The data column "total nitrogen input" accounts
for the combined nitrogen inputs from manure and commercial fertilizers.
Until the mid 1920's, spreading manure was the primary means of adding
nitrogen to agricultural fields. The amount of nitrogen from
manure for 1865 to 1925 was back calculated from available statistics
on the number of livestock units. By knowing the number of livestock,
researchers in this study were able to estimate the amount of manure
produced. Interestingly though, the nitrogen content of manure
changed during this period of time. To account for this variability,
these researchers found data on the amount of milk produced, and used
a correlation between milk produced and nitrogen content to estimate
the amount of nitrogen in the manure. The correlation they used
was "roughly for every 1000 kg increase in milk production, nitrogen
in manure increased by 20 kg". Hoffman et. al. also accounted
for the loss of nitrogen (in the form of ammonia) as it transfers
from manure to soil. Manure data after 1925 was
obtained from Statistics Sweden. Data on the use of fertilizer
nitrogen was based on sales statistics from Statistics Sweden as described
in the journal article.
In the late 19th century it was realized that
demand for manure nitrogen would exceed the supply. Just before
World
War I, the German researcher Fritz Haber discovered the commercial
process (now referred to as the Haber-Bosch process) to convert ammonia
gas into fertilizer. The heavy use of commercial nitrogen today
is a two edged sword: modern levels of agricultural production
could not be sustained without it, yet studies demonstrate negative
impacts on water quality and ecosystems, especially as nitrogen leaches
from land to water sources. As described in the reference article,
"water quality data from major European rivers, such as the Rhine
and the Daugava, indicate that a major increase in riverine loads
of N coincided with the increased use of commercial fertilizers during
the first decades of the post-war period".
It's interesting to compute the harvest-nitrogen ratios between
1865 and the present. These ratios (and the concavity of the
graph) indicate that harvest totals are increasing at a slower rate
as nitrogen input increases. Other increases in agricultural science
(genetic engineering, irrigation, use of pesticides, etc.) should
make the harvest/nitrogen ratio increase over time. Perhaps
a different nutrient is in short supply , or soil quality is being
reduced. There is a good power function fit to the data (very
close to a square root function).
Reference: "Leaching of nitrogen in Swedish agriculture - a historical perspective",
by Hoffman, Johnsson, Gustafson and Grimvall, Agriculture, Ecosystems & Environment, vol. 80 no. 3.
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