EDAPHIC OR SOIL FACTORS: PHYSICAL 



209 



A. 



so 



30 



20 



3 



p. 457). Numerous apparatus have been constructed for the deter- 

 mination of the rate of settlement of the soil particles. Burgevin 

 (1925) and Russell (1927) enumerate and discuss the advantages and 

 disadvantages of the various devices. For a simple and inexpensive 

 instrument Wiegner's sedimentation apparatus (Schldmmapparat) 

 may be recommended (P'ig. 109) . 



Soil Dispersion and Vegetation. — Many soil prop- 

 erties and the related phenomena of plant life may be 

 explained by the influence of the various grain sizes in 

 the composition of a soil. Mechanical analysis sep- 

 arates not only different physical groups but in most 

 cases also chemically different groups of substances. 

 At present, however, too few mechanical analyses of 

 soils from natural plant communities are on record to 

 give a clear picture of the conditions of interdepend- 

 ence.^ It is known, of course, that a heavy clay soil, 

 without much sand or stone, supports plant communi- 

 ties which are never found upon sandy, gravelly soils 

 (Dcschampsietum mediae in southern France, Antho- 

 ceretum in central Europe), while coarsely divided 

 sandy soils have their characteristic specific plant 

 communities such as Corynephoretum, Ammophiletum, 

 Crucianelletum. Quantitative relations between 

 degree of dispersion and plant communities have not 

 been worked out. 



Rough determinations of degree of dispersion with 

 the aid of the sieve method have been made by Gregor ^ 



Kraus (1911) in the soils of the Bromion erecti alliance ^ }?^- J-^^r^ 



^ ' Sedimentation 



near Wlirzburg. It was shown that the distribution of apparatus of 

 the coarser particles in soils of one and the same plant ^j.^® ^ 'j: ®4'. b' 

 community may be subject to considerable variations, small tube with 

 In the Bromion erecti upon loess Kraus (p. 78) found ^^^*''^^f'^ water; 

 89.2 per cent fine earth below 0.5 mm. but only 28.7 per ber; H, stopcock. 

 cent in calcareous gravel (Kalkschotter) . In the same 

 samples the coarser particles (the so-called soil skeleton), with grain 

 diameters exceeding 5 mm., made up of 10.8 and 71.3 per cent 

 respectively of the soil. 



More significant glimpses into the matter are afforded by the 

 determinations of dispersity made by Gessner and Siegrist (1925) on 

 soils of some of the brookside plant communities of the Aar in 

 Switzerland. 



1 Cf. Gessner and Siegrist (1925) and Braun-Blanquet and Pawlowski (reSI). 



