THE SOIL PARTICLE 95 



Table XVIII 



PERCENTAGE PORE SPACE IN SOILS OP DIFFERENT TEXTURE 



Sandy soil 32 . 5 



Loam 34 . 5 



Heavy loam 44. 1 



Loamy clay 45.3 



Clayey loam 47 . 1 



Clay 48.0 



Heavy clay 52 . 9 



The pore space in a normal soil is occupied by water and 

 air. If the water content is low, the air space is large, and 

 vice versa. Thus the relationships of the total pore space 

 and the size of the individual spaces to the amount of air and 

 water contained, to their movement through the soil, to soil 

 sanitation, to root extension, to bacterial action, and to crop- 

 ping conditions in general, become apparent. It is the regu- 

 lation of this pore space that is really important in any struc- 

 tural consideration. The effect on plant growth of a change 

 of pore space is the only test of its advisability. 



52. Soil particles — ^their number and surface exposed. 

 — Since soil particles run to extremely small diameters, the 

 number in any given volume is very large, especially when 

 fine-testured soils are considered. However, any calculation 

 of the number of particles present in a soil is open to great 

 inaccuracy j j&rst, because it is impossible to get a correct fig- 

 ure for the average diameter of the particles of any soil or of 

 the various groups of separates that go to make it up ; and, 

 secondly, because it must be assumed in the calculation that 

 the particles are spherical. The presence of colloidal matter, 

 especially in the heavier soil types, introduces an error the 

 magnitude of which must be very great. Nevertheless, such 

 a calculation, even if very inaccurate, gives some idea as to 

 the immense number of grains that are present even in the 



