6 PHYSICAL CONSTITUTION OF SOIL 



their fertility, and which demand in each case a distinct 

 treatment by the farmer, are primarily due to the differences 

 in size of their respective particles. 



From the time of Schiibler down to the present day the 

 soils most commonly occurring have been described and 

 classified as if they consisted essentially of various proportions 

 of two constituents, one composed of coarse, the other of fine 

 particles, namely sand, and clay. His definitions of clay, 

 loam, and sandy soils (Schiibler, Grundsdtze der Ayricultur- 

 Ckemie, 1838 ; also Jour. Roy. Agri. Soc. 1842, 156) were as 

 follows : 



In modern Textbooks the proportions of sand and clay 

 assigned to the various soils named above often differ a good 

 deal from those given by Schiibler, but in nearly every case 

 the principle of classification is the same, the soils being 

 always assumed to be composed of varying proportions of 

 two constituents. 



This simple view of the constitution of soil is unfortunately 

 both inaccurate and impracticable. Soils do not consist of 

 particles of two sizes ; they consist of particles exhibiting 

 a very wide range of size, and the preponderating groups of 

 particles in different soils occur in very different parts of this 

 series. The separation of the constituents of a soil into two 

 groups sand and clay is also impracticable, as there is no 



1 When the soil contains lime or humus the proportion of sand will be 

 correspondingly diminished. 



