SOME PHYSICAL PROPERTIES OF THE SOIL 121 



Another important property of the surface of the 

 grains is the tendency toward the retention of soluble 

 material in a partially or wholly available condition for 

 plant use. This power, designated as adsorption, is 

 one exhibited to a high degree by fine soils, in which 

 the individual pore spaces are small and the amount of 

 surface exposed is large. It is an important factor to 

 be observed in the addition to the soil of soluble fertilizing 

 constituents. Adsorption may also, by bringing materials 

 into closer contact, hasten or retard certain chemical 

 actions. Reactions may thus be expected to go on in the 

 soil that would not take place in the laboratory beaker. 

 The relation of this adsorption to bacterial activity also 

 cannot be overlooked. 



The aggregate area presented by soil particles is very 

 large,* even for the coarser soils. With the finer soils, 

 because of the immense number of particles, a figure is 

 reached that is almost beyond comprehension. When 

 the approximate number of particles and their sizes 

 in any given weight of soil are known, the internal surface 

 may be calculated by the following formula : — 



Surface = it D 2 X number of particles 



As the estimation of the number of particles in a soil is 

 so inaccurate, it is evident that a calculation of the 

 surface exposed based on such a figure must be more in 

 error. 



However, to give some idea of the internal surface ex- 

 posed by ordinary soils, the calculations made on a few of 

 the average soil classes of the United States, already 

 presented, 1 are given in the table on the following page. 



1 See Chapter VI, p. 104. 



