26 BULLETIN 1139. U. S. DEPAETVXEXT OF AGEICTTLTUBE. 
The fact that all the available moistur-e present is utilized about 
the same proportion of the time in each of the first I feet of soil 
indicates that when water is present the roots of the wheat crop 
usually completely occupy the soil to about that depth before the 
plants reach maturity. 
This does not indicate that water held at lower depths is not 
usually valuable. In most years all available moisture within reach 
of the roots of grain crops is needed, and the plat that carries the 
greater quantity of moisture in the zone of normal development of 
roots generally makes the higher yield. 
The value of moisture-storage capacity in a soil depends largely 
upon the precipitation. Thus, at Williston. where the soil is of 
sufficient depth to permit the storage of moisture to a depth of 6 feet. 
plats A and B have only occasionally been filled with moisture to a 
depth of 3 feet, and in some years the second foot of soil has not been 
filled to its carrying capacity. In spite of its greater moisture-storage 
rapacity, it is doubtful whether, in the aggregate, more moisture has 
been stored in these plats than in the corresponding plats at Edgeley. 
where the shallow soil has been filled to capacity each year. Stations 
like Dickinson, that combine a high storage capacity with a rela- 
tively large precipitation, are able to utilize moisture to a greater 
extent. 
Another evident fact is that the utilization of a large soil mass is 
not essential to a high yield. The yield depends more upon the main- 
tenance of a constant supply of available moisture at a depth where 
it can be easily absorbed by the roots than it does upon the mass of 
soil involved. The highest yields recorded at any station were pro- 
duced at Belle Fourche in 1915. when the soil was at no time wet to 
a depth greater than 2 feet. At the same station in 1920. when all the 
soil was filled to capacity with moisture and the crop prospects were 
even better than in 1915. the yield was reduced below that of 1915 
because the available moisture was exhausted too long a time before 
harvest. Yields of wheat on the shallow soil at Edgeley under favor- 
able conditions have been as high or higher than at stations where 
a much greater soil mass has been occupied by the roots. 
SUMMARY. 
With knowledge of the field carrying capacity and the minimum 
point of exhaustion of each soil unit the soil-moisture data have 
been utilized to classify into five groups the extent to which each 
foot section of soil has functioned each year in the production of 
spring wheat by three distinct cultural methods at 17 field stations 
on the Great Plains. 
On land producing a crop each year differences in cultural 
methods are not sufficient to cause major differences in the depth 
to which water is stored and from which it is recovered. Alternate 
fallowing and cropping results, on the average, in the utilization 
of a somewhat greater volume of soil. 
The depth to which available water is stored may be limited by 
the shallowness of the soil. TVhen not limited by soil character the 
surface 6 feet of soil may function in the growth of spring wheat. 
Except on soils of limited storage capacity, the depth to which 
water is stored in the Great Plains is more often determined by the 
