78 SOILS AND FERTILIZERS 



Windbreaks are effective in decreasing evaporation by 

 lessening the velocity of the wind. King found that evapora- 

 tion from a moist soil was twenty-four percent less at a dis- 

 tance of twenty to sixty feet from a black oak grove than it 

 was about three hundred feet distant. 



93. Rolling and subsurface packing. — These operations 

 are resorted to in order to bring moisture to the surface or 

 upper layer of soil. Rolling compacts the superficial layer 

 of soil and thus establishes capillary connection with the 

 moist soil below. This may be desirable in order to bring 

 moisture in contact with seeds, but although germination 

 is hastened loss of moisture results. 



Subsurface packing is designed to make more compact a 

 naturally loose soil by running wedge-rimmed wheels through 

 it. If the soil is too loose for capillary movement of water 

 to proceed effectively, this operation promotes it. Its use 

 is confined to arid or semi-arid regions. 



94. Removal of water by drainage. — Land drainage is 

 any condition, natural or artificial, that enables the surplus 

 water to escape from soils. A soil may be highly productive 

 when drained, but worthless before draining. This is but 

 another illustration of the many factors affecting soil pro- 

 ductiveness. Where natural drainage is poor, artificial 

 drainage is general^ a profitable investment. It may be 

 accomplished either by surface ditches or by underground 

 drains. 



95. Benefits of drainage. — There are many ways in 

 which good drainage benefits soils and crops. The need of 

 drainage may be very evident in the yellow color and poor 

 growth of young, plants, or it may be less readily detected, and 

 yet may be sufficiently needed to make it a profitable invest- 

 ment. Good drainage is the first requisite in enabling a soil 

 to reach its maximum productiveness. The principal ways 

 in which drainage benefits the soil and crop are as follows : 



