Transpiration as a Factor in Crop Production. 127 
fairly comparable, but that the deeper jars would not have been 
sufficiently sensitive to record rapid changes for correlating with 
hourly transpiration changes. 
RELATION OF AVAILABLE SOIL MOISTURE CONTENT TO 
GROWTH AND WATER REQUIREMENTS OF CORN. 1 
PLAN OF THE EXPERIMENTS. 
During the three years 1910, 1913, and 1914, groups of Hogue's 
Yellow Dent corn plants were grown in potometers with all 
factors uniform except the degree of available water in the soil. 
Potometers 16 by 36 inches in size, having the coil- watering 
device, illustrated on page 44, were filled anew each year with 
fertile surface loam from one of the Experiment Station fields. 
The amount of water required to saturate the soil was regarded 
as the amount retained by the soil after seepage from water 
poured on the soil surface of duplicate potometers had ceased. 
Thus, the soil was considered saturated when it contained all the 
water it would hold against percolation. The standard weight 
of each potometer was determined for its particular degree of 
soil moisture content, and this standard weight was restored 
each day. The potometers were weighed and the water added 
between 7 p. m. and 8 p. m. except in 1913, when it was added 
two hours earlier. By this practice the transpiration rate was 
low at the time of applying the water, which was in this way 
given time to distribute itself thruout the soil mass during the 
night, before heavy transpiration commenced the following day. 
DEGREES OF SOIL MOISTURE MAINTAINED. 
The various water contents of the soil during each year are 
indicated in Table 52. 
The results obtained with the different per cents of relative 
soil saturation as used in the potometers cannot be expected to 
correspond exactly to the same relative saturation under field 
conditions. The normal water-holding capacity of soil is doubt- 
less considerably disturbed by refilling and packing in potometers. 
It also varies greatly with the type of soil. Nevertheless, the 
data obtained in this way should serve to indicate general prin- 
ciples of response to variation in soil moisture. The relative 
effects of differences in soil moisture content also vary in dif- 
1 References concerning the relation of transpiration to soil moisture 
content: Fittbogen (1873), Fortier (1902), Harris (1914), Hellriegel (1883), 
Il'enkov (1865), Kiesselbach (1910), Kiesselbach and Montgomery (1911), 
Leather (1911), Maercker (1896), Ohlmer (1908), Pfeififer et al. (1912), Preul 
(1908), Schroeder (1896), Seelhorst (1899), Seelhorst and Bunger (1907), 
Thorn (1913), Widtsoe (1909), Wilms (1899), and Willard and Humbert (1913). 
