Studies in vSoil Physics. 37 



pected, therefore, that the ordinary laws of soHd friction will 

 apply to these phenomena. So little is known, however, of the 

 minute mechanical behavior of the soil that the rigid application 

 of the friction laws is scarcely possible and is certainly useless. 

 The only consideration of practical importance is the number 

 of points at which friction occurs. The soil is discontinuous 

 and friction within it can occur only at the points of solid con- 

 tact between particle and particle or between the particles and a 

 foreign body. The more such points of contact the greater the 

 friction. Now the number of such contacts is dependent directly 

 and almost exclusively upon the mechanical composition. The 

 finer the soil, the more particles in a given weight; and the more 

 particles, the more contacts. It is true that the structure will 

 have an influence which is sometimes considerable. The more 

 open the soil structure the fewer contacts (on the average) per 

 particle and consequently the less mechanical resistance. A 

 soil already well tilled and of open structure is easier to plow 

 than one which is closely packed. However, the movement 

 against which the frictions are exerted will itself tend to alter the 

 soil structure, and in any case the influence of structure is far 

 exceeded by that of mechanical composition. It is only in the 

 heavier clav soils that the decrease of contact points due to opener 

 structure is sufficient to have a marked effect upon the mechan- 

 ical resistance. * 



Organic matter. In all of the above there has been no 

 mention of the third soil constituent — the organic matter. This 

 omission is not only intentional but necessary. Practically 

 nothing is known concerning the nature of the solid organic 

 matter in the soil or its eff"ects upon physical condition. It is 

 known only (and that purely empirically) that the presence of 

 organic matter serves to loosen and make less heavy the heavier 

 clay soils. This effect is probably produced through an influ- 

 ence on the forces at the solid-liquid surface and the flocculations 

 to which they give rise. The mechanism is unknown. 



♦This is another kind of mechanical resistance which is"of some importance and is not 

 strictly frictional in character. This is the interlocking effect of the grains. It may 



happen that two soil grains are in such mutual positions that one can not move over 

 the other in the direction required by the applied force without the shearing off of frag- 

 ments from one particle or the other. The projections from the surfaces of the particles 

 are interlocked. In most cases when this happens the stress is relieved by movements 



in other directions or by movements of other particles Only when the soil particles 



are very angular and very completely interlocked does resistance of this kind become 

 practi<.ally important. 



