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| Fig. 1: Picture of Harvesting Machine. (Source: Wikimedia Commons) |
Since the industrial revolution agriculture has shifting manual labor for machinery as the one seen in Fig. 1. The use of machines and chemicals in agriculture has made energy one of its key inputs. Additionally, agriculture has always used solar energy, storing it in its produce that later serves as food, fuel, or other materials.
For farms in the U.S. (excluding Alaska and Hawaii) energy related expenditures during 2017 accounted for an approximate amount of 56.1 billion U.S. dollars or 15.6% of total farm expenditures. [1] Energy in agriculture is typically classified as direct energy and indirect energy use. Direct energy used in agriculture refers to energy that is consumed directly in agriculture activities such as, operating farm machinery, trucks, vehicles, equipment, drying operations, overhead and marketing. [2] Indirect energy used in agriculture refers to the energy used to manufacture and as chemical inputs in the production of fertilizer, pesticides and other chemicals utilized in agriculture. [2] Neither indirect or direct energy accounts for the solar energy required for photosynthesis. During 2017 direct energy expenditures accounted for 18.6 billion U.S. dollars or one third of energy related expenses, indirect energy accounted 37.5 billion U.S. dollars or two thirds of energy related expenses. [1] Although during 2012 agriculture in the U.S. utilized 914 million acres there is no economic figure attributed to the solar energy such acres received other than the rent or equivalent rent paid for the land used. [3] Aside from the rent there is an opportunity cost that could be thought of as the energy that could have been generated if such acres would have been utilized to generate solar power with photovoltaic cells.
A part of the direct, indirect and solar energy used in agriculture is transformed into produce storing it as chemical energy. The rest of the energy is wasted. In the case of direct energy, it is wasted as heat of the machinery used, indirect energy is wasted as fertilizers and pesticides miss their target. Specifically for corn if we take direct plus indirect energy as inputs and convert it to ethanol as an output, the estimated energy output:input ratio is 1.34. [4] Solar energy is mainly wasted as heat in the air, soil, water, etc irradiated. For example, in corn and soy bean only 15% and 7% respectively of the net radiation of solar energy stored in soil and canopy as heat and as carbohydrate bonds through photosynthesis. [5] Most of the portion stored as soil and canopy heat is not useful as it dissipates and does not create biomass. Thus, the energy output:input ratio taking solar into account would be significantly lower.
Even though direct energy in agriculture accounted for only 1.1% of the energy used in the U.S. during 2002 we see that the land used for agriculture activities of 914 million acres receives significantly more energy than the amount stored. [2,3,5] Such difference in energy efficiency suggests that there might be areas of improvement or levers that could be managed to improve efficiency. First, yield increase, this has been gradual year on year in low single digit numbers through better inputs and techniques. Second is the use of urban / vertical farming that provides a better use of land resources concentrating crops and liberating land. Finally, change in crop / food consumption, as we saw in corn and soybean there are significant differences from crop to crop in energy storage. In conclusion in future years as resource efficiency is demanded by society landscape in agriculture will significantly change.
© Luis Lascurain. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "Farm Production Expenditures - 2017 Summary," United States Department of Agriculture, August 2018.
[2] R. Schnepf, "Energy Use in Agriculture: Background and Issues," Congressional Research Service, RL32677, November 2004.
[3] "2012 Census of Agriculture: United States, summary and State Data - Volume 1, Geographic Area Series, Part 51," United States Department of Agriculture, AC-12-A-51, May 2014.
[4] H. Shapouri, J. A. Diffield, and M Wang, "The Energy Balance of Corn Ethanol: An Update," United States Department of Agriculture, Agriculture Economic Report No. 814, July 2002.
[5] T. P. Meyers and S. E. Hollinger, "An Assessment of Storage Terms in the Surface Energy Balance of Maize and Soybean," Agri. Forest Meteorol. 125, 105 (2004).
