58 



SOIL PHYSICS AND MANAGEMENT 



has been used quite extensively to hold the eand. This grows 

 luxuriantly as long as the sand is drifting, but dies and is re- 

 placed by other forms of vegetation as soon as movement ceases. 

 After fixation is accomplished certain varieties of trees may be 



^ 



Pia. 55. Fences being used to check the movement of sand. (U. S. Dept. of Agriculture.) 



planted, transforming these dunes into valuable forest lands. 

 Permanent or fixed dunes may be changed to migratory ones by 

 injudicious management, such as very close grazing, tillage 

 or anything that destroys or removes the protecting vegetation. 

 This h&3 occurred in some western states where close grazing by 

 sheep has destroyed the vegetation so that sand movement has 

 begun. Michigan, Illinois, Wisconsin and Indiana have consid- 

 erable areas of sand dunes. A large part of these areas is covered 

 with a scrubby growth of black oak and other trees, which furnish 

 complete protection. When this growth is removed, however, it 

 is very difficult to hold the sand and it is the part of wisdom to 

 leave even the poor growth of forest for purposes of protection. 

 The dune areas covered with prairie grasses peculiar to the sand 

 present different problems. As a general rule, there is sufficient 

 organic matter in the surface six to eight inches to hold the sand 

 particles. When the soil is cropped or pastured, some of this sur- 

 face soil may be removed by the wind in exposed places, forming 



