2 BULLETIN 1377, U. S. DEPARTMENT OF AGRICULTURE 
regardless of the producing power of the soil, whereas the importance 
of subsequent cultivation or intertiDage was not given general recog- 
nition in practical farming until near the close of the eighteenth 
century (14, pp. 148-175). \ 
Crop rotation — the growing of different kinds of crops in recur- 
ring succession on the same land — has been recognized by early 
agricultural scientists to be the foundation of the improvements 
in agriculture which took place in England, in large portions of con- 
tinental Europe and in the United States during the last part of the 
eighteenth and especially during the last century (9, p. 195). The 
benefits to be derived from the growing of leguminous crops in alter- 
nation with the cereals were distinctly recognized by the ancient 
Romans {2, 18) ; and the benefits of growing intertilled turnips or 
root crops in rotation with barley, clover, and wheat were discovered 
about, or after, 1730, in England. 
Fertilizers such as dung, marl, ashes, and green-manure crops 
were used in soil improvement in ancient times; but it was not 
until after about 1840 that chemical or manufactured fertilizers 
were known or received much recognition. In modern times, espe- 
cially in the older agricultural sections, the use of manure or chemi- 
cal fertilizers, or both, is commonly regarded as the paramount 
farm practice to assure successful crop production or to maintain 
the productivity of soils. The term fertilizer, as it is used in this 
bulletin, includes farm manure and chemical fertilizers. 
The fact that the value of fertilizers may be easily and definitely 
demonstrated on certain soils which are in need of special kinds of 
fertilizers or on soils low in producing power because of exhaustive 
cropping, has established the value of chemical fertilizers, partic- 
ularly in sections where fertilization practices have become estab- 
lished. And thus it seems logical to credit the bulk of crop yields 
in 'these sections to the fertilizers used. It is not assumed that the 
value of crop rotation is entirely overlooked. On the contrary, the 
fact that in most of these older sections, where systems of farming 
have become more or less established, crop rotation is commonly 
practiced gives evidence of the recognition of its value. 
The effects of crop rotation on yields are manyfold: Rotation 
aids in controlling weeds and certain crop pests and diseases. It 
may render manure and chemical fertilizers more effective. It 
increases the soil supply of organic matter and nitrogen, improves 
tilth, and conserves the soil reserve of plant nutrients; and the 
different crops in themselves may exert beneficial effects on those 
which follow. 
The total effects of rotation when conjoined with fertilizers may 
be measured by determining the difference in the yields obtained 
with fertilizers in rotation and with the same fertilizers in con- 
tinuous culture; that is, when a crop is grown continuously on the 
same land. The questions now arise: (1) What is the value of crop 
rotation as compared with the use of fertilizers in crop production? 
(2) When a farmer combines the use of fertilizers with rotation of 
crops, do these two farm practices when thus conjoined produce 
additive effects in promoting increases in crop yields? (3) What are 
the comparative values of crop rotation and fertilizers in maintain- 
ing and increasing soil productivity? 
1 Reference is made by number (italic) to "Literature cited" p. 68. 
