SOIL MOISTURE 11 



by the soil particles, for it is to these surfaces that the 

 soil water clings, and from them that the plant-food is 

 largely derived. It is naturally very difficult to make 

 this determination accurately, but approximate figures 

 may be given. One pound of the coarsest particles above 

 mentioned would expose an area of about 11 square feet; 

 while one pound of the finest particles would expose 

 about 110,538 square feet or more than 2% acres. The 

 surface of the soil particles in 1 cubic foot of an average 

 soil, lying between the two extremes described above, 

 would be nearly 50,000 square feet. The finer the soil, 

 the larger would be the surface of the soil particles. 

 This immense surface exposed by the particles of agri- 

 cultural soils is of the highest importance in agriculture. 



13. The soil-moisture film. The result of the attrac- 

 tion between water and rocks is that water added to a 

 soil forms a film over the surfaces of the particles. This 

 film is continuous so far as the water goes, covering every 

 particle, bridging every point of contact and filling every 

 minute opening, the diameter of which is not greater 

 than the distance through which the forces of adhesion 

 act. True, in every soil, even in those composed of the 

 smallest particles, when the soil-water film is of maximum 

 thickness, the majority of the soil pores, which are much 

 larger than the distance through which adhesion attrac- 

 tion can act, are open and free from water except as a 

 thin film may cling to their sides. The shape of this film, 

 as it fits accurately over every exposed surface, is a sym- 

 bol of multiplied complexity that completely baffles 

 human description or understanding. 



When a given quantity of water is added to a given 

 weight of soil, the thickness of the resulting soil-mois- 

 ture film depends entirely on the fineness of the particles 



