WATEB UTILIZATION BY SPRING WHEAT. 27 
quantity and character of the precipitation than by the storage 
capacity of the soil. 
Available water, when present in the soil, is removed with about 
the same degree of frequency from each of the first 4 feet. The 
aggregate quantity of water contributed by each foot section be- 
comes progressively less with increasing distance from the surface, 
both because each successive foot section is less frequently filled with 
water and because the quantity of available water that may be held 
in each successive unit is less. The fifth and sixth foot sections hold 
still less available water, and the full use of their limited capacity is 
not frequent. They contribute very little to the total quantity of 
water used by the crop, but under certain conditions this contribution 
may be important. 
In about 90 per cent of the years covered by these investigations 
the soil within the zone of normal root development was dry at 
harvest time. 
The utilization of a large soil mass is not essential to a high 
yield. The yield depends more upon the maintenance of a constant 
supply of available moisture to the depth at which it can be easily 
obtained than upon the mass of soil involved. 
