84 SOILS. 



soils contains the most finely pulverized and most highly de 

 composed portions of the other soil-minerals, and therefore the 

 main part of the available mineral plant-food, it is easy to 

 understand why soils containing a good supply of clay should 

 be called and considered " strong " land by the farmers of all 

 countries. ' Poor" clay soils are exceptional; but sometimes 

 the clay content reaches such a figure that the difficulties of 

 tillage render them too uncertain of production for profitable 

 occupation. 



Amount of Colloidal Cla\ in Soils. Any and all of the kinds 

 of clay mentioned (p. 57) as occurring naturally may, of 

 course, enter into and form part of soils. But as the amount 

 of true, plastic clay subsfmce contained in them is very in- 

 definite, it becomes necessary, in order to classify soils in re- 

 spect to their tillableness, to ascertain more definitely the 

 amount of pure, or nearly pure. o>ll<.idal chy substance con- 

 tained in the several classes of soils ordinarily recognized and 

 mentioned in farming practice. Tint this determination can 

 at best be < mly approximate, is < >hvi< ms fn mi the fact mentioned 

 above ( chapt. 4. p. ;<> ). that pure kaolinite itself is not plastic, 

 and only iK-omu^ so by the indefinite comminution and hydra- 

 tion it experiences in the processes of soil-formation. As the 

 pn>grr-> of thU process i^ aKo indefinite, the same soil con- 

 taining particles ranging from the finest to the chalky scales 

 of pure kaolinite. the drawing of a line must be more or less 

 arbitrary and empirical. 



From numerous experiments and comparisons made, the writer has 

 been led to place the limits of " plastic < lay" at ami l>rl<>\v such grain 

 sizes as will remain suspended 61 tlo.it") in a water column eight inches 

 high, during 24 hours. To go beyond this point in the examination of 

 soils for practical purposes, would render such examinations so labor- 

 ious and hem e so rare, that this kind of work would l>e pra< tically ex- 

 cluded I'M im ordinary practice. According to this view the following 

 percentages of Mich "day" correspond approximately to the designa- 

 tions placed opposite : 



Very sandy soils 5 to ;/.,' clay 



Ordinary sandy lands. . . 3.0(010^ " 



Sandy loams 10.0 to 15^ " 



Clay loams 15.010257,', " 



Clay soils 25.01035';', " 



Heavy day soils .... 35.0 to 45 ',.,' and over. 



