preparation of foods ; 

 Affects temperature of soils ; 

 Affects ventilation of soils. 



In excess : 



It may affect temperature unfavorably ; 

 It is likely to affect ventilation unfavorably. 



76. The relations of gravitational water summarized. 

 Three helpful functions of gravitational water are : 



To replenish the capillary supply ; 

 To assist in soil ventilation (in quick passages down- 

 ward through the soil) ; 



