Environmental Engineering Reference
In-Depth Information
Analogical trends were seen in all other wheat-growing countries, the only dif-
ference being that many of them, including France and the UK, started their post-
1950s yield rise from higher average levels. Average U.S. wheat yields in the year
2000 were 3.1 times the 1900 mean, and the analogical multiples are 5.8 for France
and 3.8 for China. The average yield of U.S. corn, the country's most important
i eld crop, rose more than i vefold during the twentieth century, from 1.6 t/ha to
8.5 t/ha, and by 2009 even the nationwide mean (and not just the harvests in key
Corn Belt states) had surpassed 10 t/ha. In contrast, China's average corn yield had
only doubled, to 2 t/ha. Japan's average rice yield, already relatively high in 1900
(2.2 t/ha), increased nearly three times before reaching a plateau between 5.8 and
6.5 t/ha during the 1980s. Nationwide and regional yield maxima for rice are at
6-8 t/ha in East Asia and in California. Chinese and Japanese soybean yields grew
about 1.7 times during the twentieth century, while the U.S. yield tripled between
1925 and 2000 (no soybeans were cultivated in North America in 1900).
I have also reconstructed specii c production shares for major crops grown by the
largest agricultures. The basic pattern of U.S. cropping remained relatively constant
during the twentieth century. Corn retained its primacy and output share: in 1900 its
total phytomass accounted for a bit over 40% of the entire U.S. crop production, and
essentially the same share prevailed 100 years later, when the crop's total output had
increased just over threefold compared to 1900. Wheat retained roughly one-seventh
of all output after a 2.7-fold increase in production and remained the second most
important crop until the last decade of the century, when its total phytomass was
equaled and perhaps even slightly surpassed by soybeans.
The rise in soybeans and larger hay harvests (roughly tripling in a century) were
the two primary reasons for the declining share of cereal phytomass in the total
output, from nearly 75% in 1900 to less than two-thirds in 2000. No other crop
has seen such a spectacularly rapid diffusion as soybeans: their American plantings
amounted to a few thousand hectares in the early 1930s, but since the early 1970s
they have been sown annually on more than 20 Mha, producing more than 50 Mt
of seeds a year. The increase in Brazilian soybean production has been even faster,
from a negligible harvest in the early 1960s to more than 20 Mt in the early 1990s.
The global share of sugar crops doubled, to 2% of total phytomass, while the share
of oil crops was nearly halved, to less than 3%.
To i nd the total phytomass harvests, the crop-yield i gures must be adjusted for
the production of crop residues. While the typical moistures of harvested grains have
not changed, specii c harvest indices (HI) were (as already explained) transformed
during the twentieth century. In comparison, changes in the production of other
