io8 



SOILS. 



millions ; for humus or garden land and woods earth, about 3 

 millions of pounds; for reedy swamp and peaty lands, 2 to 2^ 

 millions. 



The loose tilth and humus-content of the surface soil will in general 

 cause it to weigh less, bulk for bulk, than the underlying subsoil, even 

 when the latter is more clayey ; moreover, the continuous pressure 

 from above will tend to consolidate the subsoil and substrata. Waring- 

 ton (Phys. Properties of Soils, pp. 46, 47) gives interesting data on 

 this point from the Rothamstead fields, as follows : 

 Old pasture, first nine inches ..... 71.3 pounds per cub. ft. 



Same, fourth do. do ..... 102.3 " " " " 



Arable land, first do. do ...... 89.4 " " " " 



Same, fourth do. do ...... 101.4 " " " " 



The influence of humus and unhumified organic 

 matter, as well as of tillage, in diminishing the 

 volume-weight of soils is here strikingly shown. 



Air-space in Natural Soils. The differ- 

 ence between the specific gravity as usually 

 determined, and the volume-weight of soils, 

 is of course caused by the large amount of 

 air contained in them when dry, but which 

 in wetting them is partially or wholly re- 

 placed by water. 



Theoretically, assuming all soil grains 

 to be globular, and packed as closely as 

 possible (in oblique order), the space not 

 filled by them would be the same for all 

 sizes, whether that of marbles, or so min- 

 ute as to be hardly felt between the fingers ; 

 and would be 25.95 P er cen ^ f * ne so ^ 

 volume. 1 If the same globular particles 

 were packed as loosely as possible, i. c., in 

 square instead of oblique order (see figures 

 10 and 11), the vacant space would be 

 47.64 per cent. If however we imagine 

 each sphere to be itself composed of a num- 

 ber of smaller ones, the empty space will 



FIG. 10. Various possible .... . 



arrangements of soil particiei. obviously be greatly increased, to an ex- 



1 King, Physics of Agriculture, p. 116, ff. 



