CHAPTER II 



SOIL COMPOSITION, CLASSES AND TYPES 



Common Meanings of the Tarm " SoU.'' — In the previous 

 chapter the term ^^soil" included the total residue resulting from 

 rock weathering — being hundreds of feet deep in some places. In 

 conmion usage the term soil has restricted meanings; viz., it may 

 mean that portion of the ground which is tilled, or that portion 

 which is black or dark in color. 



Subsoil Defined. — ^That portion of the ground below the tilled 

 or dark colored portion is called '^subsoil.'' (Prefix '^sub" means 

 "under.") By some it is considered as that portion below 0i 

 inches deep and 20 inches deep. 



Soil to Mean Tilled Portion. — Soil as it is used in the following 

 chapters is taken to mean that portion of the ground which is tilled. 

 This is the more coiamon meaning since it is that portion which is 

 most important in supplying the needs of crops, and which is most 

 affected by farming operations. 



SubsoU May Differ Widely from Soil. — ^With this deJS.nition of 

 soil in mind, it is easy to see that subsoil may have extreme varia- 

 tions — it may have the same color and composition as the soil or it 

 may show very Httle or no similarity either in color or composition. 



THE COMPOSITION OF SOIL 



The physical or mechanical make-up of dry soil may be briefly 

 expressed in outline form as follows: 



[ 1. Mineral particles of various sizes (derived f (a) Sand 

 Son components from rocks) |(6) Silt^ 



^ 2. Organic matter — mostly plant remains. 



Sand particles are the coarser and heavier soil grains which do 

 not cohere when wet (Fig. 5). Between the fingers they feel rough 

 and gritty. They are the first to settle out of running water carry- 

 ing sediment — ^material washed from the upland. 



Clay particles are the finest of individual soil grains. When 

 moistened they become sticky. They settle out of quiet water very 

 slowly — ^the finest clay particles are so small that they are known 

 to remain in suspension for months, or to be scarcely discernible 

 under a powerful microscope. 



