Agriculture Reference
In-Depth Information
there will be a decent quota of this in the soil. However most of the nitrogen stored in the
field is 'organic' - ie immobilized in organic matter, inaccessible to growing crops, and
only becomes mineralized nitrogen, little by little, at a rate of about 1.5 per cent of the total
nitrogen for his crops. If he uses from this source more nitrogen than is added to it from the
atmosphere every year, then his store of inaccessible organic nitrogen - his capital, so to
speak - is depreciating, year by year, and so is his 1.5 per cent interest, and so therefore are
his crop yields. When the proportion of this declining 1.5 per cent that is being removed
from the field falls to a point equal to all the nitrogen being deposited annually on the land,
the system enters into equilibrium: the farmer is taking the same amount of nitrogen off
the land as is arriving from the atmosphere every year, and crop yields are static. The pat-
tern can be seen in the unfertilized crops of wheat that have been planted continuously at
Rothampsted since 1850 - for the first 50 years yields declined steadily and for the last 100
years they have been stable.
This state of affairs is indefinitely sustainable: but it is not productive, and if population
increases there will eventually be more people than the land can support. By 1350, yields
from the three course system, with its two years of grain and one of fallow, after gradually
declining over several centuries, had bottomed out, at about 800 kilos per hectare - just ten
per cent of a good yield today - with an equilibrium of 3,000 kilos of nitrogen stored in
the soil. According to models developed by Robert Allen, farmers at this period were de-
riving 45 kg per hectare of their nitrogen from the bank of immobilized nitrogen on their
fields, the equivalent of what was absorbed from the atmosphere, plus a pathetic four kilos
last figure is very low, and reflects a state where there was just one hectare of meadow left
for every four hectares of arable. These meadows, like the cropland, had been depleted by
the continual removal of hay crops to the point that they were only giving their minimum
yield of two tonnes of hay per hectare. There was a shortage of meadows because so much
land had been brought into cultivation and mined to the point of minimum yield.
There were, according to most historians, three routes out of this predicament - grass,
beans and clover. First, as we have seen, the Black Death, in 1350, conveniently removed
a third of the population, resulting in fewer mouths to feed, and spare land which could be
put over to pasture. This may have increased the amount of manure to a degree, but not
enough to result in anything other than a small increase in yields. Much more importantly
land managed as pasture (ie which didn't have its hay crop removed every year) absorbed
a surplus of nitrogen every year, gradually building up its bank of organic nitrogen until,
after decades or even centuries, it reached the upper equilibrium state of 7,275 kilos, where
the proportion of the 1.5 per cent of organic lost every year through the less extractive pro-
cess of pasturing equals the amount of fresh nitrogen absorbed.
