CROP ROTATION IX RELATION TO SOIL PRODUCTIVITY 57 
(4) In general, crop rotation is nearly 90 per cent as efficient as 
the use of fertilizer in effecting increases in the yields of wheat, corn, 
-and oats, above check-plot yields in continuous culture. 
(5) Considering values based on comparable results, the average 
relative value of crop rotation is higher when it is based on the effec- 
tiveness of stable manure than when based on the use of chemical 
fertilizers — being 67 per cent greater under conditions when rotation 
and the use of fertilizer are practiced apart from each other and nearly 
58 per cent greater under conditions when one practice is conjoined 
with the other. This indicates that larger increases in crop yields 
have been effected by the applications of chemical fertilizer made on 
the experimental plots than by the applications of manure. 
Excluding clover, timothy, and cowpeas, the average relative 
value of rotation is practically 20 per cent higher on soils whose 
reactions have been altered or changed by liming, than on acid soils. 
7 Except in case of barley at Rothamsted, the conjoint effects 
of rotation and the use of fertilizers are additive — being often more 
than fully additive. 
VALUE OF CROP ROTATION AND OF THE USE OF FERTILIZERS IN 
MAINTAINING AND INCREASING SOIL PRODUCTIVITY 
Thus far in this study, crop-rotation values have been calculated 
from the differences in yields obtained on fertilized and unfertilized 
plots in continuous culture and in rotation — in each case the increases 
over the yields in continuous culture being the basis for calculation. 
The discussion which foUows is of rotation and fertilizer values con- 
sidered from the points of view of maintaining and increasing soil 
productivity under the conditions of the same experiments described 
m foregoing paragraphs. 
This second method, as stated before, necessitates the determining 
of the natural producing power of the soil at the beginning of each 
experiment, which is termed the maintenance yield. This yield is 
taken as the basis from which are calculated the values for crop rota- 
tion and the use of fertilizers in maintaining and increasing soil 
productivity. 
It was thought best to determine the maintenance yield in each case 
by taking the 5-year average at the beginning of the test or before 
the experiment was begun. 
MAINTAINING SOU PRODUCTIVITY 
In Table 52 are summarized the data concerning the relative value 
of crop rotation in maintaining soil productivity, as based on the data 
given in the foregoing tables. In the fifth column are given, in each 
case, the years selected for the 5-year average — these being considered 
the best that can be obtained from the data given in the publications 
previously referred to. The South Carolina experiments are omitted 
from the table, because they cover only a short period; and the experi- 
ments on tobacco at Germantown. Ohio, are omitted because the 
annual yields at the beginning of the experiment do not appear to 
have been published. 
