TYPES OF SOILS 



27 



quite unlike one made from limestone fragments. Such soils 

 may actually need to have lime added to them to enable them 

 to produce good crops. Sandstone rocks are formed of grains 

 of sand cemented together by lime, clay, iron, or silica. When 

 the cementing material is dissolved out by water, a sandy soil 

 is left. Other residual soils may be formed from weathered 

 fragments of the original rock from which little has been car- 

 ried away by water, as in many soils derived from granite rocks. 



FIG. 12. A moving sand dune 



Photograph by A. B. Klugh 



Lacustrine soils. These differ from true residual soils in 

 having been built up in lakes and ponds by an accumulation of 

 plant and animal remains, together with fine particles of rock 

 brought in by rains or blown in by wind. In our Northern states 

 many of what were once shallow lakes have been completely 

 filled in this way and now contain a rich black soil much valued 

 for growing certain crops, such as celery. Such soils, however, 

 often lack some of the minerals needed by plants, and these 

 have to be supplied before good crops can be produced. 



