SOIL TEXTURE 



71 



by the relative value of the different sizes in determin- 

 ing the properties of the soil and its crop-producing 

 power. It is found that the fine groups exert relatively 

 much more in- 

 fluence, weight for 

 weight, than the 

 coarse ones. There- 

 fore there are more 

 divisions made 

 among the fine than 

 among the coarse 

 particles. 



25. Textural 

 groups. A num- 

 ber of systems of 

 grouping have been 

 devised. The limits 

 of these groups 

 have been deter- 

 mined by the 

 method of analysis 

 used by the investigator and by his judgment of the 

 relative agricultural importance of each group. A 

 further element which limits the number of groups is 

 the practicability of recognizing distinctions in the field 

 based upon them. The following table, from Bulletin 24 

 of the United States Bureau of Soils, exhibits the most 

 generally known of these systems of grouping employed 

 in mechanical analysis. Some of these multiply groups 

 in the small particles, while others give prominence to 

 the sand particles. 



Fig. 18. Silt soil, photomicrograph. Magnified 

 about 110 diameters. Stained so that differences 

 in mineral composition are not so distinguishable 

 as in Fig. 17. The particles have the same char- 

 acteristics as those of fine sand. Some of the 

 smallest particles are of the size of clay. 



