Energy in Agriculture and Society: Insights from the Sunshine Farm

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Farming, in industrial countries, has generally become an industry itself. As such, it has national energy demands comparable with other industries and thus competes for energy supplies, particularly petroleum. Annual global oil production has been rising for many years, but an important economic transition period for all industries will arrive when annual production peaks, then declines as discovery drops off and the remaining oil becomes harder to extract. That time is not far off, even by optimistic estimates. For example, a recent assessment by the US Geological Survey increased its previous global estimate of remaining oil, both discovered and undiscovered, by an unprecedented 37 percent.¹ During the century ending in 1990, the world consumed an amount of oil equal to this increase, 625 billion barrels of oil. In stark contrast to the 100 years, using current annual production as a guide, 27 billion barrels, the increase will last little more than 25 years during the early part of the twenty-first century. Since oil production will be approximately symmetric on both sides of the peak, the increase will delay the peak of annual global oil production by half this period, or 12 years.² Added onto current estimates of the peak year, the 12-year delay would place the peak only one human generation from now, or about three decades. The highly touted Caspian Sea oilfields in the former Soviet Union would postpone the peak only a year. The importance of national food security dictates that we should reduce farming’s dependence on fossil fuels. As part of The Land Institute’s mission to use nature as measure for developing sustainable agriculture and culture, the Sunshine Farm Project has been exploring the possibilities of farming without fossil fuels, fertilizers or pesticides. We have been conducting energy accounting on the Sunshine Farm to assess the extent to which modern farms can run, essentially, on sunlight. As late as the 1950s, many farms ran mostly on sunlight, relying on draft horses and using crop rotations for soil fertility instead of commercial fertilizers. Nowadays however, agriculture uses far more energy nowadays in the form of farm inputs, while at the same time, there are current renewable energy technologies that were not available in traditional farming. Energy accounting reveals patterns of energy consumption that can be examined to determine which changes in farming practices would save energy. For example, the use of animal manure as a source of nitrogen requires less energy than that required to manufacture commercial fertilizer. The accounting on the Sunshine Farm is done by determining the weight of everything going into and out of the farm and by entering this information into an accounting framework that we established for our computer database. The weights of inputs are converted into values of energy by computer calculations based on embodied energy factors (the energy used in industry to mine, process and fabricate raw materials and ores into farm inputs) from the professional literature. Likewise, the literature is used to provide caloric energy factors for converting our marketed farm outputs into calories, like in our daily diet. Our energy accounting shows that the Sunshine Farm could supply about 40 percent of its embodied energy needs through animal feed, electricity, and biodiesel fuel for the tractor and vehicles. That is, a 4.5-kilowatt photovoltaic array on the Sunshine Farm has been converting sunlight into electricity required for the workshop tools, electric fencing, water pumping, and chick brooding. About one-fourth of the cropland on the Sunshine Farm has been devoted to soybeans and sunflowers for biodiesel fuel that could be commercially produced to meet all our field operations and off-farm transportation. Both the array and biodiesel are renewable energy; they produce more energy than they consume.³ Almost three-fourths of the animal feed consumed on the Sunshine Farm has been supplied by the oats, grain sorghum and alfalfa produced on the farm for the draft horses, beef cattle and poultry. The remaining 60 percent of the farm’s embodied energy needs is imported onto the farm through purchased inputs including amortized capital, such as supplies, commercial feed and seed, buildings, fences, water lines, tractors, vehicles, equipment, and the photovoltaic array. These results do not include the lifestyle energy support for a farm family, a large unknown ranging from the austere Amish to typically affluent Americans, although we do keep track of human labor on the Sunshine Farm. Nor do they include some prorated part of the food processing, marketing, distribution and preparation sectors that nationally consume more energy than farming.⁴ Our focus is to determine how much energy farms can supply for their inputs. Family support and commercial sectors are not unique to agriculture but are common to all industrial activity; hence, they are social considerations beyond the scope of our project. The purpose of the renewable energy technologies in our project is to reduce our dependence on fossil fuels but not our dependence on local energy systems. Virtually all farms are part of the local community in many ways, energy being no exception. For example, from our exploration of biodiesel production, we have learned that biodiesel fuel with quality satisfactory to engine manufacturers can be produced by farmers’ co-operatives but not by individual farms producing various, unregulated home-brews of biodiesel fuel. Also, although our photovoltaic array has a bank of batteries and could stand alone, it is connected to the electric grid of the local power company to sell excess electricity. Just as important as the monetary income from the sale is the fact that the excess electricity is now on the grid for use by the local community. In other words, in an all-solar future, given certain limits of energy production by solar technologies compared to current conventional technologies (see below), it will likely be considered uncivic to own a personal electrical technology located near a grid but not connected to it. This would prevent the use of the technology’s potential excess electricity by other people, and in effect, would be squandering some of the hard-won, solar-based energy embodied in the manufacture of that technology. Since any solar technology exposed to the weather will slowly deteriorate whether or not it is used, there would be little to be gained from the selfish idea of operating a personal solar technology only when the owner needs its energy. The obligation to sell excess electricity would be quite contrary to the current popular notion of achieving energy self-sufficiency in order to disconnect oneself from the grid. This notion is made possible by the present industrial economy with its abundance of fossil fuels and mineral resources. Some important consequences for farming result from some differences between fuel crops and solar technologies such as heat collectors, photovoltaics, wind turbines and hydroelectric. The annual energy efficiency of crop production by photosynthesis is less than 1 percent, far below the range of 10-25 percent for the conversion of sunlight into useful energy by solar technologies. Hence, to produce a given amount of useful energy, fuel crops require ten to 100 times more land area than solar technologies.⁵ Solar’s smaller land requirement is why the Sunshine Farm uses a photovoltaic array as its primary source of electricity instead of a generator operated on a renewable fuel. Fuel crops’ larger land requirement explains why the national production of ethanol from corn grain for use in gasohol raised a furious ethical debate over diverting substantial cropland from food to fuel production. Moreover, solar’s smaller land requirement would make it more desirable than fuel crops for powering tractors on farms of about 100 acres or less in an all-solar future, but the technology will require further development.⁶ In the energy accounting of a farm, solar technologies are treated in the same manner as conventional energy technologies. As an imaginary example, if a farmer built a coal-fired electric plant on his/her farm, it would be ludicrous to count its marketed electricity as an output of farming. Instead, we note the amount of electricity consumed annually by the farm (absent the plant) and calculate the embodied energy of the coal required to provide that electricity through the farmer’s plant. The coal’s embodied energy is the farm input representing the electricity consumed by the farm. This is the same conventional procedure used for electrical consumption by any farm; i.e., the coal-fired plant is usually owned by a utility company and is located far away from the farm. Analogous to the imaginary example, we do not regard the output of a farmer-owned solar technology as a farm output. Instead, we note what percentage of the technology’s output is consumed by the farm. We then take the energy embodied in the mining and manufacture of that technology and adjust it by the output percentage to obtain a reduced value that becomes the farm input representing the electrical consumption of the farm. Agriculture’s potential to provide energy as well as food for society can be ascertained by the energy balance of various farms and national agricultural systems. We determine the energy balance of a farm by comparing the caloric energy of its marketed outputs with the embodied energy of its purchased inputs. Not counting the photovoltaic array, the Sunshine Farm marketed 1.7 calories for every calorie purchased. The outputs and inputs are qualitatively different types of energy, but since their units are the same, this is an energy ratio of 1.7 to 1 for outputs to inputs, or simply 1.7. Inclusion of the photovoltaic array by the above accounting procedure had little effect on the Sunshine Farm’s energy ratio, but this was because the energy ratios for the array and the farm were about the same to begin with.³ The Sunshine Farm’s energy ratio is comparable to values at the upper end of the range for mixed crop and livestock farms (Table 1).⁷ A conventional Pennsylvania dairy farm had a ratio of 1.8. Amish farms from six communities had ratios of 0.7-1.6, greater than the ratios of 0.3-0.6 for nearby conventional farms, except for a group of mixed farms in the Illinois Corn-Belt region that averaged 2.0. This strong contrast between Amish and conventional farms is all the more remarkable considering that the former also energetically provide their own field traction while the latter do not. That is, Amish farmers raise draft horses fed by their crops, while conventional farms do not make their tractors and fuel, but purchase them. The Pennsylvania dairy farm, Illinois Corn-Belt farms, and Sunshine Farm had the greatest farm energy ratios, partly because most of their marketed output (69-95 percent of caloric energy) was crops (Table 1). In contrast, crops on the other farms constituted only 14-29 percent of marketed output, and animal products accounted for the remaining share. On the other farms, feeding much of the crops to animals incurred large metabolic energy losses and thus lower farm energy ratios. Another reason for the large farm energy ratios of the Pennsylvania dairy farm, Sunshine Farm, and some communities of Amish farms was the low amount of purchased inputs per acre of cropland (Table 1). In other words, less energy-intensive production helps yield a higher farm energy ratio. The average per-acre marketed crop output of the Illinois Corn-Belt farms was so great that they had the largest farm energy ratio despite a large amount of purchased inputs per acre (Table 1). These results are corroborated by 15 hypothetical farm energy budgets computed by Gerald Leach in his 1976 book, Energy and Food Production: the greater farm energy ratios were clearly associated with fewer purchased inputs and with a larger proportion of outputs devoted to crops. National energy ratios for farming in various countries display a range of values similar to mixed farms (Table 2). Industrial countries, such as the US and those in Europe, have energy ratios near 1.0 or less, like conventional mixed farms. This is not surprising because the agricultural structure of industrial countries is generally a mix of energy-intensive crop and livestock production. For example, about three-fourths of the crops in the US are fed to livestock.⁸ While much of it is fed in large feedlots and operations, this value is a common proportion on individual mixed farms, thus resulting in similar energy ratios at the national and individual levels of farming. As an example of the effect of adopting capitalism, China’s agriculture has rapidly increased its use of commercial fertilizers and other inputs, resulting in a national energy ratio of 1.2 in 1978. This value is surely lower now and reflects an industrial economy instead of one that relied on peasant farming methods not that long ago, as described in Farmers of Forty Centuries.⁹ Although the energy ratio of 1.0 for US agriculture is based on data a quarter century old (Table 2), this data is nearly the latest available from which an energy ratio can be readily calculated. It was computed from summary data reported by Stanhill that, in turn, was based on the extensive 1974 USDA national survey of agricultural energy inputs.^10,11 The USDA survey repeated in 1978 also gives the same energy ratio.¹² Since then, less detailed estimates of some agricultural energy inputs have been made, but with no computation of the embodied energy in the inputs.^13,14 Recent econometric studies have shown that energy productivity in US agriculture has increased since 1980 due to improvement in technology and farm management stimulated by the energy price shocks of the 1970s, the withdrawal of energy-requiring marginal cropland through the federal Conservation Reserve Program, and the economy of scale accrued from continuing increases in average farm size (although the latter is not desirable for rural communities).^15,16 Hence the current energy ratio for US agriculture must be greater than 1.0, but it cannot be directly calculated from these econometric studies because the measures of energy productivity were based in part on financial data or indices. National energy ratios for farming are higher in less-industrialized countries. For example, agriculture in Egypt, Pakistan and Australia have respective ratios of 1.8, 2.9, and 3.1, again because of fewer purchased inputs and fewer crops fed to animals (Table 2). Australia relies mostly on low-input crops (e.g., wheat), free-range animal rearing, and extensive use of leys (i.e., grazed legume cover crops) instead of commercial fertilizer for cropland nitrogen needs. Likewise in America, before the tremendous increase in commercial fertilizers, pesticides and irrigation after World War II, American farming in 1940 had a national energy ratio of 2.3 (Table 2). To understand the challenge of obtaining energy from agriculture, the energy ratios for mixed crop and livestock agriculture can be compared with energy ratios that industrial society has obtained from nonrenewable and renewable energy sources. The highest energy returns have come from nonrenewable fossil fuels. Drilling for petroleum and natural gas currently yields an energy ratio of 10, well below the ratio of 100 or more in the 1940s when oil was literally gushing out of the wells (Table 3 ). The energy ratio for coal also declined during that time with a current value of 30 at the mine mouth or stripped coalfield. Including combustion of coal in electric power plants drops this value to a national average of 9. The ratio can go as low as 2.5 for western, strip-mined coal that is transported more than 1,000 miles to Midwestern power plants outfitted with pollution scrubbers. Nearly the same energy ratio applies to the current extraction and use of natural gas in electric power plants. The mining and processing of uranium ore and its use in nuclear light-water reactors to produce electricity result in an energy ratio of only 4, which includes storage of spent fuel (Table 3 ). This value certainly will not justify the future exposure of society to the inherent risks of nuclear reactors and their fuel cycle from mining to burial. The risk is simply too great for large-scale civilian or military accidents, sabotage, or mismanagement with huge economic, environmental, and social costs. None of the energy ratios in Table 3 include decommissioning of a technology (i.e., moth-balling, dismantling, or disposal) at the end of its useful life, and this energy input, relative to the other inputs, will be far larger for nuclear reactors than for the other technologies. The energy ratios for renewable liquid fuels are much lower than for extraction of petroleum. Conventional agricultural production of grain, starch, or sugar crops and some chemical processing yield ethanol fuel with small energy ratios in the neighborhood of 1-2, with methanol from tree production slightly higher (Table 3 ). The energy ratios for biodiesel fuel, which can be chemically processed from various vegetable oils, would not be much higher than ethanol and methanol. The ratios for ethanol and vegetable oil also include an energy credit for the by-product spent mash and meal cake, respectively.³ The energy ratios for renewable solid fuels, namely 6-13 for biomass production by intensive farming of crops and trees, are lower than for mining of coal, the analogous fossil fuel (Table 3 ). The energy ratios for biomass production are higher than renewable liquid fuels because the latter require chemical processing that introduces either substantial energy inputs or chemical losses for the liquid fuels and thus lowers their energy ratios. Moreover, the output of fuels derived from seed, like ethanol and biodiesel, is lower than biomass because seed is only a small portion of the above-ground plant that constitutes biomass. Subsequent use of biomass further reduces the energy ratios resulting from biomass production. Gasification of biomass crops to produce a gaseous fuel results in values of 2-5, lower than the current ratio for extraction of natural gas, the analogous fossil fuel (Table 3 ). These values are also not much better than the above ratios for renewable liquid fuels. Direct combustion of biomass crops for heat might give slightly higher ratios, including an energy input for the embodied energy in the furnace or boiler. Heat can also be obtained by flat-plate solar collectors, including storage, with energy ratios of 2-5, the lower values including fuel-operated heating systems as back-up. Direct combustion of crop residues or biomass for electricity results in energy ratios between 3 and 4, although advanced cogeneration of electricity and heat, not yet commercialized for biomass, may yield values twice as high. Another renewable fuel is biogas, mostly methane, produced from anaerobic digestion of agricultural materials such as manure, crop residues, or wastes. The range of energy ratios in a table of results from eight European countries was 1.7-5.6, the higher values in southern countries with warmer climates that reduce the energy input required to keep the digester warm enough for sufficient microbial digestion.¹⁷ However, if we use some other studies to add estimates for collection of agricultural materials and for amortized embodied energy in digesters, then the range becomes 1.5-3.1, less than the ratio for extraction of natural gas, but comparable to renewable liquid fuels (Table 3).^18,19 The lower values, associated with northern countries, do not decrease much because the two additional inputs are small compared to the heating requirement for the digesters. Finally, if an agricultural material is going to be regularly dedicated to biogas production, or some other renewable fuel, then it should be regarded not as a secondary by-product with just collection costs but as a primary product with its prorated share of the production inputs for its source. Using prorated production inputs for crop residues as representative of agricultural materials in general, the range of energy ratios for the European countries would then become 1.4-2.0. Analogous to the results for renewable fuels, most solar-related technologies for the production of electricity, including reliable back-ups, generally have energy ratios less than the national average for coal-fired electricity including coal mining. One exception is hydroelectric systems with dammed water as inherent energy storage, which commonly attain an energy ratio of about 10. Upper values of 8-10 have been reported in energy ratios for photovoltaic arrays that electrochemically convert sunlight directly into electricity and for parabolic-thermal reflectors that collect heat for driving steam or gas turbines to generate electricity (Table 3 ). In comparison, wind-electric turbines have achieved upper values twice as high. However, it remains to be seen if these upper values can be typically achieved so as to become averages. Unlike current centralized power plants, these smaller technologies will be distributed in location and thus will have less embodied energy in transmission lines. However, despite being connected to local electric grids, some technologies may require energy storage and/or back-up systems. Energy conservation and efficiency reduce the consumption of energy with the result that energy savings effectively constitute an output available for other uses. The energy ratios for conservation are comparable to nonrenewable energy sources and are often an order of magnitude greater than renewable fuels and solar technologies. For example, double-pane windows and ceiling insulation prevent losses of heat in winter and coolness in summer that are equal to 136 and 61 times, respectively, the energy expended in producing and installing the windows and insulation (Table 3 ). Passive solar design reduces heating and cooling in new houses by an amount equal to 10-25 times the energy spent in manufacturing the passive components from raw materials and in constructing new houses with them. In other words, much more energy would generally be made available from a given amount of energy inputs invested in conservation and efficiency than in renewable energy sources. As documented by the US Department of Energy, during 1979-1986 the US obtained 7 times as much new energy from savings through conservation and efficiency than from all net increases in domestic energy supplies based on fossil fuels, nuclear power, and renewable sources.^20,21 Hence conservation and efficiency should be fully developed, as well as renewable energy sources. This brief review shows that industrial society has been powered by nonrenewable energy sources with energy ratios much greater than the values of 2.0 or less for mixed crop and livestock farms. With energy ratios this low, agriculture will not be a net source of renewable fuels or electricity. Simply, if outputs from mixed farms were converted into fuels or electricity, typically half of the energy in the outputs would be lost during the conversion processes.²² In conjunction with the energy ratios of 2.0 or less, this implies that for any mixed farm the potential output of marketed fuel or electricity would be less than the embodied energy required in the manufacture of the farm’s purchased inputs. Hence, beyond the food and feed already marketed by mixed farms, there would be no net output available as energy for society. The same conclusion also applies to US agriculture for which the energy ratio is greater than 1 but probably less than 2, as elaborated above. The nation exports one-fourth of its grain production, and one might think that it could be converted into a lot of energy for society, but the resulting converted energy would provide less than one-half of the embodied energy in the farm inputs used by US agriculture.^4,8 For mixed farms or US agriculture to have some net output available as energy for society, the energy ratios must be raised by reducing purchased inputs and increasing marketed outputs. Many farmers have been using less purchased fertilizers and pesticides, mainly to cut expenses. Farms could someday, like the Sunshine Farm, energetically supply their own fuels and electricity instead of purchasing them. Inputs can also be reduced by utilizing biological efficiencies in crops and animals, such as letting animals obtain their own feed through grazing and foraging which involve no embodied energy, in contrast to feeding them machine-harvested grain and hay. As far as increasing the amount of marketed outputs from mixed farms, increased crop yields will not be an option under a regime of fewer commercial inputs in farming. It is envisioned that yields will be maintained not quite as high as current levels by diverse farming practices that will require more use of land, biological efficiencies, and human labor. Also, substitution of fuel crops in place of feed and food crops will have little effect since they have fairly similar yields under equivalent farming practices. Large increases in marketed outputs could be achieved by diverting cropland from supplemental animal feed to crops for direct human consumption. The potential increase is large because slightly more than half of US crop production is fed to animals.⁸ Since the feed conversion efficiency of animals is only 10-20 percent on a weight or energy basis, each pound less of animal products derived from supplemental feed would permit an output of 5-10 pounds of directly consumed crops substituted for the displaced feed crops.⁸There is plenty of slack for reducing the consumption of animal products in our diet because the average American currently consumes twice the average minimum daily protein recommended by the international Food and Agriculture Organization, and two-thirds of our daily protein intake is animal protein.^8,23 By these strategies for inputs and outputs, mixed farms and US agriculture should be able to increase their energy ratios to 3, perhaps 4, the former figure already achieved by Australia (Table 2). An energy ratio of 4 implies 4 units of marketed farm outputs in the numerator of the ratio for every 1 unit of embodied energy in the manufacture of purchased farm inputs in the denominator. Provision of 1 unit of embodied energy would require conversion of 2 units of farm outputs into fuel and electricity because of the aforementioned conversion losses of about 50 percent. Subtraction of the 2 converted units from the 4 units of marketed outputs would leave 2 units of farm outputs, or effectively an energy ratio of 2 after agriculture has met the embodied energy requirements of its own farm inputs. Considering that this value is double the above energy ratio of 1.0 for US agriculture, in absolute amounts this ratio should provide sufficient food and maybe a small amount of energy for the nation’s society. Hence, national agriculture must achieve at least an energy ratio of roughly 4 to be a net source of commercial energy beyond the food demands of society and the energy requirements of its own farm inputs. Otherwise, an energy ratio less than 4 means that manufacture of some agricultural inputs will require energy subsidy from society such as electricity from photovoltaic arrays or wind-electric turbines. The above review also demonstrates that nonrenewable energy sources, namely fossil fuels and nuclear power, have energy ratios that are generally greater than the analogous renewable energy sources. Furthermore, they former sources yield a lot of power from the little land area required for mining, fuel processing, and power production. In other words, fossil fuels and nuclear power yield much greater power densities, or power per acre, than renewable energy sources because the latter are derived from sunlight that is dispersed across the landscape.⁵ Hence, fuel crops and solar technologies would require much more land to meet the current energy consumption of various US sectors. An aggressive national program of energy conservation and efficiency will be required to sufficiently reduce energy consumption such that the US economy could be powered by renewable energy sources without using too much land. Absent such measures, for example, if the nation’s current transportation sector were to be fueled solely by the gross yield of ethanol from corn grain, then half of the entire US must be planted to corn.^24.25.26 This result assumes an average corn yield 90 percent of the current Midwest corn yield and that the energy inputs for corn production and ethanol processing in an all-solar future would be met by solar technologies. Since solar technologies have greater power densities than fuel crops, they require less land than fuel crops.⁵ We cannot look to America’s forests for much woody biomass unless we increase the recycling of paper and wood products. The current annual net growth of America’s forests is already entirely accounted for by the national consumption of wood and paper products.⁸ Extra biomass could be gained initially by improvement of forest productivity through conventional intensive practices such as fertilization and introduction of nonnative species, but this would result in a loss of biodiversity and a concomitant, long-term decrease in biological efficiencies that would offset the initial extra productivity.^27,28,29 Nonetheless, some energy scholars believe that energy conservation and efficiency will make it quite possible to power our current standard of living with renewable energy sources.^30,31 Solar technologies would be particularly important in meeting US energy needs since they have much greater energy ratios and power densities than renewable fuels derived from agriculture. An all-solar future with solar technologies would be radically different than one without them. The research and infrastructure needed for an all-solar future should be developed now while we have the luxury of high energy ratios from fossil fuels.

 

1. USGS. 2000. U.S. Geological Survey World Petroleum Assessment 2000 ? Description and Results. US Department of Interior, Washington, DC. Available, http://greenwood.cr.usgs. gov/energy/WorldEnergy/DDS-60 [8 Aug. 2000]. This is conventional oil that does not include tar sands and oil shales. The large increase is mainly from a new category, reserve growth, in which the USGS estimated how much the apparent sizes of known oil fields are likely to grow as drilling hits previously unrecognized pockets of oil within and just beyond the edges of fields already producing oil. By the mid-1990s, USGS analysts realized that reserve growth was substantial and thus included it in the recent assessment. In the previous USGS estimate of remaining global oil, discovered reserves and undiscovered resources (mean) were 1100 and 580 billion barrels of oil, respectively.
2. This rough estimate was confirmed by a more complex analysis in: A.A. Bartlett. 2000. An analysis of US and world oil production patterns using Hubbert-style curves. Mathematical Geology 32 (1).
3. Over its projected 20-year lifetime, we calculated that the photovoltaic array will produce 1.6 times more energy than was consumed in its manufacture and installation, including a bank of batteries and a prorated portion of the power company grid to which it is connected. The 25% of the farm’s cropland devoted to oilseeds was determined on a net-energy basis in which the gross energy content of the biodiesel fuel is reduced by the energy inputs for raising the oilseed crops and chemically converting them into biodiesel, including amortized embodied energy in machinery and buildings. It is also increased by an energy credit for high-protein meal cake, a by-product from biodiesel production that would be fed to livestock.
4. A.B. Lovins, L.H. Lovins, and M.H. Bender. 1995. Agriculture and energy. Pp. 11-18 in: Encyclopedia of Energy Technology and Environment. Vol. 1. John Wiley and Sons, NY. The proportion of energy use in the US food system is (%): farming, 18; food processing, 30; distribution, 10; commercial food service, 17; and home food preparation, 25.
5. V. Smil. 1991. General Energetics: Energy in the Biosphere and Civilization. John Wiley and Sons, NY.
6. Electricity from solar technologies would power electrolysis of water to produce hydrogen fuel that could either be spark-ignited in internal combustion tractor engines or be converted by fuel cells into electricity to run electric tractor motors. Since tractors require a lot of power, more technological advancement must be achieved in hydrogen storage tanks in either case and also in hydrogen fuel cells in the latter case. An accessible reference is: J.J. MacKenzie. 1994. The Keys to the Car: Electric and Hydrogen Vehicles for the 21st Century. World Resources Institute, Washington, DC.
7. In contrast to the actual measurement of the weight of inputs and outputs on the Sunshine Farm that were converted by energy factors, the data in the other studies were obtained through extensive interviews with farmers and farm businesses, farm financial records, and state or national farm business surveys. Some of this data is based on actual measurement of weight, but much of it was expenses that authors translated from dollars to embodied energy values by means of energy conversion factors from research publications in energy analysis.
8. USDA. 1996. Agricultural Statistics, 1995-96. US Department of Agriculture, National Agricultural Statistics Service, Washington, DC.
9. F.H. King. 1911. Farmers of Forty Centuries: Permanent Agriculture in China, Korea and Japan. Reprinted in 1973 by Rodale Press, Emmaus, PA. For a quantitative treatment of nutrient data in this book, see: M.H. Bender. 2000. Comparison of nutrient return and plant uptake in agricultural systems. Journal of Sustainable Agriculture15:89-105.
10. G. Stanhill. 1984. Agricultural labor: From energy source to sink. Pp. 113-130 in: G. Stanhill (ed.). Energy and Agriculture. Springer-Verlag, Berlin.
11. FEA/USDA. 1976. Energy and U.S. Agriculture: 1974 Data Base. Vol. 1, Part A: U.S. Series of Energy Tables. Federal Energy Administration and US Department of Agriculture, Washington, DC.
12. D. Torgerson and H. Cooper. 1980. Energy and U.S. Agriculture: 1974 and 1978. USDA Statistical Bulletin No. 632. US Department of Agriculture, Economic Research Service, Washington, DC.
13. J.R. Barse (ed.). 1990. Seven Farm Input Industries. USDA Agricultural Economic Report No. 635. US Department of Agriculture, Economic Research Service, Washington, DC.
14. M. Anderson and R. Magleby (eds.). 1997. Agricultural Resources and Environmental Indicators, 1996-97. USDA Agricultural Handbook No. 712. US Department of Agriculture, Economic Research Service, Washington, DC.
15. C.J. Cleveland. 1995. Resource degradation, technical change, and the productivity of energy use in US agriculture. Ecological Economics13:185-201.
16. B.S. Panesar and R.C. Fluck. 1993. Energy productivity of a production system: Analysis and measurement. Agricultural Systems 43:415-437.
17. M. Demuynck, E.J. Nyns and W. Palz. 1984. Biogas Plants in Europe: A Practical Handbook. D. Reidel Publishing Co., Dordrecht.
18. W. Vergara and D. Pimentel. 1978. Fuels from biomass: Comparative study of the potential in five countries: The United States, Brazil, India, Sudan, and Sweden. Advances in Energy Systems and Technology1:125-173.
19. Y.R. Chen. 1983. Biogas digester design. Pp. 23-59 in: D.L. Wise (ed.).Fuel Gas Systems. CRC Press, Boca Raton, FL.
20. EIA. 1987. Annual Energy Review 1986. US Department of Energy, Energy Information Administration, Washington, DC.
21. A.B. Lovins and L.H. Lovins. 1989. Drill rigs and battleships are the answer! (But what was the question?): Oil efficiency, economic rationality, and security. Pp. 83-138 in: R.G. Reed III and F. Fesharaki.The Oil Market in the 1990s: Challenges for the New Era. Westview Press, Boulder, CO.
22. D. Spreng. 1988. Net-Energy Analysis and the Energy Requirements of Energy Systems. Praeger, New York. See page 222 for table of bioconversion efficiencies.
23. Smil, V. 1991. Population growth and nitrogen: An exploration of a critical existential link. Population and Development Review 17:569-601.
24. H. Shapouri, J.A. Duffield and M.S. Graboski. 1995. Estimating the net energy balance of corn ethanol. USDA Agricultural Economic Report No. 721. US Department of Agriculture, Economic Research Service. National Agricultural Statistics Service, Herndon, Virginia.
25. US DOE, 1998. Annual Energy Review 1997. US Department of Energy, Washington, DC. Available:http://www.eia.gov/pub/energy.overview [7 April 1999].
26. In the USDA review in note 24, the Midwest corn yield is 122 bushels per acre, which also happens to be the average national corn yield during the past decade according to: USDA. 1999. Agricultural Statistics, 1999. US Department of Agriculture, National Agricultural Statistics Service, Washington, DC. Available:http://www.nass.usda.gov/pubs [21 Feb. 2000].
27. J.H. Cook, J. Beyea, K.H. Keeler. 1991. Potential impacts of biomass production in the United States on biological diversity. Annual Review of Energy and Environment 16: 401-431.
28. OTA. 1980. Energy from Biological Processes. US Congress, Office of Technology Assessment, Washington, DC.
29. J. Pelisek. 1975. Conifer plantations and soil deterioration. Ecologist5:332-336.
30. A.B. Lovins, L.H. Lovins, F. Krause and W. Bach. 1981. Least-Cost Energy: Solving the CO₂ Problem. Brick House Publishing Co., Andover, MA.
31. J. Goldemberg, T.B. Johansson, A.K.N. Reddy and R.H. Williams. 1987. Energy for a Sustainable World. World Resources Institute, Washington, DC.
32. J. Zucchetto and G. Bickle. 1984. Energy and nutrient analyses of a dairy farm in central Pennsylvania. Energy in Agriculture 3:29-47.
33. P. Craumer. 1979. Farm productivity and energy efficiency in Amish and modern dairying. Agriculture and Environment 4:281-299.
34. W.A. Johnson, V. Stoltzfus and P. Craumer. 1977. Energy conservation in Amish agriculture. Science 198:373-378.
35. M. Green. 1978. Eating Oil: Energy Use in Food Production. Westview Press, Boulder, CO.
36. R.M. Gifford. 1976. An overview of fuel used for crops and national agricultural systems. Search 7:412-417.
37. C.A.S. Hall, C.J. Cleveland and R. Kaufmann. 1986. Energy and Resource Quality: The Ecology of the Economic Process. John Wiley, New York.
38. C.E. Goering and M.J. Daughtery. 1982. Energy accounting for eleven vegetable oil fuels. Transactions of the American Society of Agricultural Engineers 25:1209-1215.
39. R. Herendeen and S. Brown. 1987. A comparative analysis of net energy from woody biomass. Energy 12:75-84.
40. A.F. Turhollow and R.D. Perlack. 1991. Emissions of CO₂ from energy crop production. Biomass and Bioenergy 1:129-135.
41. W. Vergara and D. Pimentel. 1978. Fuels from biomass: Comparative study of the potential in five countries: The United States, Brazil, India, Sudan, and Sweden. Advances in Energy Systems and Technology1:125-173.
42. M. Demuynck, E.J. Nyns and W. Palz. 1984. Biogas Plants in Europe: A Practical Handbook. D. Reidel Publishing Co., Dordrecht.
43. R.A. Herendeen. 1988. Net energy considerations. Pp. 255-273 in: R.E. West and F. Kreith (eds.). Economic Analysis of Solar Thermal Energy Systems. MIT Press, Cambridge, MA.
44. C.J. Cleveland and R. Herendeen. 1989. Solar parabolic collectors: Successive generations are better net energy and exergy producers.Energy Systems and Policy 13:63-77.
45. G. Tyner, Sr. and R.G. Fowler. 1992. Estimating the viability of alternative sources of energy. In: Investing in Natural Capital — A Prerequisite for Sustainability. Second Meeting of the International Society for Ecological Economics. Stockholm University, Stockholm, Sweden, 3-6 August 1992.
46. B. Sørensen. 1995. History of, and recent progress in, wind-energy utilization. Annual Review of Energy and Environment 20:387-424.
47. M.W. Gilliland, J.M. Klopatek and S.G. Hildebrand. 1981. Net energy: Results for small-scale hydroelectric power and summary of existing analyses. Energy 6:1029-1040.
48. D.M. Gates. 1985. Energy and Ecology. Sinauer Associates, Sunderland, MA.
49. R.H. Williams and E.D. Larson. 1993. Advanced gasification-based biomass power generation. Pp. 729-785 in: T.B. Johansson, H. Kelly, A.K.N. Reddy and R.H. Williams (eds.). Renewable Energy: Sources for Fuels and Electricity. Island Press, Washington, DC.
50. B. Hannon. 1981. The energy cost of energy. Pp. 81-107 in: H.E. Daly and A.F. Umaña (eds.). Energy, Economics, and the Environment: Conflicting Views of an Essential Interrelationship. Westview Press, Boulder, CO.