762 EXPERIMENT STATION RECORD. 



THE FORM OF THE SOIL TARTICLES. 



This is generally very variable. The finest particles of clay are long 

 and rounded, and ratlier uniform in shape; in sands the grains are 

 frequently more or less cubical. Most of the other soil elements are of 

 angular and irregular form. 



Molecular constitution of the soil particles. — All particles of mineral 

 origin are either crystalline or amorphous. In the former, and to a 

 certain extent in the latter case, they are nearly or quite impermeable 

 to water and gases. Some of the amorphous constituents have the 

 properties common to colloid substances of swelling up in presence of 

 water and forming a sort of jelly, and of returning to their original con- 

 dition on drying. Examples of such colloid substances are the jilastic 

 constituents of clay, the amorphous zeolitic silicates, ferric oxid, silicic 

 acid in certain conditions, and humus. These materials, although to a 

 considerable extent hypothetical, are of great significance in the study 

 of the physical and chemical properties of the soil. Upon them depends 

 the plasticity of clay soils. Decaying plant remains render soils more 

 porous and hence more permeable to gases and liquids. 



Determination of the .y/,ce of the particles {mechanical analysis), and of 

 the soil constituents. — Mechanical analysis of soils, by means of sieves, 

 or by elutriation, consists in the separation of the soil into portions of 

 varying coarseness, and the quantitative determination of the propor- 

 tions of these different grades. In this way the soil may be separated 

 into the following products: 



Grades ohtained in meclKiiiicdl analysis of soils. 



(1) Over 10 mm. (Uaiufter stones. 



(2) 10 to 5 mm. diameter coarse gravel. 



(3) 5 to 2 mm. diameter medium gravel. 



Soilskeleton . . , ^^^ 2 to 1 mm. diameter fine gravel. 



(.5) 1 to 0.5 mm. diameter coarse sand. 



L. (6) 0.5 to 0.25 mm. diameter medium sand. 



(7) 0.25 to 0.1 mm. diameter fine sand. 



(8) 0.1 to 0.05 mm. diameter coarse silt. 



Fine earth { (9) 0.05 to 0.025 mm. diameter medium fine silt. 



I (10) 0.025 to 0.005 mm. diameter fine silt. 



[ (11) 0.005 to 0.0001 mm. diameter colloid clay. 



Grades 1 to 5 are separated by sieves with circular holes, grades 

 7 to 11 by elutriation. 



In preparing a sami>le of soil for analysis care must be taken that 

 the coarser materials are freed from finer particles, and that all lumps 

 are broken up. This is effected by boiling for 10 to 20 hours with dis- 

 tilled water.^ The sample so treated is washed through a sieve with 

 round holes by means of distilled water, using a brush if necessary. 



Elutriation may be carried out in two ways. One of these depends 

 on the subsidence of solid bodies in water and may be called the decan- 



>E. W. Hilgard, Forsch. Geb. agr. Phys., 2, p. 57. 



