8 BULLETIN 1377, U. S. DEPARTMENT OF AGRICULTURE 
The practice of using manure without rotation resulted in an 
increase of 14.7 bushels over cultivation alone, and the practice of 
rotation of crops without the use of manure resulted in an increase 
of 15.1 bushels over cultivation alone. Hence, under the conditions 
of the Missouri test the practice of rotation without the use of 
manure produced practically the same effects as the practice of 
using manure without rotation. 
The practice of conjoining rotation of crops and the use of manure 
resulted in a gain of 25.3 bushels over cultivation alone, which is 
within 5 bushels of the sum of the separate effects of rotation and 
the use of manure. In other words, when under the conditions of 
the experiment the practice of crop rotation and the use of manure 
are conjoined, the effects produced on the crop yield were nearly 
fully additive, or nearly as large as the sum of their separate effects. 
The formula cr — c gives the value for the effects of rotation when 
practiced independently of the use of fertilizers. This value of 
rotation 0), in the experiment cited, is equal to 15.1 bushels of 
increase. 
When the use of manure and rotation are practiced conjointly, 
f rom cfr — cf we derive another value for rotation, being the actual 
increase effected when to the use of manure is added rotation of 
crops. This value for rotation (R) includes not only the effects of 
rotation in itself but, in addition, any effect that it may have on 
the condition of the soil or in increasing or decreasing the efficiency 
of the manure as compared with its effects when acting in the absence 
of rotation. In this case the value for rotation (R), in the illustra- 
tion used, is equal to 10.6 bushels of increase. 
The formula cf—c gives the value for the effectiveness of the use 
of manure when practiced in the absence of rotation. This value 
for manure (f), in the experiment named, is equal to 14.7 bushels 
of increase. 
From the formula cfr — cr we derive another value for the effects 
of the use of manure, being the actual increase effected when to 
rotation is added the use of manure. This value for the use of 
manure (F) includes not only its direct nutritive value (be it greater 
or less than when it is used without rotation), but, in addition, any 
interactive effects due to conjoining the use of manure and rota- 
tion of crops. In this case the value for the use of manure (F), 
in the illustration used, is 10.2 bushels of increase. 
THIRD METHOD 
The method selected for evaluating the beneficial effect of rota- 
tion and fertilizers involves only a comparison of the effects of the 
practices of rotation and of the use of fertilizers on crop yields, and 
it does not show the effects resulting from rotation and the use of 
fertilizers in relation to the maintenance and increasing of soil pro- 
ductivity. Tins necessitates a somewhat different method of study. 
Maintaining soil productivity implies holding or keeping up the 
productive power of the soil. In case of any one of the long-time ex- 
periments, the average yield of a crop at the beginning of the experi- 
ment must be taken as the yield to be maintained, or it may be 
termed the maintenance yield, from which the values of rotation 
and the use of fertilizers in fertility maintenance are to be reckoned, 
