STRUCTURE AND SOIL MOISTURE 151 



prominently in considering the capillary movement 

 of moisture. The relative adjustment and distribution 

 of the moisture between small and large particles in 

 contact is illustrated in Fig. 45. When in capillary 

 equilibrium, two soils should appear equally moist. 



73. Structure. Structure is the second factor which 

 determines the moisture capacity of a soil. If the state- 

 ments in reference to the effect of texture have been 

 fully understood, the influence of structure will be 

 readily grasped. The effect of structure is to alter the 

 effective size of the soil units or granules, and also of 

 the spaces which they form. In a coarse sand soil, 

 the general effect of rendering the structure of the soil 

 more loose is to proportionately reduce its water-holding 

 capacity, because the spaces are already so large as to 

 hold a relatively small amount of water, and that to 

 a very limited height. Change in structure further de- 

 creases that already deficient capacity. On the other 

 hand, in a fine clay soil the spaces are all very small, 

 and all have a capillary efficiency to a great height. This 

 height is much more than is ordinarily needed to bring 

 the moisture from the deep subsoil to the root zone. 

 In such a soil a more loose and open structure has the 

 effect of increasing the effective moisture capacity, 

 so long as the spaces are still able to hold water at the 

 surface of the column. But when this maximum size 

 of space is exceeded, as in a coarsely clodded soil, the 

 moisture capacity drops low, as in the case of sand or 

 gravel, when growth may be seriously interrupted. 

 Ordinarily, then, it may be said, that loosening the 

 structure of a coarse sand or gravel soil lowers its 



