CROP ROTATION IN RELATION TO SOIL PRODUCTIVITY 63 
the result of any such rigid rotation and fertilization systems as are 
practiced on the Agdell field. 
This discussion of rotation assumes diversification in cropping. 
Statistics show that cotton, corn, wheat, oats, and hay each occu- 
pies more than 30,000,000 acres on American farms. Together these 
five crops occupy 87.5 per cent of the total crop area of the country 
(17, p. 2). It is out of the question to discuss the value of rotation 
in relation to permanent productivity from the point of view of a 
rotation that is suitable for all sections of the country, because it is 
evident that a 5-year rotation including the five major crops is im- 
possible in a country so large and with such wide differences in 
climatic and agricultural conditions. However, a study of the dis- 
tribution of the important crops grown in the agricultural regions of 
the United States, within the several States, and within smaller 
areas or districts, will show that flexible rotations 4 are possible in 
most sections or localities or on most farms, the possible exception., 
being on specialized, 1-crop farms and in certain dry-land farming 
sections where, because of scant rainfall, a change from a 1-crop 
system, as from wheat and fallow, is practically impossible or 
economically hazardous. 
As regards chemical or commercial fertilizers, their use is becoming 
more and more general, this being the natural outcome as virgin, 
arable soils are kept longer and longer under cultivation. The 
richer the soil and the more skillful the farmer in his soil management, 
the longer virgin soils can produce profitable crops without the use 
of fertilizers. History shows, however, that sooner or later exhaustive 
cropping brings the best of virgin soils to a point where the use of 
manure or some other form of fertilizer becomes necessary for the 
realization of profitable yields. 
In the light of this study it may well be assumed that on productive, 
virgin soils a good rotation may for years prove more effective in 
maintaining yields than manure or single, mixed, or even complete 
commercial fertilizers. In time, however, the fertilizers give more 
and more positive results, until they approach the effectiveness of 
rotation, as the experiments on corn at Urbana, 111., seem to show. 
Cultivation in fertility maintenance, especially as regards the 
preparation of the seed bed, is generally recognized as fundamental, 
regardless of soil, climate, or economic conditions. 
In this brief survey directing attention to the general importance 
of crop rotation, the use of fertilizers, and cultivation, it must be 
recognized that on these three practices, mainly, the maintenance of 
soil productivity generally depends. The relative importance of 
each practice will necessarily vary on the various farms or soils, their 
relative efficiencies being determined by such factors as the quality 
of the soil, the character of the rotation, and the kind and quantity 
of fertilizers used. When conditions are such as to cause cultivation, 
rotation, and the use of fertilizer all to become positively effective, 
the resultant yields may be regarded as consisting of three parts. 
one portion representing the effectiveness of cultivation alone, a 
second portion to be credited to the effects of rotation, and a third 
portion representing the effectiveness of fertilizers. Thus, assuming 
* A flexible rotation may be defined as a rotation in which different kinds of crops, such as intertilled 
small-grain, and hay or legume crops, follow in definite order as named, but in which the number of years 
of growing the specific crops is not fixed. 
