THE SOIL PARTICLE 105 



It is evident that a mechanical analysis of a soil is 

 nothing more or less than an expression of class, and 

 the inferences that may be derived from either are 

 the same. This leads to a consideration of class deter- 

 mination. 



78. Determination of class. — The common method 

 of class determination is that employed in the field. It 

 consists in examination of the soil as to color, an estima- 

 tion of its humus content, and, especially, a testing of 

 the " feel " of the soil. Probably as much can be judged 

 as to the texture and class of a soil by merely rubbing it 

 between the thumb and the fingers as by any other 

 superficial method. This is a method used in all field 

 operations, especially in soil survey work. The accuracy 

 of the determination depends largely on experience. 

 Inaccuracies are likely to occur in distinguishing between 

 the various finer grades of soil; for this reason more 

 nearly exact methods are necessary at times, especially 

 in checking soil survey work or in carrying out investiga- 

 tions in which absolute accuracy is required. 



% As a mechanical analysis of a soil is really a percentage 

 expression of texture, it presents an exact method for 

 class determination. For detailed work somewhat com- 

 plicated tables * have been arranged ; but the following 

 diagram (Fig. 16), devised by Whitney, 2 presents a simple 

 method for the identification of a soil from a mechanical 

 analysis. The convenience of this triangular representa- 

 tion may be tested by the use of the average analyses, 

 already presented on a previous page. 



J Bur. of Soils, Soil Survey Field Book, p. 17, U. S. D. A., 

 Bur. Soils. 1906. Also, Bur. Soils, Bui. 78, p. 12. 1911. 



2 Whitney, M. The Use of Soils East of the Great Plains 

 Region. U. S. D. A., Bur. Soils, Bui. 78, p. 13. 1911. 



