GRAIN-SORGHUM EXPERIMENTS IX OKLAHOMA. 3 
run-off occurs except during torrential rains. The soil under opti- 
mum tilth contains 8 to 12 per cent of moisture, which can be 
reduced by plants to a minimum of about 2 per cent. The soil has 
a tendency to blow, and tillage methods must be employed to prevent 
blowing or drifting. 
CLIMATIC CONDITIONS. 
The weather data presented were recorded either by the observer 
of the Weather Bureau at Woodward, Okla., or by a representative of 
the Office of Dry-Land Agriculture Investigations at the Woodward 
Field Station. 
The principal climatic factors which influence crop production in 
this vicinity are (1) a limited and variable annual precipitation, of 
irregular seasonal distribution; (2) a relatively low atmospheric 
humidity; (3) a high rate of evaporation during the summer months ; 
(4) a wide daily range of temperature, hot days being followed by cool 
nights; and (5) a high average wind velocity. 
Fig. 2.— Airplane view of part of the Woodward Field Station. Photographed in May, 1920. 
PRECIPITATION. 
Precipitation and its distribution may be considered the limiting 
factor in crop production at Woodward, Okla. The total precipi- 
tation is usually sufficient to produce a good crop every year, but the 
distribution is frequently such that soil moisture is lacking at the 
critical period of plant growth. When this condition exists crop 
yields are low. During the eight years from 1914 to 1921, inclusive, 
there have been several seasons of poor rainfall distribution, yet the 
sorghums produced some grain, though yields were low. 
MONTHLY AND ANNUAL PRECIPITATION. 
Table 1 shows the monthly, annual, and mean annual precipitation, 
in inches, at Woodward, Okla., during the 27-year period from 1895 
to 1921, inclusive. The mean annual precipitation at W T oodward for 
this 27-year period was practically 24 inches, of which 17.4 inches 
fell during the growing season, or from April to September, inclusive. 
Table 1 shows in a general way the precipitation characteristic of 
the locality around Woodward. In the 27-year period there were 7 
years in which the annual precipitation was less than 20 inches, two 
of which occurred during the period covered by these experiments. 
