720 SOILS: PBOPEBTIES AND MANAGEMENT 



of the person by whom the survey was proposed, and the 

 kinds of soils and crops with which he dealt. Some 

 persons have used the vegetation/ especially the native 

 vegetation, as a means of classifying soils. Where this 

 is present it is an excellent means of identifying differences, 

 and pioneers as well as others have always made use of 

 the vegetation growing on a soil to detect variation in 

 its cropping capacities. Unfortunately the vegetation, 

 whether native or introduced, being a result of natural 

 causes, affords information regarding the properties of a 

 soil only when the correlation has been worked out. 

 Further, the native vegetation is now seldom present in 

 well-settled areas, so that it is inadequate as a general 

 means of classification, though very useful for some 

 purposes of comparison. 



618. Factors employed in classification. — In classify- 

 ing soils, four primary and two secondary factors are 

 employed. The former group deals entirely with the 

 soil itself ; the latter group deals with the climate or the 

 situation in which the soil is placed. The situation exerts 

 an influence on the crop value and on the properties of the 

 soil. The factors, beginning with those of the smallest 

 range of occurrence, are as follows : (1) texture, (2) special 

 properties, chiefly chemical, (3) kind of material from 

 which the soil was formed, (4) agency of formation, 

 (5) humidity and precipitation, and (6) normal and 

 mean temperature. 



The soil type is the unit of classification, and may be 

 defined as an area of soil that is essentially alike in all the 

 above characters. 



1 Hilgard, E. W. Soils, Chapters XZIV, XXV, and XXVI. 

 New York, 1906. 



