12 
The Colorado Experiment Station 
From the minimum capillary capacity, we see that a sandy 
soil would have from 5 to 7 percent of available water, amount¬ 
ing to 0.6 inch to 0.84 inch per foot. A silty soil would be dried 
under similar conditions to 7 to 12 percent, depending upon the 
physical composition of the soil, giving from 5 to 9 percent of 
available water amounting to from 0.6 to 1.08 inch of water per 
foot of soil. A clay soil under similar conditions would retain 
from 12 to 18 percent, depending upon composition, leaving avail¬ 
able from 6 to 7 per cent or slightly more, equivalent to from 
0.72 inch to about 1 inch of water per foot of soil. While there 
are great variations in individual soils, it is seen that, under dry 
land conditions, the available water is nearly the same in widel}’ 
different soil types. 
Amount of Water It Takes to Grow Crops .—Investigators in 
dry-land crops agree pretty well that it takes from 300 to 400 
and up to as high as 1,000 pounds of water to produce a pound of 
dry matter in the crop. That is, the crop uses up that much 
water in growing a pound of dry matter. 
Thus a sandy soil which contained from 0.6 to 0.8 inch of 
available water per foot would have to contain this amount of 
available water 5 to 7 feet in depth to grow a 15-bushel crop of 
wheat from the soil water unless helped out by rain. The same 
principle holds with other soils and other crops altho the limits are 
somewhat different for each soil and crop. This principle is: A 
dry-land soil must be deep in order to hold enough water to in¬ 
sure crops. Crops will dry out a shallow soil quickly. Then, if 
no rains come, the crop dies. 
The dry-farming soil must be of uniform texture, also; a 
layer of gravel or gypsum below the surface will cut off the ef¬ 
fective depth of soil. This was very well shown in the season of 
1911 at Rocky Ford. In a field of alfalfa two spots were noticed 
where the crop was not growing. Borings revealed a gravel layer 
under one spot and a gypsum layer under the other spot. These 
layers of different texture broke the continuity of the soil mass 
and prevented the roots getting water at lower depths. 
Under normal dry-farming conditions, there is very little 
movement of the soil moisture except after rains. The soil water 
does not move to the plant roots. The roots move to the mois¬ 
ture. If one will take a dish of wet soil and sprinkle dry soil on 
top, water will move up into the dry soil. If this is then scraped 
off and dry soil again sprinkled on top and scraped off when 
moistened, a point will soon be reached when the dry soil will no 
longer be moistened. We will then have a condition comparable 
to dry-farming soils (except immediately after rains). This is 
