Environmental Engineering Reference
In-Depth Information
vars stored 1.25-2.8 times as much mass in the residues
as they did in grain. In a crop of medieval European
wheat yielding 750 kg (N content 1.5%) and 2 t of
straw (0.5% N) nitrogen was about equally divided
among the grain and the residue, and the same was true
for pre-1900 Japanese rice. Only a small fraction of crop
residues were directly recycled; most of them were used
as animal feed and bedding (then recycled in manures)
as well as household fuel and raw materials for con-
struction and manufacture. Often crop residues were
simply burned in the field with virtually complete nitro-
gen loss.
Recycling of urine and excrement, practiced in Europe
and in East Asia, can support high yields but only
with much repetitive, heavy labor owing to the time-
consuming handling and treatment of the wastes (provi-
sion of bedding material, cleaning of stalls and sties of
confined animals, liquid fermentation or composting be-
fore applications). They also had low nutritional content
(@0.5% N) and large preapplication and field losses
(60%-80%) of the initially available nutrient (Smil
2001). Effective applications thus required a large mass
of manure. In China in the early twentieth century the
rates averaged about 7.5 t/ha for all farms and surpassed
10 t/ha in small holdings in the rice region (Buck 1937).
Composting and regular applications of other wastes
(from silkworm pupae to canal mud) further increased
the burden of collecting, fermenting, and distributing,
and at least 10% of all labor in Chinese farming was de-
voted to the management of fertilizers.
In the North China Plain heavy fertilization of wheat
and barley was the single most time-consuming part
of human labor (close to 20%) as well as animal labor
(30%-40%) devoted to those crops. Even in China in
the 1980s collection of urban night soil and organic
wastes and their transport to the farm took up 2-4 h/
day for those still willing or forced to continue that rap-
idly declining practice. The intensity of China's recycling
was matched by the earlier European experience. In the
eighteenth century in Flanders the annual application of
manure, night soil, oil cakes, and ash averaged 10 t/ha,
and rates up to 40 t/ha were not uncommon as organic
wastes were brought from cities and towns by a large
waste-handling and transportation industry (Slicher van
Bath 1963). But the highest known applications of
organic wastes, between 50 t/ha and 270 t/ha, of pig
and human excrements took place in South China's
Guangdong dike-and-pond region (Ruddle and Zhong
1988).
Again, a specific example illustrates the magnitude of
rewards. During the 1920s wheat cultivation in a north-
ern Chinese county yielded about 1.4 t/ha and required
307 h of human labor and 248.5 h of animal labor, of
which fertilization took, respectively, 17% and 41% (Buck
1930). Assuming, quite conservatively, that the 10 t of
fertilizer applied per hectare contained only 0.5% N,
only half of which became actually available to the crop,
and that each kilogram of N resulted in additional pro-
duction of 10 kg of grain (Hanson et al. 1982), there is
a yield increment of at least 250 kg (@4 GJ/ha) for an
investment of 36 MJ of additional human energy inputs
and animal feeding cost of 2.5 GJ. No more than about
5% of the latter, if any, would come from grains, resulting
in a net gain of at least 3.8 GJ and a more than 20-fold
return in edible cereal energy, clearly an excellent
benefit-cost ratio.
Green manuring, used in Europe from ancient Greek
and Roman times, and widely employed in East Asia
from about the sixteenth century, relied mainly on
the N-fixing legumes, above all, on vetches (Astragalus,
