2 WILTING COEFFICIENT FOE DIFFERENT PLANTS. 



uted to accidental experimental errors. If this is true, then the 

 probable error of the calculated wilting coefficient for a given soil can 

 be made to approach the probable error of the ratio as a limit simply 

 by increasing the accuracy and number of the determinations of the 

 observed quantity. 



The probable error of a single determination of the wilting coeffi- 

 cient in our experiments is given below for each method, expressed 

 as a percentage of the wilting coefficient : 



Per cent. 



Moisture-equivalent method ± 2. 9 



Hygroscopic-coefficient method ± 7.1 



Moisture-holding capacity method ± S. 3 



Mechanical-analysis method i 10. 



These errors are not to be applied to any other determinations, 

 since they represent simply the accuracy attained in our particular 

 experiments. If the number of physical measurements made upon 

 each soil had been increased, the error would have been reduced. 



FORMULAS SHOWING RELATIONSHIPS BETWEEN PHYSIOLOGICAL AND 

 PHYSICAL MEASUREMENTS OF MOISTURE RETENTIVITT. 



For convenience in reference the formulas for determining the 

 wilting coefficient in a given soil by indirect methods are here pre- 

 sented in collected form, together with the probabie error, 



m. 14 . rn ■ . moisture equivalent 



U llting coerncient= — i : — . 



1.84 (1±0.007) 



ttu^ ~ „^&.„;^+ hygroscopic coefficient 

 Wilting coefheient= ^ 68 f 1±(x018) ■ 



Wilting c efficieiit= moi5tare - IloldiD g ca P acitr - 21 . 

 2.90 (1±0.021) 



Wilting coefficient^ ' 01 and '^^+ -' 07 clay 

 SUBSIDIARY FORMULAS. 



The subsidiary formulas which follow as the result of the interrela- 

 tionships established are also here included. The probable error has 

 been omitted, since its determination from the formulas would 

 always include the experimental errors of the wilting coefficient 

 determination, due to the fact that the physical measurements are 

 not directly compared. 



DETERMINATION OF THE MOISTURE EQUIVALENT. 1 



Moisture equivalent=wilting coefficientXl-SL 

 Moisture equivalent=hygroscopic coefficient X 2. 71. 

 Moisture equivalent = (moisture-holding capacity — 21) X 0.635, 

 Moisture equivalent=0.02 sand+0.22 silt+1.05 clay. 



i These equations refer to moisture-equivalent determinations made with a centrifugal force equal to 

 1,000 g. (gravity) and should nor be confused with the equation given by Briggs and McLane in which a 

 force of 3,000 g. was employed. 

 230 



