Environmental Engineering Reference
In-Depth Information
subsidies result in higher productivities, then the conver-
sion efficiency of a cropping system is clearly increasing.
This trend can be illustrated by data on corn, the leading
U.S. crop, heavily subsidized. In 1945 average subsidies
of nearly 6 GJ/ha helped to produce about 2.2 t of grain
(conversion efficiency merely 0.3% when assuming PAR
of 11 TJ/ha). By 2003 subsidies of about 18 GJ/ha
aided in harvesting 9 t/ha (conversion efficiency 1.23%).
The energy subsidy rate had tripled, but the efficiency of
converting solar radiation into harvested grain had more
than quadrupled.
A second key error often made in deploying energy ra-
tios is a simplistic focus on energy output. Cropping aims
to maximize the productivity of particular cultivars, but
not the conversion efficiency of sunlight into phytomass.
If the latter were the case, we would cultivate only C
4
species: silage corn in temperate regions (whole plant
harvested and used) and sugarcane in the tropics (highest
producer of edible phytomass with year-round growth).
Crops are clearly grown not just for their gross energy
content but for their unique combinations of nutrients
(carbohydrates and proteins in cereal and leguminous
grains, vitamins and minerals in fruits and vegetables);
processing potential (gluten-rich wheat flour has excel-
lent dough-making properties, which are entirely absent
in cornmeal); palatability, storability (cereals vs. tubers);
and even the presence of
as a source of vitamin C, minerals, antioxidants, and di-
etary roughage, but most of us eat them simply for their
unique taste. This nonenergetic consideration is even
more prominent when we drink wine. Only terminal
alcoholics may derive most of their energy from it, but
for everyone else drinking wine has nothing to do with
ensuring a daily supply of food energy.
More fundamentally, such reasoning would mean that
a society trying to minimize food subsidy ratios would
have to run on tubers, but these plants have no, or hardly
any, protein or lipids, and they spoil much faster than dry
grains. For this reason, every advanced civilization was
based on cereals, the Incas being the only partial excep-
tion because corn supplemented the dominant potatoes.
Tuber cultivation among the New Guinean shifting gar-
deners has a high energy ratio of 16 (see section 6.1) in
contrast to the low energy ratios (2-5) of modern cereal
agricultures, but few would argue that we should choose
the former practice as the energetic foundation of mod-
ern society. Energy ratios can never capture these qualita-
tive considerations, and hence they are relevant only to
systems that produce energy (wood, ethanol, methanol).
The third fundamental weakness of energy ratios is
their disregard for time-energy and space-energy trade-
offs. Energy subsidies in farming not only produce higher
yields but do so while dramatically cutting strenuous,
tedious human labor and supporting higher population
densities at higher nutritional levels. Is it desirable to pro-
duce staples with energy ratios 15-30 but requiring
heavy exertion involving up to 80% of the labor force,
including most children? Or is it better to produce sta-
ples with ratios 2-5 and requiring mostly light work and
the participation of 2%-10% of the population? These
work and time gains are inevitably accompanied by a
indigestible but beneficial
roughage.
What does it matter if grapes contain barely more
energy than the fuels and electricity needed for their
cultivation (Heichel 1976)? Obviously, the energies
embodied in viticultural production cannot be digested,
and grapes are not cultivated primarily because of their
energy content. A nutritionist may point out their value
