34 BOTANICAL GAZETTE [JANUARY 
COMPARISON OF THE ACCURACY OF THE INDIRECT METHODS FOR 
DETERMINING THE WILTING COEFFICIENT 
Since the numerical value of the ratio used in calculating the 
wilting coefficient by indirect methods varies considerably according 
to the method employed, it is necessary for purposes of comparison 
to express the probable error in each case as a percentage of the 
ratio which it affects. This comparison is given in the accompany- 
ing table (table VI). 
TABLE VI 
SHOWING oo COMPARATIVE ACCURACY OF THE RATIOS USED IN THE INDIRECT 
WILTING COEFFICIENT 
PROBABLE ERROR OF MEAN RATIO 
MeEtTHOD Ratio 
Absolute Percentage cf 
value ratio 
Moisture equivalent....... 1.84 +0.013 Soy 
Hygroscopic coefficient... .. 0.68 +0.012 +1.8 
Moisture holding capacity 2.90 +0.06 2.1 
hanical analysis ....... 1.00 +0.025 2.5 
The probable error of the mean ratio shows the degree of uncer- 
tainty that is attached to the value given for the ratio. That is to 
say, if the moisture equivalent series were repeated, the chances are 
even that the mean ratio would fall between 1.827 and 1.853. In 
other words, in a soil having an observed moisture equivalent of 
18.4 per cent, the chances are even that in so far as the accuracy 
of the ratio is concerned the wilting coefficient lies between 9.93 
and 10.07 per cent. This corresponds to an uncertainty of +0.7 
per cent in the value of the wilting coefficient calculated by means 
of the ratio 1.84, as shown in the last column of the table. 
The last column of the table shows the probable error of the 
mean ratio expressed as a percentage of the ratio itself. This 
affords at once a means of comparing the accuracy of the different 
ratios. It will be seen that the probable error arising from the 
uncertainty of the ratio in calculating the wilting coefficient by the 
moisture equivalent method is about 0.7 per cent; by the hygro- 
scopic coefficient method 1.8 per cent, or over twice as great; by 
the moisture-holding capacity method 2.1 per cent, or three times 
as great; and by the mechanical analysis method 2.5 per cent, oF 
nearly four times as great. 
