24 BOTANICAL GAZETTE [JANUARY 
placed in perforated cups and moistened with an amount of water 
in excess of the amount they can hold in opposition to the 
centrifugal force. After standing 24 hours, the cups are placed ina 
centrifugal machine, which is operated at a constant speed so chosen 
as to exert a force 1000 times that of gravity upon the soil moisture. 
Each soil then rapidly loses water until the capillary forces are 
increased sufficiently to establish equilibrium with the centrifugal 
force employed. The moisture content of each soil is now not only 
in equilibrium with a force 1000 times that of gravity, but is also 
in capillary equilibrium with every other soil which has been simi- 
larly treated, so that if the soils are placed in capillary contact 
in any combination whatever, no movement of water from one 
soil to another will occur. The moisture content of each soil under 
these conditions is the moisture equivalent of that soil. This 
method, then, provides a means of determining and comparing 
the retentiveness of different soils for moisture when acted upon by 
a definite force, which is measured in absolute terms and is repro- 
ducible within narrow limits. 
In the accompanying table (table IT) is given a comparison of the 
wilting coefficient and the moisture equivalent for a series of soils 
ranging in texture from a coarse sand to a clay. The names 
applied to the soils have been determined from the mechanical 
analyses in accordance with the soil classification table used by the 
Bureau of Soils. The soils are arranged in the order of increasing 
moisture equivalents. For the moisture equivalent determinations 
we are indebted to Mr. J. W. McLane. All moisture determina- 
tions are expressed as percentages of the dry weight of the soil 
used. 
The moisture equivalent determinations given in the table 
represent in each case the mean of two determinations. The num- 
ber of wilting coefficient determinations made upon each soil is 
shown in the fourth column, and the mean of these determinations 
is given in the fifth column. The last column gives the ratio of the 
moisture equivalent to the wilting coefficient for each soil. 
It will be seen from an inspection of the table that the soils 
used in the comparison show a wide range in moisture retentiveness, 
8 Soil Survey Field Book, 1906. : 
RE A ns or A raeEwaer a et Det ee ee eae Sa 
ONES ESS) USS ys SO ee ee 
