THE PHYSICAL PROPERTIES OF THE SOIL. 7G0 



TllK COIIKSION OF THK SOIL. 



Cohesion is the force with wiiich soil ]);irti('les jidlieie to one another. 

 As a measure of this we may use tlie resistance wliicli the soil opposes 

 to a separation by tension (relative firmness) or pressure (absolute firm- 

 ness), or to the penetration of a wedge-shaped body (resistance to sep- 

 aration). Cohesion depends on the mechanical constitution and the 

 humidity of the soil as well as on the presence of difleient salts. Of 

 the soil constituents clay possesses the greatest coliesive power, while 

 the particles of quartz, humus, and lime are comparatively weak in this 

 respect. The cohesion of a soil, therefore, increases with the quantity 

 of clay it contains and diminishes as the quantity of quartz, humus, 

 or lime increases. Under otherwise similar conditions the attraction 

 between the soil particles is greatest when the parti(-lcs are smallest. 

 It also depends on their arrangement. Soils with separate grain 

 structure, other conditions being the same, have a higher cohesion than 

 crumbly soils. By pressure the cohesion is increased in ])roportion to 

 the force acting. The influence of water is apparent in the, fact that 

 in clays and soils rich in clay the cohesion is greatest Avhen the quan- 

 tity of water present is least, while in humus, quartz, and chalk the 

 constituents of the soil cohere most strongly when a definite propor- 

 tion of water is present, either more or less than tliis quantity causing 

 a decrease in cohesion. The freezing of water in the soil causes a 

 remarkable increase in the resistance offered to the penetration of a 

 wedge-shaped body (resistance to separation), and also to forces tend- 

 ing to break up the mass (absolute firmness). By mixing caustic lime 

 with clay the firmness of the latter is considerably diminished for all 

 states of moisture, while potassium hydrate or alkaline carbonates 

 increase the cohesion in the dry state and seem to decrease it when 

 the mass is very moist. Other conditions being equal, the resistance 

 to the entrance of wedge-shai)ed bodies is much greater when the soil 

 is covered with plants than when bare.^ 



THE ADHESION OF THE SOIL. 



This property is apparent in the fact that the soil adheres to a greater 

 or less extent to the wood and iron parts of agricultural implements. 

 From the investigations on this subject, it appears that a number of 

 circumstances seem to influence the force which causes adhesion.^ It 

 is proportional to the surface in contact and increases with the amount 

 aud duration of pressure. 



In sandy soils (quartz and calcareous sand) adhesion is greater the 



' W. Schumacher, 1. c, p. 125. G. Schiibler, 1. c, p. 74. F. Haberhindt, Wissen- 

 schaftlich-practische Untersiichuugen auf dem Gebiete des Pflanzenbaues, 1875, vol. 

 1, p. 22; Forsch. Geb. agr. Phys., 1, p. 148. H. Puchner, Ibid., 12, p. 195. 



-G. Schiibler, 1. c, p. 76. F. Haberlandt, Der allgemeine Inudwirthscliaftliclie 

 Pflanzenban, 1879, p. 393. F. Schachbasian, Forsch. Gob. agr. Phys., 13, p. 193. 

 16352— No. 9 2 



