14 SOILS AND FERTILIZERS 



8. Form of Soil Particles, Soil particles are ex- 

 tremely varied in form. When examined with the 

 microscope they show the same diversity as is observed 

 in larger stones. In some soils the particles are spher- 

 ical, while in others they are angular. The shape of 

 the particles is determined by the way in which the 

 soil has been formed, and also by the nature of the 

 rock from which it was produced. 



The form and arrangement of the particles are im- 

 portant factors to consider in dealing with the water 

 content of soils. In the wheat lands of the Red River 

 Valley of the North, the particles are small and spher- 

 ical, being formed largely from limestone rock, while 

 the subsoil of the western prairie regions is composed 

 largely of angular silt particles, which are intermingled 

 with clay, forming a mass containing only a minimum 

 of inter soil spaces. The silt particles being angular 

 and embedded in the clay, the soil has more the char- 

 acter of clay than of silt. While these two soils are 

 unlike in physical composition, the form and arrange- 

 ment of the particles give each nearly the same water- 

 holding power. Two soils may have the same mechan- 

 ical composition and yet possess materially different 

 physical properties because of a difference in the form 

 and arrangement of the soil particles. In some soils 

 10 per cent, of clay is of more agricultural value than 

 in other soils. Ten per cent, of clay associated with 

 60 or 70 per cent, of silt makes a good grain soil, 

 while 10 per cent, of clay associated largely with sand 

 makes a soil poorly suited to grain culture. 



The classification of the soil particles into sand, silt, 



