208 PLANT SOCIOLOGY 



sand 2 to 0.2 mm. ; fine sand 0.2 to 0.02 mm. ; dust or silt 0.02 to 0.002 

 mm.; and fine silt or clay less than 0.002 mm. (2)u). 



The degree of division of the soil determines the most important 

 physical soil properties, such as absorptive capacity, aeration, and 

 water conduction. The activity of the plant roots is also greatly 

 affected by the size of the soil particles. 



The upper limit of capillary water movement and water storage is 

 reached when the particles are 2 mm. in diameter. If the grains are 

 less than 0.02 mm., the root hairs are no longer able to penetrate 

 through the spaces between them, if the soil is of single grain structure. 

 All particles of this size, even quartz grains, assume claylike properties 

 (Ramann, 1928). Between 0.02 and 0.002 mm., electrolytes, partic- 

 ularly the H ions, effect flocculation; OH ions, by contrast, maintain 

 the fine dispersity. 



Coarse, sandy soils are good conductors of water but are also 

 subject to intensive leaching and therefore are poor in nutritive sub- 

 stances. Finely divided raw clay soils are impervious, subject to 

 little leaching, therefore rich in nutritive substances; but root activity 

 is impeded in these tough soils. Raw clay (grain diameter less than 2n) 

 is said to stop the movement even of bacteria. 



Determination of Particle Size: Mechanical Analysis of Soils. — While 

 none of the customary methods can be called precise (see Mitscherlich, 

 1923, pp. 51-58), they are regularly used and give practical results 

 (Fig. 108). 1 



For the segregation of the coarser soil particles the sieve methods 

 can be used (see Ramann, 1911, p. 287; Mitscherhch, 1923; Burgevin, 

 1925). For the determination of stone and gravel content, the soil is 

 dried at 100°C. and then sifted. But air drying is preferable for the 

 analysis of the finer parts of the soil (Burgevin, 1925, p. 480; Russell, 

 1927). 



The medium-fine soil particles may be determined by the elutriation 

 method of Schone, Kopecky, etc., but the sedimentation methods are 

 better because by them the finer particles may also be measured. 

 At present the sedimentation methods are used almost exclusively in 

 many government institutions. They are based upon the principle 

 that the rate of settling of the various soil particles in a mixture depends 

 chiefly upon the size of the grain, the largest particles settling first. 

 There is a definite ratio between the rate of settling and the size of the 

 soil particles. According to Stokes's formula, the size of the particles 

 may be calculated by this method (see Wiegner, 1926; Burgevin, 1925, 



* A full description of the various methods employed in the mechanical analysis 

 of soils has been given by Keen (1931) together with a bibliography of the subject. 



