104 BOTANICAL GAZETTE [FEBRUARY 
Many of the Illinois experiment fields include three check plots 
in each series. These check plots are all untreated and are only a 
short distance apart, yet some of them differ widely in crop yields. 
It is reasonable to assume that neighboring plots receiving the 
same fertilizer treatment would differ as widely. These differences | 
due to factors not under the control of the investigators make the 
probable error large, and when only one plot of each treatment is 
used, the differences between plots with different treatments must 
be great before one can assume that the treatment has been effec- 
tive. Where the differences are as great as in the work of REIMER 
and Tartar (58) and of Otson (54), there is no doubt that the 
treatment has been effective, but in many of the field experiments 
in different parts of the country the differences are too small to 
justify the conclusions drawn from them, as the probable error is 
so great. Where a number of plots of each treatment are used, the 
uncontrollable factors tend to neutralize each other and the prob- 
able error is reduced. As the number of plots of each treatment 
increases, smaller average differences are necessary to be signifi- 
cant. It seems probable that three plots of each treatment are 
necessary if satisfactory results are to be obtained. In the past 
investigators have had a tendency to scatter field experiments over 
a number of widely separated fields on the same soil type. It seems 
probable that more satisfactory results would be obtained if the 
work were confined to one field on each soil type, and each field 
had from three to five plots of each treatment. 
Summary 
1. Composite soil samples from Indiana, Kentucky, Michigan, 
Ohio, and Wisconsin were analyzed for total sulphur, total phos- 
phorus, and volatile matter (loss on ignition), and cooperative 
fertilizer experiments with gypsum were conducted in fields in 
Indiana and Kentucky. 
2. The analytical data show a general relation between the sul- 
phur content and loss on ignition in soil samples from the same soil 
type or closely related soil types, but the relation is not apparent 
when different soil types are compared. 
