Environmental Engineering Reference
In-Depth Information
Box 8.4
English Wheat Yields, 1200-1800
The earliest estimates of European wheat yields are derived from information on rela-
tive returns of the planted seed: during the early medieval period these ratios were only
twofold and during the years of inclement weather harvested grain could be less than
the planted amount. By 1200 the ratios ranged mostly between 3 and 4, and could
surpass 5 (Bennett 1935; Stanhill 1976). Conversions of these (usually volumetric)
ratios to mass averages can be done only approximately; they indicate yields of just
over 0.5 t/ha by 1200 and an irreversible doubling to more than 1 t/ha only half a
millennium later.
After reducing the average yield of 0.5 t/ha by at least 10% to account for post-
harvest (storage) losses to fungi, insects, and rodents, and then by a third to be set
aside for the next crop's seed, only about 300 kg of whole grain (or 4.5 GJ) are
available for food consumption (the production of white l our would reduce the mass
by another 15%); a yield of 1 t/ha, a storage loss of just 5%, and a quarter of the
harvest set aside for seed would leave about 700 kg (10.5 GJ) for food. Because in
Europe's traditional agricultural societies as much as 85% of all food energy (at least
8 MJ/day, or nearly 3 GJ/year) came from staple grains, medieval harvests of 0.5 t/ha
could support only about 1.5 person/ha (150/km
2
), while those of around 1 t/ha
(reached by 1700) could sustain between 3 and 4 persons/ha (300-400/km
2
) planted
to cereals.
empire, and Asian doubling was only a bit faster, from the time of China's Han
dynasty to the late Ming period.
And no traditional agriculture was able to produce enough food to assure more
than basic (and overwhelmingly vegetarian) diets even during the best years and to
prevent recurrent periods of hunger and famine that in some societies persisted even
into the nineteenth century (Ireland 1845-1852, China and India 1876-1879 and
1896-1902). Low crop yields also limited the use of draft animals (particularly
horses, whose heavy draft required concentrate feed; in contrast, the much less
demanding cattle could be fed only grass and crop residues) and the production of
animal foods. As a result, the global area of cropland grew only marginally during
the i rst millennium of the Common Era (from 130 to 150 Mha), and it then took
another seven centuries to double, to 300 Mha (HYDE 2011).
If we assume average food yields equivalent to 300 kg of grain/ha in the year
1000 and 400 kg/ha in 1700, the global harvests amounted, respectively, to roughly
50 Mt and 120 Mt of grain equivalent, enough to feed (assuming that plant foods
had supplied 90% of all food energy needs averaging 3.5 GJ per capita per year)
