1881 .] 
AMERICAN AGRICULTURIST. 
471 
Eolation of Crops. 
BY DR. MANLY MILES, DIRECTOR OP EXPERIMENTS, 
“HOUGHTON FARM,” N. Y. 
In a well planned system of farming, the 
subject of crop rotations should be carefully 
considered, as one of the essential elements 
of success in its highest and best sense. It 
seems to be the prevailing opinion that the 
alternation of crops, in systematic order, is a 
modern invention that was gradually de¬ 
veloped as a direct result of the applications 
of science to the art of agriculture. The early 
writers on agriculture, even from the times 
of the Greeks and Romans, have, however, 
quite uniformly urged the advantages of a 
succession of crops from the teachings of ex¬ 
perience. They were satisfied that a variety- 
of crops grown in succession, all other con¬ 
ditions being equal, would give a greater 
aggregate yield than could otherwise be ob¬ 
tained. The reasons for the success of the 
system could not, it is true, be given, but 
practical men were fully agreed in urging its 
importance, and many systems of rotation, 
more or less perfect, were planned, some of 
which became the prevailing rule of farm 
practice in particular localities. That these 
practical rules of alternating crops of differ¬ 
ent habits and modes of growth are based on 
correct, but not fully explained, principles, 
has been shown by direct experiment. 
At Rothamsted, where many lines of ex¬ 
perimental investigation have been so suc¬ 
cessfully conducted for many years, Drs. 
Lawes and Gilbert obtained nearly as much 
wheat in eight crops, alternated with eight 
crops of beans, as in sixteen crops of wheat 
grown consecutively, without manure, in 
another field ; and also nearly as much whea^ 
as in eight crops alternated with bare fallow. 
When it is remembered that Nitrogen, as a 
fertilizer, gave the best results with wheat in 
the experiments with different fertilizers ; 
and that the crops of beans alternated with 
the wheat, drew from the soil something like 
twice as much Nitrogen, each year, as a crop 
of wheat on the plats where it was continu¬ 
ously grown, the results are not only remark¬ 
able, but of great practical interest. The 
yield of wheat each year was, in fact, nearly 
doubled by its alternation with a crop that 
made a draft upon the soil of more than 
twice as much Nitrogen, the very element 
of fertility that gave the best results in its 
own growth. Experiments like this are val¬ 
uable, as they show conclusively that what 
are called the exhausting effects of a crop 
are not to be measured exclusively by what 
is removed from the soil. 
Many theories have been advanced to ex¬ 
plain the well-established influence of one 
crop upon the growth of another; but as the 
laws of plant growth became better known, 
and greater exactness in the means of investi¬ 
gation were discovered, they were found in¬ 
sufficient to account for all the observed 
facts, and even at the present time a com¬ 
plete and satisfactory theory of rotations is 
wanting. When it was observed that the 
yield of a grain crop was diminished, when 
grown continuously on the same land for a 
number of years, and that a marked increase 
of the crop was obtained after some other 
crop had been grown, the idea that the soil 
was “tired” so far as the particular crop 
was concerned, and needed “resting,” be¬ 
came the accepted explanation. In 1566 
Camillo Tarello presented, to the Senate of 
Venice, a plan of an improved system of 
agriculture, in which he urged the import¬ 
ance of better cultivation of the soil; an in¬ 
crease of cattle food for a better supply of 
manure; and the ‘ ‘ resting ” of the soil for grain 
crops by alternating them with the grasses 
and clover. The next theory worthy of par¬ 
ticular notice was presented by De Candolle, 
who assumed that plants threw off excretions 
from their roots that poisoned the soil for 
the same species of plant, but served as 
nutritive material for other plants. This ex¬ 
cretory theory, although for the time a popu¬ 
lar one, was finally disproved, and Liebig’s 
mineral theory, as it was called, was quite 
generally accepted as giving the best explana¬ 
tion of the known facts of crop rotations. 
According to this theory, plants derived their 
mineral, or ash constituents, from the soil, 
and obtained from the atmosphere their sup¬ 
ply of Carbon and Nitrogen. As crops dif¬ 
fered in their ash constituents, it was as¬ 
sumed that their demands upon the soil 
would differ, and that this largely explained 
the advantages of rotations. The assumed 
source of Nitrogen was, however, a more im¬ 
portant factor, and crops were classified as 
exhausting when their mineral constituents 
predominated, and as restorative when they 
contained a larger proportion of Nitrogen, 
which they were supposed to draw from the 
atmosphere by means of their broader leaves 
which characterized them. The cereals, in¬ 
cluding our wheat, oats, and barley, were 
thus placed in the group of exhausting crops; 
while clover and other leguminous plants, 
were placed in the restorative group. 
As a full crop of clover removes from an 
acre of soil more of the mineral or ash con¬ 
stituents than a full crop of wheat or oats, 
it is difficult to understand why, in accord¬ 
ance with this theory, the wheat and oats 
should be classed as exhausting crops, and 
the clover and its allies as restorative crops, 
if their mineral constituents are alone con¬ 
sidered. Moreover, in regard to the source of 
the Nitrogen of what are called restorative 
crops, like clover and beans, there seems to 
be evidence that a smaller proportion is 
drawn from the atmosphere than was form¬ 
erly supposed, and that the soil furnishes the 
most important supply. From investigations 
at Rothamsted, and also in France and Ger¬ 
many, it seems probable that the combined 
Nitrogen of the air available for plant growth 
would not exceed eight or ten pounds per 
acre in the year in Western Europe. At Roth¬ 
amsted, however, the wheat on the un¬ 
manured plots gave for 32 years an average 
yield of Nitrogen of 20.7 lbs. per acre ; bar¬ 
ley without manure for 24 years yielded, on 
the average, 18.3 lbs. of Nitrogen per acre; 
and beans, for 24 years, gave an average 
yield of 31.3 lbs. of Nitrogen per acre when 
unmanured, and 45.5 lbs. per acre when 
dressed with mineral manures containing no 
Nitrogen. A plot of clover grown for 27 
years in succession on garden soil, was found 
to yield, on the average, at the rate of nearly 
two hundred pounds of Nitrogen per acre 
each year. As but 8 to 10 lbs. of Nitrogen 
per acre could be obtained from the atmos¬ 
phere by these crops ; and, as direct experi¬ 
ments had proved that nearly, if not quite 
an equal amount of Nitrogen per acre was 
wasted by drainage at Rothamsted, it will 
be seen that these crops must have obtained 
from the soil a very large proportion of their 
Nitrogen. The grain crops, it will be seen, 
removed from the soil but a comparatively 
small amount of Nitrogen, and yet from the 
experiments with fertilizers it had been 
shown that nitrogenous manures had a 
marked influence on their growth. 
On the other hand, the leguminous crops, 
like clover and beans, that yielded a much 
larger amount of Nitrogen per acre, were not 
to the same extent benefited by nitrogenous 
manures; but gave a marked increase in 
their yield of Nitrogen on the application of 
purely mineral manures. The influence of a 
highly nitrogenous clover crop upon the 
grain crop which follows it, so often noticed 
by farmers, was well shown at Rothamsted 
in the experiments with barley, and particu¬ 
lar attention should be given to the amounts 
of Nitrogen drawn from the soil by each of 
these crops. In 1873 a field that had grown 
six successive grain crops, was divided into 
two parts ; on the one a crop of barley was 
grown, and on the other clover. The yield 
of barley gave 37.3 lbs. of Nitrogen per acre ; 
and the clover, of which three cuttings were 
made, yielded 151.3 lbs. of Nitrogen per acre. 
The next year barley was grown on the 
whole field, and its yield of Nitrogen was 
39.1 lbs. per acre after the barley of the pre¬ 
ceding year; and 69.4 lbs. of Nitrogen per 
acre where the clover had been grown ; that 
is to say, the Nitrogen yielded by the barley 
was “30.3 lbs. per acre more, after the re¬ 
moval of 151.3 lbs. in clover, than after the 
removal of only 37.3 lbs. in barley.” 
In regard to this remarkable result, Dr. 
Gilbert says: “It is quite consistent with 
agricultural experience that the growth and 
removal of a highly nitrogenous leguminous 
crop should leave the land in high condition 
for the growth of a gramineous corn (grain) 
crop, which characteristically requires nitro¬ 
genous manuring; and the determination 
of Nitrogen in numerous samples of the 
soil taken from the two separate portions 
of the field, after the removal of the 
barley, and the clover, respectively, con¬ 
curred in showing considerably more Nitro¬ 
gen, especially in the first 9 inches of 
depth, in the samples from the portion where 
the clover had been grown, than in those 
from the portion whence the barley had been 
taken. Here then, the surface soil, at any 
rate, had been considerably enriched in Nitro¬ 
gen by the growth and removal of a very 
highly nitrogenous crop.” 
From the high price and great value of 
nitrogenous manures, and the pertinent sug¬ 
gestiveness of facts like these, would it not 
be best for farmers to ponder well the extent 
to which these crops may be profitably 
grown, as a means of improving the condi¬ 
tion of the soil, and thus increasing the yield 
of other crops that are not as gross feeders. 
i’oml Mud.— There have been a number 
of inquiries of late, as to the value of pond 
mud, and, if of worth as a fertilizer, the best 
means of making it available. Pond mud is 
a very indifferent compound. Some of it is 
rich in vegetable matters, and when properly 
composted with barn-yard manure, or with 
lime, makes a valuable fertilizer. Other mud 
from the bottom of ponds, is not worth the 
time required to get it ready for the land 
needing manure. In general, if the mud is 
made of decayed and decaying vegetable 
matter, and can be reached with little ex¬ 
pense, it will pay to get it out, after which it 
should be composted as above stated. 
