770 EXPERIMENT STATION KECORD. 



smaller the ])arti('les; in powdery soils (separate jirain strncture) adhe- 

 siou is considerably lHi;lier than in the same soil in a crumbly condition. 

 Of the different soil constituents, clay shows the strongest adhesion to 

 wood and iron and liumus the weakest, while quartz is intermediate. 

 The soil, thcreibre, adheres to the imjdements most strongly when it is 

 rich in clay and less strongly when quartz and humus predominate. 

 Tiie influence of water vaiies in differently constituted soils. In sandy 

 soils adliesion increases with an increase of Avater up to complete sat- 

 uration. In soils rich in clay adhesion to wood and iron reaches a 

 maximum at a definite humidity (SO per cent of the quantity required 

 for saturation), below and above which adhesion decreases. Humus 

 seems to act lilie quartz, in respect to the influence of moisture. In the 

 dry state, adhesion in jill kinds of soils is zero. 



The addition of caustic lime has no appreciable effect on tlie adhesion 

 of soil to agricultural implements, but caustic alkalies seem to cause a 

 firmer adhesion of clay. 



Clayey soils adhere to wood more firmly than to iron, while with soils 

 rich in quartz and humus the reverse is true. Adhesion of soils to rusty 

 iron is almost invariably less than to i^olished iron, and in case of soils 

 rich in clay, less than to wood. Only in soils containing much (juartz 

 are the adhesive values greater for rusty iron tlian for wood. 



THE I-UICTIOX OF SOILS OX WOOD AN1> IKON. 



The resistance which is offered by friction to the sliding of imple- 

 ments over the soil is expressed by the ratio of the foice which is neces- 

 sary to overcome the resistance to friction to the weight of the moving 

 or sliding body (coetticient of friction). This coefficient varies, accord- 

 ing to the nature of the materials concerned, between 0.33 and 0.8U. 

 Other properties being the same, tiue-grained soils show a greater resist- 

 ance when moist than when dry. If the soil is moist, the resistance 

 offered to wooden and iron implements is greater when the soil has the 

 separate grain structure than when it is crumbly, but if the earth is 

 dry the reverse is true. As regards the behavior of the various soil 

 constituents, experiments' show that the coeflicient of friction is great- 

 est in quartz sand or calcareous sand, and smallest in clay, while humus 

 possesses an intermediate value. The coefficient of friction diminishes 

 with the quantity of moisture in the soil, and is greater for wood than 

 for iron. Eust increases considerably the coetticient of friction for iron. 



THE SPECIl'lC GKAVITY OF THE SOIL. 



This is the ratio of the weight of the soil to that of an equal volume 

 of Avater considered as unity. For different soil constituents the fol- 

 lowing values have been found (E. Wollny): 



IF. Schachbasian, Forscb. Geb. agr. Pbys., 13, p. 214. 



