EXPERIMENTS WITH FALLOW IN MONTANA 13 
conditions than have prevailed during the seven years under study. 
The figures given are only for the year immediately following the 
fallow and do not compare the yields of the third and fourth crops 
in rotations Nos. 118, 119, 171, and 172. 
WATER STORED DURING THE FALLOW PERIOD 
Soil-moisture determinations were made each year from 1917 to 
1923, inclusive, on the two plats that are alternately fallowed and 
cropped to spring wheat. From these determinations the increase 
in the water content of the soil between harvest and the following 
seeding date can be calculated. This is approximately 21 months 
when the land is fallowed and is here called the fallow period. The 
soil samples were not always taken on the exact harvest and seeding 
dates, but with the exception of 1918, when the harvest samplin 
was delayed until December, they were taken within a few days 
these dates. 
Samples were taken in 1-foot sections to a depth of 6 feet with a 
tube 20 millimeters in diameter. As the determinations for the lower 
3 feet show no appreciable changes in water content, the data from 
the surface 3 feet only are used in the present study. Four separate 
cores, or samples, were taken from as many different locations on 
each plat on each date. These were placed in two cans, the water 
content of each determined, and the percentage of water in the two 
averaged. The soil-moisture determinations for each foot were, 
therefore, the averages of four samples. The percentages of soil 
moisture were determined on the basis of dry weight. The weight 
of the soil per cubic foot was determined by calculation from the 
weight of the cores taken with the soil tube. The averages of large 
numbers of samples were used for these determinations, and it is 
believed that the volume weights determined by this method are 
fairly accurate. 1 
With the increase in the percentage of soil moisture and the weight 
per cubic foot of soil as known quantities, the increase in percentage 
is calculated as inches of water. 
Table 9 presents detailed data regarding the quantity of water 
stored in the soil during five fallow periods between August 11, 1917, 
and May 21, 1923. This table gives the weight per cubic foot of soil, 
the date of the first determination (called harvest date) , the date of 
the determination at or near seeding time (called seeding date), the 
percentage of water in each foot of soil on each of these dates, the 
increase in percentage between these dates, the equivalent in inches 
of this increase, the total increase in inches of water, the total pre- 
cipitation during the period, and the percentage of the precipitation 
that was stored in the soil. 
The total quantity of water stored in the soil varied from 1.77 
inches for the period from August 11, 1917, to May 1, 1919, to 4.25 
inches for the period from August 2, 1920, to May 22, 1922. The 
total precipitation from harvest to the following seeding date varied 
from 11.60 to 20.46 inches. The percentage of the precipitation 
that was stored in the soil varied from 10.1 to 22.8. 
1 Cole, John S., and Mathews, O R. Use of water by spring wheat on the Great Plains. U. S. Dept. 
Agr. Bui. 1004, 34 p., 10 figs. 1923. 
