6i8 
Journal of Agricultural Research voi. xxvii, No. 9 
SOIL WATER 
In an irrigated field the soil water that is of chief concern to the fanner 
is the water that exists in the surface layer of the soil from 4 to 10 feet 
deep, depending on the character of the crop for which the field is used. 
This surface layer of soil serves as a reservoir in which to store water from 
time to time by irrigation, so that it may be continuously available to 
the crop plants during the growing period. 
The water that is put into this surface layer of soil may not all be used 
by plants. On the one hand, if the soil is saturated with water, the roots 
of most crop plants do not function properly. If the roots have been 
established before the soil becomes saturated they may die below the 
level of saturation. If the soil is already saturated the roots do not 
penetrate the saturated zone. Consequently, a soil may contain too 
much water to support the growth of crop plants. This condition is not 
uncommon in irrigated lands, and such lands are referred to as “water¬ 
logged.” 
On the other hand, some of the water in the soil is not available to 
plants because it is held so firmly by the soil that the plants can not use 
it. When the moisture content of the soil is so low that plants can not 
obtain from it enough water to maintain their growth there still remains 
an appreciable quantity of water in the soil. It is customary to refer to 
the moisture condition of the soil at which plants can not obtain water 
for growth as the wilting point. It is only the water that is in excess 
of the moisture content at the wilting point that is available to crops or 
that constitutes the available reservoir capacity of the soil. 
The upper limit of the reservoir capacity of the soil is a point well below 
the saturation point. This upper limit is commonly referred to as the 
maximum field carrying capacity. The maximum field carrying capacity 
of a soil is not susceptible of accurate determination because it is condi¬ 
tioned by forces that are constantly changing. 
THE WATER-HOLDING CAPACITY OF SOIL 
It is generally assumed that the specific gravity of soil material is 2.65, 
so that if we could conceive of a cubic foot of soil so compressed as not 
to contain any voids it would weigh 165 pounds. As a matter of fact, 
as it occurs in the field, soil is found generally to range in weight from 75 
to 105 pounds per cubic foot, with a few exceptional cases in which it 
weighs less or more than the figures given. These figures refer to the 
dry weight of the volume of soil. 
If we assume that the soil material has a specific gravity of 2.65, then 
it is possible to compute the pore space in a volume of soil, e. g., a cubic 
foot, when this volume has various weights. Thus if 1 cubic foot of 
soil without pore space weighs 165 pounds and a cubic foot of soil with 
pore space weighs 85 pounds, then: 
_ 165—85 
Pore space=—^—=48.5 per cent, 
or a cubic foot of dry soil which weighs 85 pounds may be said to have 
51.5 per cent of its volume occupied by soil material and 48.5 per cent 
of its space existing as voids. If this void space were filled with water, 
that is, if the soil were saturated, but without changing the volume, then 
the soil would be said to hold 35.6 per cent of water. That is to say, if a 
cubic foot of water weighs 62.4 pounds, the water which occupies 48.5 
