The Storage and Use of Soil Moisture 
25 
Charts I to IV show graphically the amount of water stored 
by summer tilling. Chart I shows the work done in four fields. 
Two of these fields were summer tilled during the summer of 
1908. which was a good year for storing water. The other two 
fields were summer tilled during dry seasons. In this chart the 
base line represents 8 per cent, or practically the lower limit of 
available water. All water added to the soil above this point 
and held within reach of the plant roots can be used by the crop. 
The foot sections of soil are shown on the left of the chart. This 
chart further illustrates the fact that the amount of water which 
can be stored by summer tilling in this soil is largely governed 
by the amount and distribution of the precipitation. 
Chart II shows the average amount of water in the upper 
six feet of soil in three fields, and the amount of precipitation 
during the interval. 
Each square on the chart, as Ave go upward from the bottom, 
represents 1 per cent of water in the soil or one inch of rain. As 
we go from the left to the right on the chart, a square represents 
a period of ten days time. According to work done by Drs. 
Briggs and Shantz of the Bureau of Plant Industry, the ap- 
proximate lower limit of moisture available for plant growth is 
somewhat too low as represented on this chart. 
In the spring each of three fields here represented had about 
the same average water content. The water content of the field 
sown to wheat increases slightly, under the heavy May rains, 
until the latter part of May. After this date the crop not only 
uses the rain that falls, but draws on the soil for almost four 
inches of stored water. After harvest an increase is shown, and 
at the last sampling the wheat and cornfields have about the 
same amount of water. That the wheatfield did not gain in 
water content as rapidly in the spring as either of the other 
fields was due to the fact that the crop was already using a good 
portion of the rain that came. 
It will be noted that the field listed to corn increased in water 
content almost as rapidly to July 1 as did the summer tilled 
field. Up to this time the corn plants, few in number and small 
as compared with the wheat, were not making much demand for 
water. Almost five inches of water had been retained from the 
rains up to this time. At this period the corn began to use more 
water and not only used the rain that fell, but used more than 
five inches of water that had been stored in the soil. The sum- 
mer tilled field retained this and remained thru the winter 
with practically all the water the soil would carry in the upper 
six feet. 
