THE SOIL PARTICLE 87 



While the principles of series identification are too com- 

 plicated to be expanded farther at this time, enough has been 

 said to establish the importance of accurate soil classification. 

 Unless soils are accurately named in soil survey work, the map 

 and its accompanying report are useless. 



Soil texture and class are thus the basis for practical soil 

 study, whether regarding some particular property or a gen- 

 eral condition, such as crop adaptation. No matter what the 

 phase of soil study may be, texture and class are sure to have 

 some important influence and must be considered in the in- 

 vestigation. 



47. Soil structure. — While texture is of great importance 

 in determining the general characteristics of a soil, it is evi- 

 dent that the arrangement as well as the size of the particles 

 must exert some influence. The term structure is used to refer 

 to this arrangement or grouping. It is at once apparent that 

 soil conditions — such, for example, as air and water move- 

 ment, heat transference, and the like — will be as much affected 

 by structure as by texture. As a matter of fact, the great 

 changes wrought by the farmer in making his soil better 

 suited as a foothold for plants are structural rather than 

 changes in texture. The compacting of a light soil or the 

 loosening of a heavy one is merely a change in the arrange- 

 ment of the soil grains and in the condition and nature of the 

 colloidal complexes 1 thereof. 



From the standpoint of size and arrangement of particles 

 there are really two classes of soils, those of single grain struc- 

 ture and those which are complex, the particles both large 

 and small being bound together by indefinite colloidal com- 

 plexes. The former condition is of course best exemplified 

 by a sand. Such a soil is loose and open with large individual 

 pore spaces and ready circulation of air and water. The com- 



1 Material in a colloidal state has a great deal to do with all soil 

 phenomena. Its characteristics and influence must be kept constantly in 

 mind in soil study. 



