Environmental Engineering Reference
In-Depth Information
about 200 and 500 million people; this compares to global population estimates of
about 300 million people in the year 1000 and 600 million people in the year 1700,
leaving the remainder (respectively, about a third and roughly 15%) to be fed from
foraging, shifting cultivation, and animal grazing. Similar approximations show that
the eighteenth century brought still relatively small but clearly noticeable gains in
average yields and population densities. The global area of cultivated land rose from
300 Mha in 1700 to about 420 Mha in1800, a 40% expansion, while the popula-
tion increased from 600 million to nearly one billion, close to a 65% gain, and
hence a 20% decline in the per capita availability of arable land, which implies a
similar increase in average productivity.
Expanded cultivation also produced more crop residues. Annual seed crops had
evolved to be tall (Donald and Hamblin 1984): traditional rice varieties grew to be
up to 1 m tall, wheats as much as 1.5-1.8 m, corn up to 3 m, and sorghum to
around 4 m. This tallness was valued because their stems and leaves provided
residual phytomass that could be used as animal feed and bedding, as fuel, and as
raw material, and hence they were usually harvested as carefully as the crop itself.
Crop residues make inferior fuels: their bulk and their relatively low energy content
mean that i res require a large volume of the material and frequent stoking, but they
were the only readily available phytomass in deforested regions.
Traditional cultivars had invariably low harvest index (HI), the ratio of crop yield
and the total aboveground phytomass, including harvested parts and residual stalks,
stems, twigs, leaves, or pods. The HI for traditional wheat cultivars was as low as
0.2-0.3, for rice cultivars no higher than 0.36 (Donald and Hamblin 1984). Those
residues that were not removed to be used elsewhere were left on the ground to
decay naturally (sometime after they were grazed by animals), but a more common
choice was either to plow them in to replenish soil organic matter or burn them.
Although crop residues are low in nitrogen (usually no more than 0.5% of dry
weight), some straws are relatively high in potassium, and in any case, their recycling
is a rich source of organic carbon.
Farming intensii cation continued in most European countries after the recovery
from an overproduction-induced depression in the early nineteenth century. Two
German examples illustrate these changes (Abel 1962). In 1800 about a quarter of
German i elds were fallowed, but the share was less than 10% by 1883. The average
annual per capita meat consumption was less than 20 kg before 1820, but it was
almost 50 kg by the end of the century. Three-crop rotations were replaced by a
variety of four-crop sequences: in a popular Norfolk cycle, wheat was followed by
turnips, barley, and clover, and the practice of six-crop rotation was also spreading.
