96 NATURE AND PROPERTIES OF SOILS 



coarser soils. A few figures are given in Table XIX for some 

 of the average soil classes * established by the Bureau of Soils : 



Table XIX 



APPROXIMATE NUMBER OF PARTICLES TO A GRAM OF VARIOUS SOIL 



CLASSES 2 



An important property of the surface of the grains is the 

 tendency toward the retention of soluble material in a par- 

 tially or wholly available condition for plant use. This power, 

 designated as absorption, is exhibited to a high degree by 

 fine soils, in which the individual pore spaces are small and 

 the amount of surface exposed is large, due to the presence 

 of considerable colloidal matter. This capacity is an especially 

 important factor in the economical use of fertilizer salts. Ab- 

 sorption may also, by bringing materials into closer contact, 

 hasten or retard certain chemical actions. Changes may thus 



*The mechanical analyses of these particular classes are given on 

 page 83. 



"The number of particles in any soil sample may be arrived at from 

 a mechanical analysis and the diameters that limit each group. Using 

 the average diameter of each group together with the percentage of 

 the groups in a given sample, the number of particles may be calculated 

 by the following formula: 



, T . « . . , , M ., Weight of sample in grams 

 Number of particles in a sample of soil = — r fa  



-I/O j^ U X u.Ot) 



The formula 1/6 jr, D 3 is that used for determining the volume of a 

 sphere, the diameter in this case being expressed in centimeters. When 

 multiplied by the average specific gravity of soil particles the weight of 

 an average particle is obtained in grams. In the above calculations, 

 2.7 was used instead of 2.65. 



