6 BULLETIN 1377, U. S. DEPARTMENT OF AGRICULTURE 
The increase effected in the yield of a particular crop by rotation 
alone when it is combined with cultivation may not be equal to the 
increase effected when rotation is added to the combined practices 
of cultivation and the use of fertilizer. In like manner the increase 
effected by the use of fertilizer may differ. Thus, small r will indi- 
cate the effects of rotation when it is combined with cultivation 
alone, and capital R will indicate its effects when rotation is added 
to the combined practices of cultivation and the use of fertilizer. 
The value for R (with fertilizer) may be either greater or less than 
the value for r (without fertilizer) . In a similar manner, small / 
will indicate the effects of fertilizer when it is used without rotation, 
and capital F, the effect when the use of fertilizer is added to rota- 
tion. In the data which follow, it will be shown that the value for 
F (with rotation) is often greater than the value for / (without 
rotation) . 
The different values for rotation and for the use of fertilizer may 
be illustrated in the following manner, using the same results with 
manure on corn at Columbia, Mo. : 
Case I 
Bushels 
Yield in continuous culture without any fertilizer (c) 22. 4 
Gain effected by use of manure in absence of rotation (/) 14. 7 
Additional gain effected when to use of manure is added rotation (R) 10. 6 
Yield resulting by adding rotation to the use of manure 47. 7 
Case II 
Bushels 
Yield in continuous culture without any fertilizer (c) 22. 4 
Gain effected by rotation in absence of manure (r) 15. 1 
Additional gain effected when to rotation is added the use of manure (F) _ _ 10. 2 
Yield resulting by adding the use of manure to rotation 47. 7 
FIRST METHOD, INVOLVING ASSUMPTIONS 
Ordinarily, the evaluations of rotation and of fertilizers, as effect- 
ing increases in crop yields, are calculated as follows : 
Manure without rotation, 37.1-22.4; that is, cf—c, or 14.7 
bushels. 
Rotation without manure, 37.5-22.4; that is, cr-c, or 15.1 
bushels. 
Manure and rotation conjoined, 47.7-22.4; that is, cfr-c, or 25.3 
bushels. 
And further, the effect of manure in rotation is ordinarily measured 
as the difference between 47.7 and 37.5; that is, cfr — cr, or 10.2 
bushels. Since the conjoint action of rotation and manure has 
effected an increase of 25.3 bushels in the yield, it seems reasonable, 
in order to determine what portion of this total increase should be 
credited to rotation, to take the difference between 25.3 bushels and 
10.2 bushels, thus arriving at a difference of 15.1 bushels as a value 
for the effect of rotation (R) when conjoined with the use of manure. 
Thus, according to this method, rotation without manure effected 
an increase of 15.1 bushels, whereas manure without rotation effected 
an increase of 14.7 bushels. When acting conjointly, rotation is- 
given a value of 15.1 bushels of increase, and manure, a value of 10.2 
bushels. 
