1918] 



Waynick: A Statistical Study of Nitrification in Soil 



255 



tions, it is desired to consider briefly the error in making the readings 

 on the colorimeter due to the inability of the eye to detect small 

 changes in the depth of color between the standard employed and the 

 unknown solutions. All workers in soil chemistry are familiar with 

 the Kenicott- Sargent colorimeter, so that the instrument itself nerds 

 no description here. To check the readings on the unknown solutions, 

 a solution was prepared of the average strength of the residual nitrate 

 determined in the first series reported. Sixteen equal volumes of this 

 solution were taken and treated exactly as the soil extracts were treated. 

 The average amount of nitrate nitrogen found in the sixteen portions 

 was the same as that for the series referred to above, namely. 2.7 

 milligrams. The actual determinations, together with the calculated 

 statistical constants, are reported in table 6. The probable error of 

 the mean of sixteen samples, together with the error to which both 

 larger and smaller numbers of samples are subject, is given in table 7. 

 The calculations have been made by the use of the formulae already 

 ^iven. It is evident that the calculation of the probable error from 

 sixteen, instead of a larger number of samples, makes it less reliable 

 than if a larger number had been used, but since the error from 

 this source is relatively small as compared to the error due to field 

 sampling, it is deemed of sufficient accuracy for the purpose in hand. 

 This error will be referred to as the laboratory error to distinguish it 

 from the error due to sampling. 



TABLE 6 



Coefficient of Variability and Probable Error of Colorimeter Readings 



No. 



1 

 2 

 3 



4 

 5 

 6 



7 

 8 



Nitrate 

 nitrogen 



Milligrams 



2.5 

 2.8 

 2.8 

 2.9 

 2.5 

 2.6 

 2.8 

 2.6 



Deviation 



from mean + 



Milligrams 



0.2 



0.1 



0.1 



0.2 



0.2 



0.1 



0.1 



0.1 



a = .13 ±.01 

 C.V. =4.S±..V, 



II..S'", 



