Geoscience Reference
In-Depth Information
Vegetation cover change, mechanical changes in the uppermost soil layer,
changes in water retention capacity and changes in soil biota due to practices such
as manuring and marling taken together are the most profound change in soil
architectures which happened in the Holocene. This is linked to reverberations in
human societies: sedentary lifestyle, higher population densities, and the develop-
ment of strati
ed societies with uneven distribution of surplus all are in one way or
other connected to soil changes.
Interventions into the nitrogen balance of soils are of particular importance,
because they change the linkage between soils and atmosphere. Earlier nitrogen
sources were limited to naturally existing oxidized (as nitrate) or at least chemically
bound (as ammonia) forms. Airborne nitrogen became available only through the
deposition of nitrous oxides produced by lightning in thunderstorms in minute
quantities. The
rst such intervention was the use of leguminous plants (nitrogen
xing species) as domesticated plants; their ability to bind nitrogen from the air,
thus greatly enlarges the nitrogen pool for cycling. Legumes provide nutrient-rich
food for humans and their livestock; beans, peas and lentils are common around the
world. A major innovation was the much larger in
ux of oxidized airborne nitrogen
produced by the deliberate use of nitrogen
xing species as green manure, by
plowing under the entire plant.
To give but one example of the historical signi
cance of this technique, the
cultivation of nitrogen-xing clover in England dates from the early 17th century.
Although data on the chronology and location of their introduction is patchy we
know for example that by the 1740s about half the farmers in Norfolk and Suffolk
were growing turnips and about a quarter had clover on their farms.
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It seems
likely that it was not until after the mid-eighteenth century that these crops were
having much effect on cereal yields. They were important in enabling some light
lands, like the chalk downlands of southern England and parts of Norfolk, to be
brought under the plow for the
rst time. A third of the increase in arable pro-
ductivity in northern Europe between 1750 and 1850 has been attributed to legumes
such as clover.
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Nitrates were created in the soil by nitrogen-
xing bacteria living in symbiosis
with leguminous roots. The largest reservoir of nitrogen, the air, could however not be
tapped directly before the invention of the catalytic reaction of nitrogen and hydrogen
gas, developed into an industrial process called the Haber-Bosch-Synthesis, patented
in 1910. If we look at human interventions into the nitrogen balance in soils, the
invention of legumes as domesticated plants, the invention of green manure and the
invention of an air-based synthesis of nitrogen are the main turning points.
As to the two other key nutrients necessary for plant growth, the restoration of
potassium by means of wood or other plant ashes was developed in antiquity,
whereas the mining of potassium-containing minerals as well as the mining of
73
Overton (
1984
).
74
Chorley (
1981
).
