SPRING WHEAT IN THE GREAT PLAINS AREA. 41 
On shallow soil, where the development of roots and the recovery 
of water is limited by underlying shale, or on shallow soils under- 
lain by gravel impervious to plant roots, one can not expect to get 
any material benefit from systems of tillage calculated to increase 
the storage of water in the soil. The shallowness of the soil will of 
itself limit the amount of water that can be stored in it. It may be 
that one heavy rain or rainy period will be sufficient to fill such a 
soil completely. After it has been filled to its carrying capacity it is 
obvious that no amount of cultivation will increase its water content. 
Crops grown on such soils are dependent upon seasonal rainfall. 
The soils upon which these investigations have been made are in 
the main fertile. In most cases they have been but recently broken 
from the native sod and in no case has the fertility been dissipated 
by long-continued cropping. Unless some abnormal factor enters in, 
such as hail or injurious insects, the yield obtained is largely deter- 
mined by the available water. Since the water available to the 
crop on shallow soils can not be materially increased by cultivation, 
and since on these soils water is the chief limiting factor, it is 
unreasonable to expect much increase in yield from one method of 
tillage over another. 
On uniform soils of sufficient depth to allow the accumulation of a 
surplus of water, wider variations in yields are to be expected from 
the various methods, under climatic conditions favorable to the 
storage of water. Such results have been obtained at several of 
the stations. The differences in yields, however, from different 
methods of tillage, have not been the same from year to year, even 
on the most responsive soils, but have varied with the climatic con- 
ditions. In some years comparatively wide differences are obtained. 
In another year the climatic conditions may be unfavorable and 
little or no differences in yield are shown. The rainfall might be so 
distributed that it could not be accumulated in the soil by one 
method more than by another. If only light showers came, or dry 
weather prevailed during the practice of some system calculated 
to accumulate water in the soil, it is obvious that little or no water 
would be stored. If the rains came later in sufficient amount and 
falling slowly enough to avoid run-off, the soils under all methods 
would be filled with water, which would tend to equalize the result- 
ing yields. If, on the other hand, little or no rain came and none 
had been stored, the results would be equalized in failure. It must 
be borne in mind that cultivation of itself does not accumulate the 
water in the soil. There must first be rain. The cultivation can 
assist only in getting the water into the soil and in preventing its 
loss through weeds, by run-off, or by vaporization and loss through 
shrinkage cracks. 
