INDIRECT DETERMINATION OF THE WILTING COEFFICIENT. 69 



were adjusted to give the best calculated values for the wilting co- 

 efficient, but the same proportion among the coefficients was main- 

 tained. The formula used is as follows : 



0.01 sands +0.12 silt +0.57 clay= wilting coefficient. 



In this formula the " sands" refer to the percentage of particles 

 ranging from 2 to 0.05 millimeter in diameter, the "silt" to particles 

 from 0.05 to 0.005 millimeter in diameter, and the "clay" to par- 

 ticles smaller than 0.005 millimeter in diameter. Table XXII * gives 

 the mechanical composition of each of the soil types, the computed 

 value of the wilting coefficient as determined by the above formula, 

 the observed value of the wilting coefficient, and the residuals or 

 differences between the observed and the computed values. 



Table XXII. — Comparison of the observed wilting coefficient with that found by com- 

 putation from the mechanical analysis. 



SoU. 



Types of soil. 











Coefficient. 





Coarse 



Fine 



Silt. 



Clay. 







Resid- 







sand. 



sand. 



Com- 



Observ- 



uals. 











puted. 



ed. 





Per ct. 



Per ct. 



Per ct. 



Per ct. 









60.4 



37.1 



0.8 



1.6 



1.8 



0.9 



+0.9 



28.2 



64.4 



4.7 



3.9 



3.1 



2.6 



+ -5 



35.4 



55.1 



4.8 



4.5 



3.6 



3.3 



+ .3 



29.9 



56.7 



5.0 



8.2 



3.8 



3.6 



+ .2 



33.1 



50.0 



8.6 



7.5 



4.9 



4.8 



+ .1 



2.8 



59.8 



30.2 



6.9 



10.3 



9.7 



+ .7 



3.4 



55.5 



21.8 



19.1 



9.5 



10.3 



- .8 



32.4 



28.8 



26.7 



11.8 



9.9 



9.9 



.0 



15.8 



42.4 



28.7 



12.9 



10.7 



10.8 



- .1 



19.2 



35.6 



30.6 



14.7 



11.4 



11.6 



- .2 



2.0 



48.8 



37.7 



12.3 



13.5 



13.9 



- .4 



3.6 



35.2 



41.4 



14.4 



14.6 



15.2 



- .6 



5.1 



27.0 



35.2 



32.5 



14.5 



16.2 



-1.7 



3.2 



43.7 



45.1 



17.1 



16.0 



16.5 



- .5 



4.4 



20.5 



52.6 



22.0 



16.6 



16.3 



+ .3 



Ratio of 

 observed 

 to com- 

 puted. 



Coarse sand 



Fine sand 



do 



....do 



Sandy loam 



Fine sandy loam. 



Loam 



Sandy loam 



Fine sandy loam . 



do 



Loam 



....do 



Clay loam 



do 



do 



0.50 



.84 



.92 



.95 



.98 



.94 



1.08 



1.00 



1.01 



1.02 



1.03 



1.04 



1.12 



1.03 



.98 



The ratio of the observed to the computed value of the wilting 

 coefficient is given in the last column of the table in order to provide a 

 basis of comparison as regards accuracy with the other physical 

 measurements. The mean ratio 2 is 1, with a probable error of 

 ±0.025. This large probable error is due mainly to soil No. 1, which 

 has a departure no greater than some of the other soils but which on 

 account of its very small wilting coefficient gives a ratio which is 

 widely divergent from the rest of the series. The formula for com- 

 puting the wilting coefficient, when affected with the probable error, 

 then becomes: 



0.01 sands+0.12 silt+0.57 clay =wilti coefficient. 

 1+0.025 



i The writers are indebted to the Bureau of Soils for the mechanical analyses. No mechanical analyses 

 were available for samples Nos. 10 and 13. 



2 In determining the values of the sand, silt, and clay coefficients so as to give a mean ratio equal to unity, 

 soil No. 1 was disregarded, since a better general agreement was obtained in this way. This is virtually 

 what would have happened if the method of least squares had been applied to the experimental data. In 

 all calculations of probable error, however, this soil has been included with the rest. 

 230 



