Feb. 28, 1916 
Correcting for Soil Heterogeneity 
1043 
In the case of a variety test the yield calculated by this contingency 
method may be regarded as the most probable yield of any given plot 
if we suppose the whole field had been planted with a single variety whose 
average yield was the same as the observed average of all the plots. 
The deviation of the calculated yield of a given plot from the mean of 
the field may be taken as a measure of the influence of the soil of that 
plot as compared with the whole field. Thus, if the calculated yield 
of a given plot is 10 bushels above the average of the field, it may be 
taken to mean that the soil on this plot is capable of producing 10 bushels 
more grain than the soil on the field as a whole. 
This figure may be used to correct the observed yield of the corre¬ 
sponding plot. Thus, if the observed yield in a given plot is 80 bushels 
and the calculated yield is 5 bushels above the, average of all the plots, 
then to make the yield of this plot comparable with the average of the 
field it would be necessary to reduce the observed yield by 5 bushels. 
Thus, we may obtain for this plot a “corrected” yield of 75 bushels. 
Likewise, where the calculated yield is below the average, it is necessary 
to add a corresponding amount to the observed yield in order to take 
account of the deficiency in the soil of that plot. 
Expressed in a formula, we may let O equal the observed yield and D 
the deviation of the calculated yield from the mean of the field. Then the 
*‘ corrected * 3 yield = 0 —D 
In fields where there are comparatively small differences between the 
yield of individual plots the direct method of correcting the yield as 
given above may be used. The corrected yields given in figures 2 and 3 
were obtained by this direct method. 
In the case of variety tests or experiments where there are likely to be 
marked differences between individual plots, it will be better to make 
corrections on a relative rather than an absolute basis. To do this, the 
deviation of the calculated yield from the mean of the field is deter¬ 
mined as before. Next the percentage which each deviation is of the 
mean is determined. Then this percentage of the observed yield is 
added to, or subtracted from, the observed yield to obtain the corrected 
yield. An example will make this clear. Suppose the mean yield of the 
plots in a field is 70 bushels. The observed yield on a given plot is 80 
bushels and the calculated yield of this plot is 77 bushels. Thus, the 
deviation of the calculated yield from the mean is + 7 bushels, which is 
10 per cent of the mean (70 bushels). The corrected yield will then be 
10 per cent less than the observed; or 10 per cent of 80 equals 8 bushels. 
The resulting corrected yield will be 72 bushels. By the absolute method 
the corrected yield would have been 73 bushels. The corrected yields 
given in figure 4 and Table I have been obtained by this method. 
