WATER UTILIZATION BY SPRING WHEAT. 6 
Wheat is grown continuously on plat A. No cultivation is given 
the plat in the fall after a wheat crop is removed or in the spring 
until nearly time for seeding, when it is given a shallow plowing 
and usually one harrowing. No effort is made to control weed 
growth or to enable the plat to take up or conserve moisture. The 
plowing is usually only about 3 inches deep and the harrowing after 
plowing represents a minimum of cultivation in preparing a seed 
bed. This plat was originally designed to serve as a contrast to other 
plats receiving thorough cultivation for the purpose of storing water. 
Wheat is likewise grown continuously on plat B. This plat is 
given a deep plowing, usually 8 inches, as early in the fall as possible 
after the wheat crop is removed. It may be either worked down 
immediately or left rough for a period, depending upon whether 
it seems advisable to conserve the moisture already in the soil or to 
leave the ground rough in order to catch precipitation more readily. 
No expense is spared in either the number or character of opera- 
tions necessary to keep the ground free from weeds and in good 
tilth. The plat usually receives two double diskings and one or more 
harrowings. It represents what should be the greatest efficiency in 
storing moisture between harvest and seeding, and indicates the 
maximum results from the use of moisture-conservation methods 
where a crop is grown every year. 
Wheat is grown on land alternately cropped and fallowed on plats 
C or D. One of these plats is in crop each year and the other in 
fallow. By the use of two plats it is possible to obtain a record each 
year of wheat grown on fallowed land. The fallow plat is plowed 
in the spring to a depth of about 8 inches. It is kept free from 
weeds during the summer and fall, and an effort is made to main- 
tain the surface of the soil in condition to take up moisture readily. 
These plats represent the approximate maximum that may be expect- 
ed in moisture storage, when a year's crop is sacrificed in order to 
accumulate moisture for a crop the following year. Unpublished ex- 
periments in methods of fallow conducted by the Office of Dry-Land 
Agriculture Investigations have shown this to be one of the most 
efficient methods of fallowing. Plat C or D in this bulletin always 
refers to each plat in the year when wheat is grown. 
RANGE OF MOISTURE CONTENT OF SOILS. 
The quantity of water that may be held within any unit of soil is 
subject to a definite limitation. When the moisture content of any 
section of soil increases to a point where the molecular attraction 
of the moisture for the soil particles is exceeded by the pull * of 
gravity, there is a movement of water downward through the soil. 
It is in this manner that water falling upon the surface of a soil is 
conducted to the lower depths. Each successive layer of soil must 
become wet to a certain degree before water can reach the layer 
below. The addition of water in any considerable quantity to any 
foot section of soil necessitates that all of the soil above must have 
been wet enough to permit the passage of water through it. The con- 
dition of a soil holding all of the moisture that it is capable of holding 
against the pull of gravity is called its " field carrying capacity." 
Burr describes this condition of the soil as follows : 
