106. 



This table shows that the finer the soil the li^iter its abso- 

 lute -weight. Clay soils may range from 60 to 90 pounds in weight, accor- 

 ding to their fineness and state of granulation, linn a soils weigh from 

 90 to 110 pounds. In practice, oils are spoken of as "light 1 * and "hea- 

 vy", but tli is use of these terras does not apply to the weight of the 

 soil. The term light is applied to sanoy soil bec-iune the particles 

 move freely; on the other hand, a clay is termed heavy because of its 

 cohesiveness, ^nd difficulty in working by the fanner. 



A dry sand may weigh 89//J a dry clay may weigh 



e > . Pore space and arrangement of particles in various soils. 



.re space is apace riot occupied by soil pnrticles. In a mass 

 of particles there is some unoccupied or pore space. It the particles 

 are fine, then the intervening spaces are correspondingly small; if 

 large, the spaces are large. Theo.r e t i cally . pore space is independent of 

 the ni^e of the particles, with any given arrangement. There would be 

 as much pore sr>ace in a cubic foot of uackshot as in one of marbles. 

 But in the coil this ie not true. For, the finer the particles, the lar- 

 ger the proportion of pore space is f o md to be. 



A clay has much more total pore space than a sand, 41tho the in- 

 dividual spaces or openings between the particles are much smaller in 

 the clay. The approximate ^ of pore space in a soil may be calculated 

 by use of the following formula; 



Vw 



* 100 . x 100 



>re P rPer cent of pore space 



Vs * Volume in c.c. occupied by the soil 

 Vw Weight of water equal to weight of soil in grams 

 Vp r Volume in c.c. of pore space in soil 

 2.65 = Specific gravity of soil particles. 



Another and more simple formula which may be used in the calcu- 

 lation of the pore space is aa follows: 



P s 100 - Ap. SP. _Kr. x 100 

 Ab . sp . gr . 



Where P * Per cent of pore space 



Ap. sp. Apparent specific gravity or volume weight 

 Ab. sp. s Absolute specific gravity of soil material 

 Total space occupied by soil mass. 



The different amounts of pore space in soils change/ the charac 



ter of the soil materially. Hilgard gives the following figures which 



may be used as convenient standard figures, for % of pore space in 

 various soils: 



Packed sand soil has less than JO/ pore spce, 



Ordinary * " " " " 6c;: " w 



Clay aoilft * A 47- 50;' 



Forest soil has 50- 6 



Farm aoil has 35- J> " " 



