CROP ROTATION IX RELATION TO SOIL PRODUCTIVITY 5 
this study results from complete fertilizers have been selected, 
because it is 10 be observed that, in these long-time experiments, 
complete fertilizers are more consistently effective in maintaining 
and increasing soil productivity than those in which one or two of 
the major fertilizing constituents are lacking. Thus it seems logical, 
especially since results from manure are also included, to compare 
the value of rotation with the effectiveness of those chemical ferti- 
lizers which are most efficient in soil-fertility maintenance. The 
details concerning the composition of the fertilizers and the quantity 
applied are brought out in the discussion of each of the experiments 
which have been selected for study. 
EXPERIMENTS SELECTED FOR STUDY 
From all published, official data it has been possible to select six 
groups of long-time experiments (Table 1) which satisfy the condi- 
tions discussed in the foregoing paragraphs. They are named in 
chronological order according to the dates when comparable yields 
begin, though in some cases the experiments were begun several 
years before the dates indicated. The results of these experiments 
are discussed in the order named. 
Table 1. — Long-time continuous-culture and rotation experiments 
Location > Dates of .comparable Crops involved 
Rothamsted, Hertford County, England ; 1851 to 1921, inclusive.. Wheat and barley. 
Columbia, Mo 1889 to 1918, inclusive.. Indian corn, oats, wheat and timo- 
thy. 
Wooster, Ohio 1894 to 1918, inclusive.. Indian corn, oats, and wheat. 
Germantown, Ohio ' 1903 to 1918, inclusive.. Tobacco. 
Urbana, El. (Morrow plots)... 1904 to 1917, inclusive.. Indian corn. 
Florence, 5. C... > 1914 to 1919, inclusive.. Cotton. 
DISCUSSION OF EVALUATION METHODS 
In evaluating the effects of rotation and of fertilizers on crop yields 
and in maintaining and increasing soil fertility, at least three methods 
suggest themselves. A discussion of these methods follows. 
In order to make the discussion concrete, the average of the results 
obtained with farm manure on corn at Columbia, Mo., will be used, 
as follows : 
Yield without manure and rotation (cultivation alone, c) — 22.4 
bushels. 
Yield with manure but without rotation (c and fertilizer,/) — 37.1 
bushels. 
Yield in rotation without manure (c and rotation, r) — 37.5 bushels. 
Yield in rotation with manure (c, r, and/) — 47.7 bushels. 
For convenience, continuous culture without fertilizer or manure 
will be called simply cultivation, including the tillage necessary in 
the preparation of the seed bed and any subsequent cultivation, 
whose effect will be indicated by small c. Rotation and fertilizer 
effects will be indicated by small r and/, or capital R and F : respec- 
tively, indicating different values for rotation and fertilizers. In 
all cases the effects of cultivation, rotation and fertilizers will be 
evaluated in terms of yield units, as absolute values, representing 
either actual yields or increases effected. 
