36 The Plant World. 



All this amounts to saying that if an un flocculated clay soil is 

 moist enough for the setting up of water-films it will be nearly 

 saturated, and this is obviously true. Every farmer knows that 

 a clay soil must be flocculated to be useful, and this flocculation 

 must result from the forces on the solid-liquid surface, not from 

 the water-films. The later are here inoperative because they 

 can not exist. 



It should be noted in passing that the importance of the 

 forces at the solid-liquid surface is not confined to the visible 

 flocculations to which they give rise. It is obvious that the 

 water-film forces can draw together the solid grains only because 

 the film is firmly attached to them. There must be between the 

 water film and the soil grain an adhesion stronger than any other 

 forces operative in the system. More than this it is impossible 

 to say. The surface forces of the solid-liquid surface are still so 

 little understood that nearer analysis of the phenomena is not 

 now possible. 



It may be well to note also the eff"ect of different mechanical 

 compositions upon the position of the critical moisture content. 

 It is apparent that a soil composed of finer particles has a larger 

 total surface for a given mass of soil. Consequently to set up 

 in this soil mass a water-film system of equal efficiency will re- 

 quire relatively more water. The critical moisture content, 

 when expressed in per cent of water present, will be greater the 

 finer the mechanical composition. In actual fact, experiment 

 has shown that the critical moisture content varies from about 

 three to five per cent (by weight) for sands to thirty or forty per 

 cent for clays. Ordinary loams run from ten to twenty per cent. 

 It is noteworthy that in the finer soils the critical moisture con- 

 tent is less sharp and less well marked than in the coarser — 

 quite what is to be expected, since in these finer soils the water 

 films are relatively inefficient. The forces of the solid-liquid 

 surface (which are then in control) are probably not dependent 

 upon the amount of water present. 



Mechanical resistance. The effects which we have 

 agreed to call mechanical resistance are referable practically 

 in toto to the friction of the soil particles upon each other or upon 

 a foreign body. It may be that the water-film tensions discussed 

 in the last section are of some effect, but they are certainly far 

 less important than the purely frictional forces. It is to be ex- 



