| | |
'54 and '55 |(20-1/2)|54-7/8|49-3/8|47-5/8
------------------------------------+--------+------+------+------

The yield of barley after turnips is less than it is after grain crops,
and it is evident that this is due to a lack of available nitrogen in
the soil. In other words, the turnips leave _less_ available nitrogen in
the soil than grain crops.

After alluding to the facts given in the foregoing table, Messrs. Lawes
and Gilbert say:

"There is evidence of another kind that may be cited as showing that it
was of available nitrogen that the turnips had rendered the soil so
deficient for the after-growth of barley. It may be assumed that, on the
average, between 25 and 30 lbs. of nitrogen would be annually removed
from the Rothamsted soil by wheat or barley grown year after year
without nitrogenous manure. But it is estimated that from the
mineral-manured turnip-plots there were, over the 10 years, more than 50
lbs. of nitrogen per acre per annum removed. As, however, on some of the
plots, small quantities of ammonia-salts or rape-cake were applied in
the first two years of the ten of turnips, it is, perhaps, more to the
purpose to take the average over the last 8 years of turnips only; and
this would show about 45 lbs. of nitrogen removed per acre per annum. An
immaterial proportion of this might be due to the small amounts of
nitrogenous manures applied in the first two years. Still, it may be
assumed that about 1-1/2 time as much nitrogen was removed from the land
for 8, if not for 10 years, in succession, as would have been taken in
an equal number of crops of wheat or barley grown without nitrogenous
manure. No wonder, then, that considerably less barley has been grown in
3 years after a series of mineral-manured turnip-crops, than was
obtained in another field after a less number of corn-crops.

"The results obtained in Barn-field afford a striking illustration of
the dependence of the turnip-plant on a supply of available nitrogen
within the soil, and of its comparatively great power of exhausting it.
They are also perfectly consistent with those in Hoos-field, in showing
that mineral manures will not yield fair crops of barley, unless there
be, within the soil, a liberal supply of available nitrogen. The results
obtained under such very different conditions in the two fields are, in
fact, strikingly mutually confirmatory."

CHAPTER XXX.

MANURES FO