The Energetics of Agroecosystems - Agroecology: The Ecology of Sustainable Food Systems - page 262

The changes that have occurred since World War II
in the way cultural energy is used to produce corn is a
good example of how energy use has changed in agricul-
ture in general. Between 1945 and 1983, corn yields in
the U.S. increased threefold, but energy inputs increased
more than five-fold. In 1945, the estimated ratio of energy
output to energy input in corn was between 3.5:1 and
5.5:1. By 1975, this ratio had declined to between 3.2:1
and 4.1:1, and in the early 1990s, it stood at 2.53:1
(Pimentel and Pimentel, 1997). During the last decade,
this ratio of return has probably remained about the same,
with the continued intensification of inputs to agriculture
balanced by tailoring of inputs to measured crop needs
(“precision agriculture”).
Energetically speaking, industrial cultural energy is of
a higher quality than both solar energy and biological
cultural energy. It is more concentrated — calorie for
calorie, it has a greater capacity for doing work than solar
energy or biological cultural energy; 1 kcal of energy in
the form of fossil fuel, for example, is able to do about
2000 times as much work as 1 kcal of solar radiation.
But even though industrial cultural energy is generally
of very high quality in terms of the work it can do, each
form of this energy varies in the amount of energy that
was required to give it this higher quality state. A kilo-
calorie of electricity, for example, can do four times the
work of a kilocalorie of petroleum fuel, but much more
energy was expended to create the electricity. As the laws
of thermodynamics tell us, humans must expend energy
in order to concentrate energy, and no new energy can be
created in the process. So we are as much concerned with
the absolute amount of work that can be done by each
kilocalorie of a certain form of energy as we are with the
total amount of energy that is expended to transform it
into that energy form. To compare industrial cultural
energy inputs in these terms, we can calculate their energy
costs. Table 18.3 presents a range of energy costs for some
commonly used industrial energy inputs.
Industrial cultural energy is used either directly or
indirectly in agriculture. Direct use occurs when industrial
cultural energy is used to power tractors and transport
vehicles, run processing machinery and irrigation pumps,
and heat and cool greenhouses. Indirect energy use occurs
when industrial cultural energy is used off the farm to
produce the machinery, vehicles, chemical inputs, and
other goods and services that are then employed in the
farming operation. This energy is sometimes referred to
as embodied energy, or emergy, in order to emphasize the
energy costs that are often overlooked when we calculate
the direct energy consumed in a farming system (Odum,
1996). In the typical conventional farming system, about
one third of energy use is direct, and two thirds is indirect.
The production of fertilizers — especial nitrogen fer-
tilizer — accounts for the great majority of indirect energy
use in agriculture. Nearly one third of all the energy used

TABLE 18.3
Approximate Energy Costs of Commonly Used
Industrial Cultural Inputs

Machinery (average for trucks and
tractors)

18,000 kcal/kg

Gasoline (including refining and
shipping)

16,500 kcal/l

Diesel (including refining and
shipping)

11,450 kcal/l

LP gas (including refining and
shipping)

7700 kcal/l

Electricity (including generation
and transmission)

3,100 kcal/kwh

Nitrogen (as ammonium nitrate) 14,700 kcal/kg

Phosphorus (as triple
superphosphate)

3000 kcal/kg

Potassium (as potash) 1860 kcal/kg

Lime (including mining and
processing)

295 kcal/kg

Insecticides (including
manufacturing)

85,680 kcal/kg

Herbicides (including manufacturing) 111,070 kcal/kg

Source: Fluck, R.C. (ed.) 1992. Energy in Farm Production . Vol. 6.
Elsevier: Amsterdam.

in modern agriculture is consumed in the production of
nitrogen fertilizer. This energy cost is high because nitro-
gen fertilizer is used so intensively and because a large
amount of energy is required to produce it. In corn pro-
duction, for example, about 152 kg/ha of nitrogen fertilizer
is applied to the field, which represents 30% of the total
energy input per hectare (Pimentel and Pimentel, 1997).
This energy input could be reduced greatly by using
manures, biological nitrogen fixation, and recycling.
Another 15% of indirect energy use occurs in the
production of pesticides. When formulation, packaging,
and transport to the farm are included, the energy cost is
somewhat higher. Although newer pesticides are usually
applied in smaller quantities than those common a few
decades ago, they are typically higher in energy content.
Most of the industrial cultural energy inputs in agri-
culture, both direct and indirect, come from fossil fuels
or are dependent on fossil fuels for their manufacture.
Other sources of industrial energy play a very small role
in agriculture overall, even though they may be significant
on a local basis. An analysis of the energy budget for corn
production in the Midwestern U.S. showed that more than
90% of the industrial energy inputs came from fossil fuels,
and less than 1% of the total energy needed for production
came from renewable biological cultural energy in the
form of labor (Pimentel and W. Dazhong, 1990). When
crop production depends so fully on fossil fuels, anything
that affects the cost or availability of such energy can have
dramatic impacts on agriculture.