CROP ROTATION IN RELATION TO SOIL PRODUCTIVITY 6 
There is another aspect of the question regarding the effects of 
rotation on crop yield, which is perhaps the more important one; 
namely, the value of crop rotation in relation to the national food- 
production problem. From this point of view, cultivation of the 
soil, crop rotation, and the use of fertilizers are still to be regarded 
as the dominant farm practices not only in maintaining but in increas- 
ing our Nation's food supply. As regards its maintenance, much 
will depend on good cultivation and judicious use of fertilizers. 
But what of the value of crop rotation? As regards increasing our 
food supply, how much can the average yield of wheat or corn, for 
example, be increased by improving present methods of cultivation? 
How much additional increase can be effected by establishing more 
systematic crop rotation or by improving the rotations now being 
practiced? And how much can these increases be augmented still 
more by a more general and intelligent use of manure and chemical 
fertilizer ? 
PRIMARY OBJECTS OF STUDY 
Inasmuch as any attempt to answer these questions, especially as 
regards crop rotation, without any specific knowledge of the value 
of rotation may result in bare speculation, it seems logical to study 
experimental data with a view, in each case (1) to determinining 
some more or less definite measure of the value of rotation in crop 
production, and (2) to comparing its beneficial action with that of 
fertilizers in maintaining and increasing soil productivity. These, 
briefly stated, are the primary objects of this study. 
METHOD OF STUDY 
Whatever method may be applied to experimental data in evalu- 
ating absolute or relative values of crop rotation and the use of 
fertilizers in crop production and in maintaining and increasing soil 
fertility, at least four conditions must be met experimentally before 
such values can be ascertained: (1) The value of the effects of crop 
rotation and the use of fertilizers must be based on long-continued 
fertility experiments; (2) in any particular case a crop must be 
grown with and without fertilizers in continuous culture and in rota- 
tion and on the same type of soil; (3) comparable yields must repre- 
sent the same seasonal effects ; and (4) the fertilizer treatment given 
a crop in rotation should be similar to that given in continuous 
culture. 
The fact that crop rotation is a system of cropping which extends 
over a longer or a shorter period of years necessitates a consideration 
of the long-time fertility tests. A fertilizer may be applied before, 
at, or after planting time, and the results may be measured, in part 
at least, the same year. Such a demonstration can not be made 
with crop rotation. A fertilizer is a definite, physical object which 
can be measured, weighed, and applied to a soil. A rotation, on 
the other hand, is something abstract, in that it possesses no material- 
ity or has no physical reality as does a fertilizer. It is, rather,* a 
concept connoting the attributes of a particular system of cropping, 
whose effects, in general, may be measured only after a series of 
years. 
From the four differently treated plots furnishing the basic data 
for this study are obtained, in each case, the following comparable 
