CHAPTER VII 



EDAPHIC OR SOIL FACTORS: PHYSICAL 



1. TEXTURE OF THE SOIL 



The colloid chemist deals chiefly with finely divided systems and 

 their smallest particles: the microns, molecules, and ions. On the 

 other hand, the physical study of soils has to do mainly with the 

 coarser particles. There is, however, no fundamental distinction 

 between the two fields of research. 



The importance of the degree of division of the soil particles for 

 plant life has long been recognized. Thurmann (1849) expressed it 

 clearly in his classification of soils. His pelitic soils correspond to 



flaw Clay 



Infernafi'onal Method (Afferberg) 



0.-.. . J Medium Coarse _ ., , 



Sirr Fine Sana Sand fSand^Qrrr orayel 





TTanv 



Zfi 



l^ 



If^A^ 



+.■+■+■.+!■ 





^ 



Sfi. 20(1 60fL 200(i250fi 500(1 tmm 2mm, Smm. 



United States Burecm of Soils Metfioci 



FiQ. 108. — Classification of soil particles on the basis of size. 



finely divided clay soils; his psammitic soils to the sandy, coarsely 

 divided ones. Schlosing proposed a classification based upon the 

 quantitative determination of the amount of the various sizes of grains 

 in the soil. The division of soil particles according to size-classes, 

 adopted officially in France in 1898 and in Great Britain in 1928, was 

 based upon the work of Schlosing. Slightly different size-classes are 

 used in North America, but it is hoped that the classification proposed 

 by Atterberg, and internationally adopted in 1914, will lead shortly to a 

 unification of the methods of classification (Fig. 108). 



Soil Granulation. — The diameters of the grains of the various 

 classes according to the international method (Atterberg, 1912) are: 

 coarse gravel (rubble) more than 20 mm.; gravel 20 to 2 mm.; coarse 



207 



