22 PHYSICAL CONSTITUTION OF SOIL 



to those in which the finest groups largely preponderate. 

 With these differences in physical constitution, the agricultural 

 value of the soils, and their suitability for the growth of 

 different crops are plainly connected. We could hardly have 

 a better illustration of the great influence of physical structure, 

 and of the extent to which this can be revealed by the methods 

 of mechanical analysis. 



The alluvial soils, which cover large areas of the central and 

 western States of America, are not represented in the above 

 table. Ancient alluvial soils, often described as Loess l , have, 

 like their modern representatives (see River Deposit, Table I) } 

 a very simple constitution, the currents of water which brought 

 them to their present resting place having deposited in one 

 spot particles of similar hydraulic value. The Loess soils of 

 Illinois and Nebraska contain according to Whitney (Soils, 

 Bulletin 5, p. 13) from 50 to 70 per cent, of silt, the particles 

 of which are mostly -0105 mm. diameter. The so-called 

 ' Plains Marl ' of the same district in America is a soil of 

 apparently similar origin, but composed of rather coarser 

 particles ; the samples examined by Whitney contained 7 1-75 

 per cent, of very fine sand, diameter of particles '0510 mm. 

 Both these kinds of soil contain but little clay, yet their 

 water-holding power is considerable, and they make good 

 wheat land. We shall see presently (p. 99) that it is in soils 

 of this class that the capillary movement of water takes place 

 to the most beneficial extent. 



By assuming an average diameter for the particles in each 

 of the separated groups, and knowing their specific gravity, 



1 The typical Loess of the Rhine is an extensive deposit apparently formed 

 at the close of the glacial period, when the rivers in the northern hemisphere 

 greatly exceeded their present bounds. The Loess of America has probably 

 a similar origin. 



