THE PHYSICAL PEOPEETIES OF SOILS 61 



that of surface tension. Any excess of water beyond this 

 amount is drawn downwards and a state of equilibrium 

 between the two forces is again established. 



The amount of water which a soil can retain depends 

 on the size and approximation of the particles. The 

 smaller particles present a larger internal surface to be 

 covered with films of water, their points of contact are 

 more numerous and the interstices are smaller. If the 

 particles are very small the interstices remain full and the 

 soil continues saturated to the limit of its capacity. When 

 the particles are larger the force of gravity overcomes that 

 of surface tension and the interstices are emptied, but the 

 films covering the particles remain. 



Mayer found that quartz sand consisting of approxi- 

 mately uniform particles of less than 0*3 m.m. diameter 

 retained practically the whole of the water required to 

 saturate it. 



Schloesing tested different kinds of soil and found the 

 following quantities of water retained per 100 parts of 

 dry soil: 



Kind of Soil. Water Retained. 

 Coarse sand ....... 3 per cent. 



Fine sand ........ 7 ,, 



Calcareous sand 32 ,, 



Clay soil 35 



Forest soil ....... 42 ,, 



In general the water-holding power of sandy soils is too 

 small and that of clays too great, but in both cases it can be 

 modified by treatment. The retentive power of sands is 

 sensibly increased by rolling, and that of clays is reduced 

 by pulverising and loosening. Consolidation diminishes 

 the size of the interspaces and increases the number of 

 points of contact, while pulverising has the opposite effect. 

 The water-holding power of sands can also be increased 



