CROP ROTATION" IN RELATION TO SOIL PRODUCTIVITY 9 
and from which rotation and fertilizer values in increasing productiv- 
ity are to be measured. 
A concrete illustration may help to make clear the application of 
this method. The same results obtained with manure on corn at 
Columbia, Mo., as used in the previous discussions, will be used. In 
Figure 1 are given the average yields per acre for the period, 1894 
to 1918, in a semidiagrammatic form, showing the comparable aver- 
age yields and their relative positions with reference to the main- 
tenance yield of 33.5 bushels, the 5-year average yield at the begin- 
ning of the experiment, for the period, 1889 to 1893, inclusive. 
It is to be ooserved that cultivation alone (c) fell short 14.5 bushels 
in maintaining the yield obtained at the beginning of the experi- 
ment; the practice of combining the use of fertilizer (manure) with 
cultivation (cf) resulted in a slight gain of 0.3 bushel, above the main- 
tenance yield; the practice of combining rotation with cultivation 
c/r Ores 
cr 0323- 
^ = ^ /*£>?/// T£rt s4MC£ c/ O 33.0 y/£L 
S3.S — S3. S 
cQ/9.0 
Fig. 1. — A representation of the results ofaseries of long-time fertility tests on corn at Columbia, 
Mo., showing the relation of comparable yields to the maintenance and the increasing of soil 
productivity. Small c indicates the effects of cultivation alone; /, the effects of fertilizer 
(manure) ; and r, the effects of crop rotation 
(cr) increased soil productivity by 5.8 bushels per acre; and the 
practice of combining cultivation, rotation of crops, and the use of 
manure (cfr) increased the productivity of the soil by 15.4 bushels 
(48.9-33.5). 
The difference between the yields of 48.9 bushels (cfr) and 19.0 
bushels (c) simply gives a measure of the difference in the producing 
power of the soil as effected by the two different practices indicated. 
On the other hand, the difference between the yield of 48.9 bushels 
(cfr) and 33.5 bushels (maintenance yield) measures the effects of the 
conjoint action of cultivation, rotation, and fertilization in increasing 
soil fertility. Likewise, the true measure of the value of manure in 
increasing productivity during the period of the experiment is not 
the difference between the yields of 33.8 bushels (cj) and 19.0 bushels 
(c), but the difference between 33.8 bushels (cj) and 33.5 bushels 
(maintenance yield). The same argument holds in case of rotation. 
In this particular experiment, it is to be noted that both rotation 
(r) and manure (/) maintained soil productivity. Since the yield to 
60635— 26f 2 
