254 



SOILS AND SOIL WATER RELATIONS 



Let us now assume that another two inches of rain falls on this same 

 soil before any appreciable amount is lost from its upper layers by evaporation 

 or transpiration. After a new equilibrium has been attained the upper foot 

 of soil will not have increased in water content, but a layer of the soil approxi- 

 mately two feet in depth will now be moistened up to its field capacity (Fig. 

 67, B). In other words addition of a second increment of water equal in 



Fig. 67. Diagram illustrating distribution of percolating water in a soil under certain 



field conditions. 



volume to the first does not increase the water content of the already moist 

 layer of the soil above its field capacity, but results, due to further capillary 

 movement, in raising to its field capacity a second layer of soil, lying directly 

 under the zone which had been moistened by the addition of the first incre- 

 ment of rain, and approximately equal to it in thickness. Although the exact 

 mechanics of the distribution of water under these conditions is probably more 

 complicated, the effect produced is as if the second increment of water simply 



