10 BULLETIN 1377, U. S. DEPARTMENT OF AGRICULTURE 
be maintained is 33.5 bushels, the full effectiveness of rotation alone, 
in its relation to fertility maintenance, may be expressed as 117.3 
per cent; and that of manure alone, 100.9 per cent. Thus the rela- 
tive value of rotation, as compared with manure, in maintaining 
productivity may be expressed as 116.2 per cent. 
As regards increasing soil productivity, when the use of manure is 
combined with cultivation the increase in yield, resulting from these 
combined practices, is only 0.3 bushel (33.8—33.5). When rotation 
is combined with cultivation the increase is 5.8 bushels. But when 
rotation and the use of manure are conjoined, the increase in yield is 
15.4 bushels, which is nearly 152.5 per cent greater than the sum of 
the increases resulting from rotation and fertilization when practiced 
independently of each other. 
The long-time experiments included in this study have to do with 
six major crops, and the average yields at the beginning of the experi- 
ments vary from rather low to medium — yields which are accepted 
as indicating the fertility of soils that have gone through the " vir- 
gin" period of cropping, and which make possible a study of the 
values of crop rotation and the use of fertilizers in increasing soil 
productivity. 
EFFECTS OF CROP ROTATION AND THE USE OF FERTILIZERS ON 
CROP YIELDS 
ROTHAMSTED EXPERIMENTS WITH WHEAT AND BARLEY 
The more systematic fertility experiments at Rothamsted were 
begun in 1843; those with wheat grown in continuous culture were 
begun on the Broadbalk field in 1843, and those with barley in con- 
tinuous culture, on the Hoos field in 1852. The rotation experi- 
ments were begun on the Agdell field in 1848 — the rotation consist- 
ing of rutabagas (Swedish turnips), barley, clover (or beans) or fal- 
low, and wheat (4, pp. 31, 70, 190). In this study only the four- 
crop rotation has been considered: Rutabagas, barley, legumes, and 
wheat, grown in the order named. From the above dates it is to be 
observed that comparable wheat yields begin with the year 1851, 
and those of barley with 1853. Since the rotation plots are not 
repeated, wheat and barley are grown every fourth year on the Agdell 
field; on the Broadbalk and Hoos fields, respectively, they are grown 
every year. On the rotation plots rutabagas receive all the ferti- 
lizer; on the continuous-culture plots, wheat and barley are ferti- 
lized annually. 
Having determined the values of crop rotation and of fertilizers 
in increasing soil productivity, these values, although calculated 
from the comparatively low maintenance yields of the long-con- 
tinued experiments, should serve just as well in emphasizing the 
values of rotation and of fertilizers in fertility maintenance under 
conditions of higher productivity. 
One more word in reference to rotation and fertilizers in fertility 
maintenance: Just how long an experiment should run to determine 
the true values of rotation and the use of fertilizers in maintaining 
productivity is difficult to state; probably 30 or 40 years would suf- 
fice. More carefully planned experiments seem necessary and advis- 
able, to enable the gathering of more facts on a problem which is so 
intimately associated with the Nation's food-production problem. 
