SOME FBTSICAL FROPBUTim OF THM SOIL 117 



The pore space in any of these soils is, of course, subject 

 to considerable fluctuation, especially in the surface 

 soil, due to tillage and the incorporation of organic matter ; 

 hence a sandy loam might under certain conditions present 

 more pore space than a silt or a clay loam. When soils 

 are in the physical condition for the best plant growth, 

 however, the rule holds that, the finer the soil, the greater 

 is the pore space. The differences in pore space between 

 the surface soil and the subsoil in Wisconsin are shown 

 by King -^ as follows : — 



First foot 

 Second foot 

 Third foot 

 Fourth foot 

 Fifth foot 

 Sixth foot 



Weight peb 



Percentage of 



Cubic Foot 



Pore Space 



79.0 



52.2 



92.6 



44.0 



104,6 



36.8 



106.2 



35.8 



111.0 



32.9 



111.1 



32.8 



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 mce 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 cropping conditions in general, 

 become apparent. It is the regulation of this pore space 

 that is really studied in any structural consideration. 

 The effect on plant growth of a change in pore space is 

 the final test of its advisability. 



iKing, F. H. Physics of Agriculture, p. 111. Published 

 by the author, Madison, Wisoousin. 1910- 



