* ::•:; 



INDIRECT DETERMINATION OF THE WILTING COEFFICIENT. 57 



in perforated cups and moistened with an amount of water in excess 

 of the quantity they can hold in opposition to the centrifugal force. 

 After standing 24 hours the cups are placed in a centrifugal machine 

 which is operated at a constant speed so chosen as to exert a force 1,000 

 times that of gravity upon the soil moisture. Each soil then rapidly 

 loses water until the capillary forces are increased sufficiently to estab- 

 lish equilibrium with the centrifugal force employed. The moisture 

 content of each soil is now not only in equilibrium with a force 1,000 

 times that of gravity, but is also hi capillary equilibrium with every 

 other soil which has been similarly treated, so that if the soils are 

 placed in capillary contact in any combination whatever, no move- 

 ment of water from one soil to another will occur. The moisture con- 

 tent of each soil under these conditions is the moisture equivalent of 

 that soil. This method provides a means of determining and com- 

 paring the retentiveness of different soils for moisture when acted 

 upon by a definite force, which is measured in absolute terms and is 

 reproducible within narrow limits. 



Table XYII gives 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. 1 The soils are 

 arranged in the order of increasing moisture equivalents. For the 

 moisture-equivalent determinations the writers are indebted to Mr. 

 J. W. McLane. All moisture determinations are expressed as percent- 

 ages of the dry weight of the soil used. 



Table XVII. — Relation of the wilting coefficient to the moisture equivalent of soils 



ranging from sand to clay. 





Types of soil. 



Moisture 

 equiva- 

 lent. 



Coefficient. 



Ratio of 



Soil. 



Xumber 

 of deter- 

 mina- 

 tions. 



Average. 



equiva- 

 lent to 

 coeffi- 

 cient. 



7 





1.6 



4.7 



5.5 



6.7 



9.7 



11.9 



18.1 



18.9 



19.6 



19.9 



22.1 



25.0 



27.0 



27.4 



29.3 



30.0 



30.2 



11 0.9 

 16 5.6 



1 75 



2 





1.81 



8 



do 



3 



3.3 



1.67 



9 



do 



2 

 9 

 3 



13 

 3 

 1 

 1 

 1 



12 

 1 

 2 



3.6 

 4.S 

 6.3 

 9.7 

 10.3 

 9.9 

 10.8 

 11.6 

 13.9 

 15.2 

 14.fi 



1.86 



3 



Sandv loam 



2.02 



10 



do 



1.89 



4 



Fine sandv loam 



1.87 



12 





1.83 



A 



Sandv loam 



1.98 



B 



Fine sandy loam 



1.84 



c 



do 



1.90 



5 



Loam 



1.80 



D 



do 



1.78 



13 



Clav loam 



1.88 



H 



do 



4 16.2 

 1 16.5 

 16 16.3 



1.81 



E 



do 



1.82 



6 



do 



1.85 





Mean 















1.84 













± .013 

















Soil Survey Field Book, Bureau of Soils, U. S. Dept. of Agriculture, 1906, pp. 17-18. 



230 



