CROP ROTATION IN RELATION TO SOIL PRODUCTIVITY 61 
on the increases over the maintenance yield; or the average yield 
obtained at the beginning of the experiments, yet the general results 
as regards the value of rotation in relation to soil productivity all 
point in the same direction. 
The evaluation of crop rotation as calculated from the maintenance 
yield seems to be the more logical and scientific method, especially 
from the point of view of maintaining and increasing the producing 
power of the soil. However, in determining the values for rotation 
and the use of fertilizer when one practice is combined with the 
other, or in determining the relative value of R. the first method 
referred to above has been found to be very useful. 
Taking all the results into consideration, as based on the average 
results of the published data of the long-time fertility experiments 
herein considered, the following important facts as regards the value 
of crop rotation have been brought out : 
(1) Rotation of crops is practically 75 per cent as effective as the 
use of fertilizer in effecting increases in crop yields — being nearly 
90 per cent as effective as the use of fertilizer when the results on 
wheat, corn, and oats, only, are considered. 
(2) As based on the, average yields at the beginning of the experi- 
ments involving fully comparable yields, rotation has been shown 
to be 91.5 per cent as effective as the use of fertilizer in maintaining 
the producing power of the soil. 
(3) In increasing soil productivity, the effects of rotation alone 
may equal or exceed the effects of the use of fertilizer without rotation. 
(4) The effects of rotation and the use of fertilizer apparently are 
not the same, as is shown by the fact that their conjoint effects on 
crop yields are additive — being more than fully additive in most of 
the cases considered. 
(5) In permanent crop production, high productivity levels are 
possible only when rotation and the use of fertilizer are conjoined. 
It is not the object of this study to emphasize primarily the 
important place that fertilizers assume in permanent soil productivity, 
the value of which is fully recognized; but rather to stress the im- 
portance of crop rotation in relation to profitable crop production 
and to show the necessity of conjoining rotation and the use of ferti- 
lizers in the establishment of permanent agriculture. Xo attempt 
will be made in this discussion to formulate any rules for the use of 
fertilizers for greater efficiency; but attention is directed (1) to some 
of the conditions under which these experiments are conducted; 
(2) to a reasonable interpretation of the above facts in relation to 
the Nation's food-production problem; and (3) to a practical appli- 
cation of the principles involved to efficient and profitable soil 
management. 
In all the fertility tests included in this study the experimental 
plans are rigid or fixed, there being no modification whatever in 
the rotation involved in any particular experiment, and but little 
or no alteration is made in the fertilizers applied. Of course, in the 
long-continued fertility tests, rigidity of plans is generally accepted 
as necessary; but in practical experience the farmer usually adopts 
more or less flexible plans. He may alter or even change the rota- 
tion, if necessary, or lie may vary the fertilizer treatment to better 
meet the soil and crop requirements, or he may lime the soil only 
when crop or soil conditions indicate the necessity of liming. A 
