72 



THE TEXTURE OF THE SOIL 



[chap. 



half an inch in diameter which possess the same volume. 

 Hence it follows that the surface of an ordinary soil 

 must be extremely extensive, and since many of 

 the properties of the soil are dependent upon the surface 

 it becomes important to arrive at some measure of 

 this quantity. By calculation only a very rough idea 

 of its extent can be formed, both because every 

 departure of the soil grains from the spherical form 

 will increase the surface without affecting the weight, 

 and also because the mechanical analysis of a soil 

 gives only a generalised statement of the distribu- 

 tion of soil particles of various sizes in the soil. But 

 the surface of the soil grains in the case of a sandy 

 soil where the grains are all free, may be calculated 

 from the observed rates of flow of fluids like air or 

 water through a measured portion of the sand ; and by 

 using this method King has computed the surface of 

 the constituent particles of various types of soil with 

 the results set out below : 



As a rough figure to remember, the surface of the 

 particles in one cubic foot of an ordinary light loam 

 may be taken as about an acre ; this will increase 

 as the soil approaches more and more to clay, and 

 diminish as the soil becomes increasingly sandy. The 

 extent of surface exposed by the soil particles is im- 

 portant because it is their active part ; other conditions 



