PHYSICAL COMPOSITION OF SOILS. 



99 



Number of soil grains per gram. It is of some interest to 

 consider the number of grains of different sizes that may be 

 contained in, c. g., a gram of soil. If for this purpose we as- 

 sume all the soil grains to be spherical, \ve shall obtain the 

 minimum figures, for most other shapes will pack more closely. 

 King (Physics of Agriculture, p. 117) calculates such figures 

 for different grain-sizes, assuming the density to be that of 

 quartz (2.65), with the result that while with a diameter of one 

 millimeter (1-25 inch) the number of grains would be 720, 

 and with one-tenth of a mm. 720,000; if made of the finest 

 particles only, viz., one thousandth of a mm., the number 

 would be 720,000 billions. Probably few of the clayey soils 

 we ordinarily deal with are of this order; it is doubtless 

 approached in certain fine plastic clays. 



Surface afforded by various grain-sizes. The amount of 

 surface afforded by a similar amount of soil must naturally be 

 considered in this connection, since upon it depends not only the 

 amount of moisture which the soil may hold in the form of 

 superficial films, but also the extent of surface upon which the 

 weathering agencies as well as the root hairs of plants may act. 

 Quoting again from King's work, we find on the same premises 

 given above for the number of grains, that their surface would 

 in the case of grains of one mm. diameter be eleven square feet 

 per pound (about half a pint) of material; while in the case of 

 the finest grade we should have 110,538 square feet, or more 

 than two and a half acres. 



From actual experiments made with the flow of air through 

 various soils, King calculates that while in ordinary loam soils 

 the total surface is about an acre per cubic foot, in fine clay 

 soils it rises to as much as four acres. If we imagine this 

 large surface to be covered with even a very thin film of water, 

 it is readily seen how large an amount may be present in a cubic 

 foot of moist soil. 



E. A. Mitschcrlich (Bodcnkunds fur I^and-und-I-'orstwirthe ; Berlin, 

 1905) attributes to the surface offered by the soil particles supreme 

 importance in determining the productiveness of soils. According to 

 him the internal soil-surface determines directly the ea^e with which 

 roots can penetrate the soil ; and he proposes the determination of this 

 factor by means of the heat produced in wetting the soil ( ' Benet- 



