Feb. 16,1914 
Indicator Significance of Vegetation 
367 
composite of four cores. The soils were usually sampled to a depth of 4 
feet and occasionally to a greater depth, the cores being taken in i-foot 
sections. 
Determining the Soil-Moisture Content. —Numbered tin boxes of 
uniform weight and with tight-fitting covers were used to receive the 
soil samples directly from the sampling tubes. The whole sample was 
used as a basis for the moisture determinations and after the initial 
weighing was dried in a water oven to constant weight. The moisture 
content is in all cases expressed as a percentage of the dry weight of the 
sample. 
Determining the Moisture Equivalent. —In studying soil moisture 
in relation to plant growth it is important to have some standard for 
measurement of the retentivity of the soil for moisture. As two of the 
authors have previously shown, 1 this may be conveniently accomplished 
by the method of moisture equivalents. This method consists in sub¬ 
jecting a moist sample of soil to a constant centrifugal force equal to 
1,000 times that of gravity until the moisture content of the soil is 
reduced to the point where it is in equilibrium with the centrifugal 
force employed. The residual moisture content of the soil is then 
determined. This value, expressed as a percentage of the dry weight 
of the sample, is the moisture equivalent. A direct measure of the 
retentiveness for moisture of the various soils is thus obtained, and, 
since the same force is employed throughout, all of the determinations 
are directly comparable. 
Determining the Wilting Coefficient. —It has been shown by two 
of the authors 2 that the moisture equivalent serves as a useful indirect 
means of determining the wilting coefficient. The latter term designates 
(as a percentage of the dry weight of the soil) the quantity of water 
remaining in the volume of soil occupied by the active roots of a plant 
which is beginning to wilt. 3 
These determinations (moisture equivalent and wilting coefficient) 
serve to give an idea of the texture of the soils occupied by the different 
plant associations, as indicated by their retentiveness for moisture. By 
subtracting the wilting coefficient from the actual moisture content a 
measure is obtained of the percentage of moisture available for the active 
growth of plants at the time the soil samples were taken. 
Determining the Salt Content.— The total salt content of each 
soil sample was determined by the electrical-resistance method developed 
1 Briggs, L- J., and McLane, J. W. Moisture equivalents of soils. U. S. Dept. Agr., Bur. Soils Bui. 45, 
23 p., 1 fig., 1 pi. 1907. 
Briggs, L- J., and McLane, J. W. Moisture-equivalent determinations and their application. Proc. 
Amer. Soc. Agron., v. 2,1910, p. 138-147, pi. 6. 1912. 
3 Briggs, L. J., and Shantz, H. L. The wilting coefficient for different plants and its indirect determi¬ 
nation. U. S. Dept. Agr., Bur. Plpnt Indus. Bui. 230, 83 p., 9 fig., 2 pi., 1912. 
8 “ Wilting” in this case must be understood as permanent wilting—i. e., a condition from which the 
plant can not recover its turgidity until the soil receives additional moisture, no matter how great the 
humidity of the atmosphere. 
