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IOWA ACADEMY OF SCIENCE Voi,. XXVII, 1920 
different soils by subjecting them to centrifugal force, and have 
called the percentage of moisture retained the moisture equivalent. 
Briggs and Shantz have worked out the method which has come 
to be most generally used in comparing the . available moisture 
supply of various types of soils. They have called the percentage 
of water remaining in soils when wilting occurs the wilting coef- 
ficient. Fuller has termed the amount of moisture contained in 
soils above the wilting coefficient growth water. Again, it has 
been shown that the wilting point of plants is not constant for 
the same plants and soil types. Brown,^® has studied its relation 
to the evaporating power of the air and has found that the wilting 
point is reduced with reduction in evaporation. Briggs and 
Shantz have found in another study that it varies also, although 
to a less degree, for the same soil with different types of plants. 
Therefore, a comparison of the total amounts of moisture con- 
tained by various soils is not a criterion of the ability of those 
soils to maintain plant growth. 
It is necessary to recognize the above facts in view of the data 
presented here. The wilting coefficients have not been determined. 
The chief purpose is a study of the water holding capacity of 
the various soils and the relation of the movements of the soil 
water. It is evident that an increase in the total moisture content 
of an ordinary soil would make it more nearly capable of sus- 
taining plant life without regard to the wilting coefficient. 
Relation or Forests to Water Table 
McGee in his work explains how the general level of the water 
table of a region affects the total soil moisture content, and also 
the resultant effect that its recession has upon the amount of 
moisture available for the growth of plants in the upper soil 
layers. This, of course, in turn affects the ability of the vegeta- 
tion to withstand periods of excessive drought and evaporation. 
He discusses further the causes for this recession, as does also 
Schwartz,^® and it is not necessary to review the field here. A 
matter of especial note, however, is the general statement at 
which McGee arrives, namely, that it is clear that the lowering of 
the ground level of water is due mainly not to consumption of 
the accumulated stock, but to the cutting off of the natural source 
23 Brown, Wm. H., The relation of evaporation to the water content of the soil at 
time of wilting: Plant World, 15, 121-134, 1912. 
24 Briggs, L. J., and H. L,. Shantz, Bot. Gaz., 53, 229-235, 1912. 
25 McGee, W J, U. S. Dept, Agric., Bur. Soils, Bull. 92, pp. 7-11, 1913. 
26 Schwartz, G. Frederick, U. S. Dept. Agric., Bur. Forestry, Bull. 44, pp. 13-15, 
1903. 
