Transpiration as a Factor in Crop Production. 207 
conditions. The amount of available soil fertility, soil moisture, 
and other conditions in potometers never are and probably can- 
not be made to be strictly comparable to field conditions. This 
fact does not, however, prevent the establishment of general 
principles concerning the response of the plant to variable con- 
ditions under control in potometers nor concerning the relative 
water requirement of different crops. 
(1) It was seen in Table 1 that as an average for four years 
the potometer plants had 5 per cent greater leaf -area and weighed 
21 per cent more than the average of plants grown in the field at 
the rate of two plants per hill or 7,112 plants per acre, during three 
years, and three plants per hill during one year. Assuming that 
a corn plant in the potometers used as an average the same amount 
of water (132 kilograms) as was used by a corn plant in the field 
spaced two plants per hill, in hills 3.5 feet apart, we find that an 
acre of such c:>rn would use 9.14 acre-inches of water. It is fairly 
certain that the total amount of water used per plant would not 
be greater under field conditions than the above figure. 
With the customary rate of three plants per hill, the total 
water transpired per acre would doubtless be greater, altho not 
proportional to the increase in the number of plants unless 
sufficient available fertility and soil moisture were present to 
support fully the increased number. The increased self-protection 
of a thicker stand would probably tend to reduce transpiration. 
(2) The 5-year average water requirement per pound of ear, 
as given in Table 31, is seen to be 629 pounds. Allowing 16 per 
cent for cob, we have a water requirement of 750 pounds per 
pound dry weight of grain. Assuming a 50-bushel yield of moisture- 
free grain, 1,050 tons or 9.3 acre-inches of water would be used, 
which is practically the same result as obtained by the previous 
method of calculation. The actual average yield of moisture-free 
corn at the Experiment Station during the past five years has been 
37 bushels per acre, grown three plants per hill. According to the 
method just discussed, this yield would have had a water require- 
ment of 6.9 acre-inches. This amount is evidently too low, since 
it does not take into consideration the water used by barren 
stalks, which are ordinarily rather numerous. The proportion of 
ear to stalk was also lower in the field than in the potometers. 
The average yield of moisture-free corn at the Experiment Station 
during 13 years has been 46 bushels per acre. 
(3) The amount of water used in the production of a given 
yield of grain in potometers cannot be applied directly to the 
grain yield under field conditions without qualifications, because 
there are so many variable factors, all of which affect the water 
requirement. 
