4 BULLETIN 1139;, U. S. DEPARTMENT OF AGRICULTURE. 
The maximum water-holding capacity of a soil is the amount of water that 
soil will retain against the pull of gravity. When water is added to a soil in 
sufficient amount the film of water around each soil particle becomes thickened 
and the small spaces between the soil particles filled with water. In the addi- 
tion of water a point is reached where the attraction of gravity overcomes the 
force that holds the water to the soil particles, and the water is drawn down- 
ward into the lower soil. If no more water is added a point is reached where 
the pull of gravity is equalled by the force that holds the water to the soil and 
there is no further movement except by the slow action of capillarity. The 
amount of water in the soil at this time is termed " maximum water-holding 
capacity " or " saturation point." 1 
It will be noted that Burr used the term " maximum water-holding 
capacity." This term describes the condition of the soil accurately, 
but has been used by other investigators in a different sense; there- 
fore the term " field carrying capacity " is preferred. 
The fact that the moisture content of a soil must be at its field 
carrying capacity before water is conducted to the lower depths does 
not indicate that it remains in that condition for any long period of 
time. The water held within a soil is removed by direct evaporation, 
by the action of plants, and to a limited extent may be transported 
by vaporization and recondensation. 
The field carrying capacity of each individual foot section of soil 
at the 17 stations included in this publication has been determined for 
every foot section that has been wet often enough to permit accurate 
determination of the point. It has been determined by averaging the 
moisture content of each individual foot section of soil every time it 
has been wet enough to permit moisture to move through it into the 
foot section below. 
The averages show that the field carrying capacity of a soil de- 
pends upon the physical character of the soil and bears a linear rela- 
tion to its moisture equivalent and other physical constants. It is a 
little lower than the moisture equivalent as described by Briggs and 
McLane. 2 
The moisture equivalent of each foot section of soil in the plats 
from which data are here presented has been determined. 
Loss of water from the soil takes place in various ways, but the 
principal loss in a soil upon which a wheat crop is growing is the 
water taken up by the roots of the crop and transpired by the plant 
tissues. Not all of the water in the soil is removed, as there is always 
a residue not available to plants. A soil whose moisture content is 
at a point where plant roots can no longer remove water is in a con- 
dition termed its "minimum point of exhaustion." This condition is 
described by Burr as follows : 
The minimum point of exhaustion of water from the soil by the plant is the 
point at which the force exerted by the plant in obtaining water is equalled by 
the attraction of the soil for the water. At this point the plant can obtain no 
more water from the soil and will suffer until water is supplied. 1 
The minimum point of exhaustion of a soil bears a direct relation 
to the wilting coefficient as determined by Briggs and Shantz. 3 Al- 
1 Burr, W. W. The storage and use of soil moisture. Nebraska Agr. Exp. Sta. Re- 
search Bui. 5, 88 pp., illus. 1914. 
2 Briggs, Lyman J., and McLane, John W. The moisture equivalent of soils. U. S. 
Dept. Agr., Bur. Soils Bui. 45, 23 pp., 1 fig.. 1 pi. 1907. 
3 Briggs, Lyman J., and Shantz. H. L,. The wilting coefficient for different plants and 
its indirect determination. U. S. Dept. Agr., Bur. Plant Indus. Bui. 230, 83 pp., 9 figs., 
2 pis. 1912. Bibliographical footnotes. 
