﻿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 againsl ( lie pull of gravity. When water is added to a soil hi 

 sufficient amounl 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 r . The storage and use of soil moisture. Nebraska Agr. Exp. Sta. Re- 

 search Bui. 5, 88 pp., Plus. 1014. 



2 Briggs, Lvman 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, Lvman J., and Shantz, II. L. The wilting coefficient for different plants and 

 its indirect determination. I". S. Dept. Agr., Bur. Plant Indus. Bui. 230, 83 pp., 9 figs., 

 2 pis. 1912. Bibliographical footnotes. 



