98 SOILS: PBOPUBTim AND MANAGEMENT 



73. Physical character of the separates. — It is im- 

 mediately apparent that as these groups vary in size they 

 must exhibit properties, especially physical ones, which 

 are widely different. These properties should in turn 

 be imparted to the soil of which the separates form a 

 part. If we are conversant with these various values, a 

 mechanical analysis should reveal to us at a glance cer- 

 tain soil conditions which may or may not be conducive 

 to the best plant growth. 



The clay particles are very minute ; many of them are 

 so small as to be invisible under the ultramicroscope. 

 They are really shreds and fragments of minerals, and 

 are jagged and angular in outline. They are highly 

 plastic, and when rubbed together they become sticky 

 and impervious. They shrink much on drying, with the 

 absorption of considerable heat. On being wet again 

 they swell with the evolution of the heat already taken 

 up. Many of the particles exhibitthe Brownian move- 

 ment and will remain in suspension for an indefinite 

 period. The finer part of the clay makes up a portion of 

 that indefinite group of material in the soil called colloids, 

 which because of their fineness of division (molecular 

 complexes) exhibit certain well-defined properties, of 

 which adsorption of moisture and salts in solution, and 

 high plasticity and cohesion, are the most important 

 from a soil standpoint. Silt exhibits the same qualities 

 as clay, but to a much less marked extent. The presence 

 of clay in a soil imparts to it a heavy texture, with a 

 tendency to slow water and air movement. Such a soil 

 is highly plastic, but becomes sticky when too wet and 

 hard and cloddy when too dry. The expansion and the 

 contraction on wetting and drying are very great. The 

 water-holding capacity of a clay soil is high. 



