Transpiration as a Factor in Crop Production. 203 
competition, and the roots may continue to grow into the moister 
soil between the rows. 
Our experimental data concerning the effect of increased fer- 
tility also suggest that it may be practicable on very fertile soil 
to grow a smaller variety of corn. The increased fertility will 
result in production at a relatively low water cost, while the total 
amount of water transpired will be held within reasonable limits 
by avoiding a luxuriant vegetative growth. These suggestive 
methods appear encouraging and are deserving of thoro investi- 
gation in practical field tests. 
Our experience would indicate that much corn is planted at 
too thick a rate. The reduced yields from the thicker rates are 
due primarily, under our conditions, to a shortage of moisture, 
and secondarily to a shortage in available fertility for so large a 
number of plants. Farmers in the drier regions of the Great 
Plains area have recognized this principle and their rate of seeding 
is markedly lower than is practiced in more humid sections. 
Ordinarily the reduction in total water used will not be propor- 
tional to the reduction in stand, because of greater vegetative 
growth when plants are more favored. 
MOISTURE CONTENT OF SOIL 
The fact that a shortage of moisture below the optimum for the 
individual plant reduces the total water requirement and some- 
what reduces the water requirement per unit dry matter of crop 
has no great practical value for application. This reduction can- 
not be brought about without a reduction in yield. 
Soil with a surplus of moisture needs drainage. An over- 
abundance of soil moisture reduces the yield. This is doubtless 
due to some injurious effect which decreases the amount of 
available fertility. 
Under irrigation, there is no advantage in applying a surplus of 
water. Water will not take the place of soil fertility, nor is the 
common saying true that fertility may replace water. Increased 
fertility merely causes a more responsive growth, which is a 
factor in plant nutrition and not in transpiration. An excessive 
application of water results in a greater amount being transpired 
per unit dry matter produced than under optimum moisture 
content, but this is due, in part at least, to a less normal growth. 
The largest total amount is used under optimum soil moisture, 
when the largest yield is also harvested. There may be a large 
variation in the water content without materially affecting the 
growth relationships. Thus, the lowest soil saturation conducive 
to maximum growth is the most practical, for then less water 
is lost by surface evaporation than with a wetter soil. 
