56o 



INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



Within all of the classes except No. I, subclasses may be established 

 according to the nature of the most important character limiting 

 use-intensity. Thus, soils that are subject to erosion are placed in 

 an ' erosion subclass ', indicated by adding an ' e ' to the designation, 

 as, for example, ' Class IVe '. Similarly, soils with a problem 

 primarily of water-control are placed in the ' w ' subclass of the 

 appropriate class. Other usual subclasses are of soil-deficiencies 

 and aridity, while salinity adds yet another. 



CLASS YII LA«0 



Fig. i8o. — Illustration of the eight land-use capability classes. (Courtesy of U.S. 

 Soil Conservation Service.) 



The separation of the ditferent land-use capability groups and, 

 particularly, classes, naturally depends on different manifestations 

 and criteria in different regions ; yet to a considerable extent it 

 tends in some circumstances to take account of floristic and vegeta- 

 tional features or effects, and, consequently, to be phytogeographical 

 in basis. For, as we have already observed, the plant life gives us 

 a living expression of the total environment such as nothing else 

 can replace. It even provides us with ready-made experiments on, 

 and something of an interpretation of, what happens beneath the 

 soil surface. Especially where established communities are con- 

 cerned, suitably enlightened study of the plant life not only affords 

 a more exact indication of the combined action of all site factors 



