66 RELATIONS OF SOIL TO WATER 



Flugge's experiments with gravel, sand, and a mixture of 

 equal parts of both, furnish a good illustration of the very 

 similar results obtained with masses composed of very different 

 sized particles, and of the diminution in the volume of the 

 interstices when large and small particles are mixed. 



Volume of Interstices per cent, of Total Volume 



Gravel 384-40.1 



Sand 35-6-40.8 



Gravel and Sand . . . 23-1 - 28-9 



In ordinary soils, the volume of the interstices will generally 

 be somewhat greater than in the case of sand or gravel ; this 

 is owing to the presence of porous or compound particles in 

 the soil. Particles of chalk or limestone are porous ; particles 

 of humus are highly porous. Compound particles occur 

 abundantly, as we have already seen, in all soils in a con- 

 dition of good tilth. As the proportion of porous particles 

 in the soil increases, so also will its capacity for containing 

 water. 



A soil abounding in porous or compound particles has its 

 capacity for water diminished if the soil is reduced to a fine 

 powder in the laboratory. Zenger found that the soil from 

 a peaty meadow, TOO parts by weight of which were capable 

 of holding 178 parts of water, had its capacity for water 

 reduced to 103 when finely powdered. 



There is one factor which influences the water capacity of 

 soil, which, however, has no relation to the volume of the 

 interspaces ; this is the action of the colloid constituents. 

 Colloid bodies placed in contact with water take up a con- 

 siderable quantity, and swell considerably. We have already 

 noticed (p. 35) the increase in volume in peat and clay which 



