CROP ROTATION IN RELATION TO SOIL PRODUCTIVITY 11 
Such a study of crop rotation and the use of fertilizers from the 
points of view of maintaining and increasing soil fertility gives added 
value to all long-time experiments involving continuous-culture plots. 
To return to the first question proposed, concerning the compar- 
ative effects of rotation and the use of fertilizer on crop yields: 
The soil of the three named Rothamsted fields consists of " 'rather 
a heavy loam resting upon chalk.' * * * Notwithstanding the 
irregularity of the subsoil, the agricultural character of the soil is 
fairly uniform all over the estate; some fields work rather more 
heavily than others, and the proportion of stones lying on the sur- 
face varies somewhat, but these differences are comparatively unim- 
portant. The soil passes into the subsoil without any sharp line of dis- 
tinction, and the distribution of flints in the subsoil is very irregular, 
while the solid chalk is reached at depths varying between 8 and 12 
feet. 
"In the Rothamsted arable soils * * * there has always been 
sufficient carbonate of lime to keep up a neutral condition and put 
out of action any acid as fast as it was produced. However, it was 
observed later that one of the Rothamsted fields did contain plots 
on which the soil had become acid through the application of ammo- 
nium salts year after year for a long period; this was the Park grass 
field, which is cut for hav every year," a field not included in this 
study U, pp. 24, 25, 292, 298). 
The Rothamsted soil is very old, agriculturally. In 1881 Sir John 
Lawes said of it: "At what period my land was first brought into 
arable cultivation it is impossible to say, but at Rothamsted I have 
records which prove that wheat and other corn crops (meaning small 
grains) were grown 250 years ago upon these same fields which are 
now under experiment; there are, however, no data to show how often 
a field was cropped in succession." (8, p. 12.) 
EXPERIMENTAL DATA 
The data presented in Table 2 show the comparative effects of 
crop rotation and the use of fertilizers on the average yields of wheat 
and barley for a period of 72 years on the old arable soil at Rotham- 
sted, when rotation and the use of fertilizer are practiced independ- 
ently of each other. The first wheat yield in rotation on the Agdell 
field was obtained in the year 1851, and other yields were obtained 
every fourth year up to and including 1919. This gives a record of 
18 crops of wheat grown in rotation, the average yield of which is 
compared with the average yields obtained in the same years on nine 
different plots on the Broadbalk field where wheat is grown in con- 
tinuous culture. Each of the latter averages, therefore, represents 
18 yields obtained during the same years that wheat has been grown 
on the rotation plots. 
Since the experiments with barley in continuous culture were not 
begun until 1852, the first comparable yields of this crop were ob- 
tained in 1853, the second year of barley in rotation. Comparisons 
in average yields of barley are made in a similar manner as in case of 
the wheat. Thus each of the average yields of barley on the Hoos 
field represents 18 crops which were grown during the same years in 
which barley has been grown in rotation. 
