PHYSICAL PROPERTIES OF SOILS 183 



ment of the soil particles upon wetting, especially in soils low in 

 organic matter. 



The porosity may be determined by dividing the difference be- 

 tween the real and the apparent specific gravity by the real, or by 

 the following formula : 



Real Sp, gr. App. Sp. gr. 

 Porosity = - ^ . - - 



Real Sp. gr. 



The porosity of different grades of sand has been determined 

 in this way and the results are given in the following table : 



Porosity of Different Grcules of Sand 4 



The porosity of the soil under field conditions is of much more 

 importance than that of the laboratory sample, after having been 

 finely ground. The laboratory determination gives comparative re- 

 sults, however, that are of some value. The porosity may best be 

 obtained by taking the apparent specific gravity under field con- 

 ditions, a.s given on page IT."), and the real specific gravity of tin- 

 soil and use these in the formula above. Soils in good tilth have 

 high porosity. It is not unusual for the same type of soil from 

 different fields to have a difference of 10 per cent or even more 

 of pore space in favor of the soil of good tilth. The total pm- 

 space varies inversely, while the si/e of the individual pores varies 

 directly as the size of the particles. The total pore space of coarse 

 soils as sands is small but the pores are large. The sectional areas 

 of individual pores vary as the squares of the diameter of the soil 

 particles. In figure SSA there are 1(5 pores per -square inch, while 

 if the particles are one-half as large they number <H. The sectional 

 area can be only one-fourth as large in the latter case as in the 

 former. If coarse sand whose particles have a diameter of one- 

 twenty-fifth of an inch is compared with clav whose particles are 

 one twenty-five-thousandth of an inch it will be S<HMI that for sand 



