38 BULLETIN 140, U. S. DEPARTMENT OF AGRICULTURE. 



The film of moisture which surrounds every soil particle up to the 

 point where saturation begins varies in thickness according to the 

 amount of water contained at any particular moment by a given vol- 

 ume of the soil. The soil-film moisture is removed, not by drainage, 

 but only by transpiration through growing plants and by evapora- 

 tion. As the last factor can be held under control to a considerable 

 extent by the dust mulch system of crop cultivation, or by artificial 

 mulches, the amount of soil moisture available to growing plants and 

 trees depends upon the film moisture contained in the soil, and the 

 amount of this depends in the first analysis upon the texture of the 

 subsoil and to a lesser degree upon that of the surface soil. As every 

 soil particle is surrounded by a film of moisture, it follows that the 

 finer the soil the greater is the number of films, and likewise the 

 greater the area or amount of moisture in a given volume of soil. 



Whitney 1 found that the surface area of the soil particles in a 

 cubic foot of the subsoil in the pine barrens land was about 24,000 

 square feet, in silty and fine sandy river terrace subsoils the area was 

 100,000 square feet or 2 t 3 q acres, and in the much more clayey lime- 

 stone subsoils 200,000 square feet. In commenting on this data, 

 Wilej^ 2 states: 



This great extent of surface and surface attraction gives the soil great power 

 to absorb moisture, and thus the soluble mineral ingredients, of which most 

 soils contain only a little, are held too closely to allow of rapid loss by drainage, 

 and still sufficiently available to answer the needs of vegetation, provided the 

 store is large enough. 



And again: 



The porosity of a soil depends upon the size of the soil particles (texture), 

 the way in which these particles are grouped together (structure), and upon 

 the space between the particles or groups of particles. If a soil be cemented 

 together into a homogeneous mass, its porosity sinks to a minimum ; if it be 

 composed, however, of numerous fine particles, each preserving its own physical 

 condition, the porosity of the soil will rise to a maximum. The porosity of a 

 soil may be judged very closely by the percentage of fine particles it yields on 

 mechanical analysis. A finely divided soil has a high capacity for absorbing 

 moisture and holding it. The adaptation of a soil to different crops depends 

 largely on the sizes of the particles composing it. 



This is illustrated in the case of a certain soil containing about 30 

 per cent of clay, " which is strong enough and sufficiently retentive 

 of moisture to make good grass land, but too close in texture and too 

 retentive of moisture for the production of a high grade tobacco or 

 to be profitable for market vegetables." 



Cameron and Gallagher 3 found that the optimum moisture con- 

 tent — i. e., the particular content at which a given soil can be put into 



1 Whitney, Md. Agr. Expt. Sta., 4th Ann. Rept, p. 282. 



2 Agricultural Analysis, pp. 131—132. 



3 Bui. 50, Bureau of Soils, U. S. Dept. of Agr. 



