Fertilizer Consumption and Energy Input for 16 Crops in the United States
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- Amenumey, S.E. & Capel, P.D. Nat Resour Res (2014) 23: 299. doi:10.1007/s11053-013-9226-4
Fertilizer use by U.S. agriculture has increased over the past few decades. The production and transportation of fertilizers (nitrogen, N; phosphorus, P; potassium, K) are energy intensive. In general, about a third of the total energy input to crop production goes to the production of fertilizers, one-third to mechanization, and one-third to other inputs including labor, transportation, pesticides, and electricity. For some crops, fertilizer is the largest proportion of total energy inputs. Energy required for the production and transportation of fertilizers, as a percentage of total energy input, was determined for 16 crops in the U.S. to be: 19–60% for seven grains, 10–41% for two oilseeds, 25% for potatoes, 12–30% for three vegetables, 2–23% for two fruits, and 3% for dry beans. The harvested-area weighted-average of the fraction of crop fertilizer energy to the total input energy was 28%. The current sources of fertilizers for U.S. agriculture are dependent on imports, availability of natural gas, or limited mineral resources. Given these dependencies plus the high energy costs for fertilizers, an integrated approach for their efficient and sustainable use is needed that will simultaneously maintain or increase crop yields and food quality while decreasing adverse impacts on the environment.
KeywordsFertilizerenergy efficiencyenergy inputcrop productivityglobal food security
Increases in crop yield in the United States (U.S.) have been partially attributed to greater mechanization and use of fertilizers, both of which depend mainly on fossil fuels (Pimentel and Pimentel 2008; Cruse et al. 2010; Woods et al. 2010). With the anticipated growth in U.S. and global populations, it will become necessary to increase food productivity. The area of cultivated land will not increase substantially, even though demand for food will increase as population increases (Fixen 2007; Bruinsma 2009). Fertilizers will be increasingly important to improve crop yields. Fertilizers supply plant nutrients that play an important role in sustaining agricultural output for food, feed, fuel, and fiber. Nitrogen (N), phosphorus (P), and potassium (K) are primary nutrients and are supplied either singly or in various combinations and ratios to the crops. A portion of the nutrients applied are lost from the field from harvest and through run off and leaching. Nitrogen is also lost through volatilization and denitrification. Consequently, agricultural soils need to be re-fertilized regularly to replenish nutrients in the soil. The large quantities needed to meet the demand are obtained through mining (P, K) and industrial processes (N). P and K for fertilizers are produced by mining and refining naturally occurring ore deposits, while most of the N for fertilizer is manufactured from sequestered atmospheric nitrogen.
Nitrogen fertilizer is important to plant growth, including the production of amino acids and proteins. Anhydrous ammonia, a reduced nitrogen compound, is an important N fertilizer for many crop producers and is the main starting material in the production of most other N fertilizers. Anhydrous ammonia is manufactured by combining N2 from the atmosphere with hydrogen at very high temperatures and pressures using the Haber–Bosch process. Depending on the price of natural gas and size of the manufacturing plant, natural gas accounts for about 72–90% of the total cost of production of anhydrous ammonia (Schnepf 2004; Huang 2007). To produce 1 metric ton of ammonia, about 35 × 106 kJ of natural gas are required (Huang 2007). It is estimated that N produced from the Haber–Bosch process is responsible for about 40% of the protein found in humans (Smil 2002).
Phosphorus is an essential nutrient that promotes critical root growth, crop maturity, and the production of seeds. It is mined from sedimentary and igneous rocks that contain a high content of phosphate minerals. Phosphate rocks are non-renewable. The currently known reserves are found mainly in four countries—U.S., China, South Africa, and Morocco. The U.S. is the largest producer and consumer of P and has approximately 40 years of reserves remaining (Vaccari 2009). Agriculture uses about 95% of the world’s production of P for fertilizer, pesticides, and animal feed (Cisse and Mrabet 2004). Based on the current global P consumption growth rate of 1–2% per year, it has been predicted that depletion of global P reserves could occur within 50–100 years (Cisse and Mrabet 2004; Cordell et al. 2009).
Potassium is the third major nutrient essential for plant growth. Potassium helps plants to withstand extreme cold, heat, drought, and pests. It increases the efficiency of water use and facilitates sugar movement in plants. About 90% of the world’s K production is used for fertilizer (USGS 2013). K is mined or recovered from surface brine deposits from seas and salt lakes. The world’s largest deposits of K are found in North America, especially the Canadian provinces of Saskatchewan and New Brunswick. Other important K deposits are located in Russia, Belarus, and Germany. Canada is the world’s largest producer of potash, the primary source of K (Stone 2008). Potassium reserves are anticipated to last for a long time, but extracting, processing, and transporting K are energy intensive.
Energy inputs and outputs of crop production in the U.S. have been investigated for various crops by several authors (Pimentel 2006; Pimentel and Patzek 2008; Pimentel and Pimentel 2008). Technological advances in the early part of the twentieth century brought about mechanical innovation in agriculture. Since then, farmers have been constantly looking for more efficient and reliable sources of power to run their farm operations. Consequently, fuel-efficient farm machines have gradually replaced horses and mules on the farm (Uri and Day 1991; Conkin 2008). Direct energy inputs into crop production include electricity, diesel, gasoline, tractors, irrigation pumps, and other types of equipment. However, there is energy use associated with the production, packaging, and transport of fertilizer, in addition to the energy required for fertilizer application on the field. It has been estimated that about a third of the total energy input to crop production goes to the production of fertilizers, one-third to mechanization, and one-third to other inputs including labor, transportation, pesticides, and electricity (Pimentel 1993; Schnepf 2004; Pimentel 2006). Since agricultural inputs depend heavily on energy, challenges to meet the increasing energy needs of agriculture are dependent on the availability of fossil fuels (Hill et al. 2006; Woods et al. 2010; Cruse et al. 2010).
In 2011, the U.S. imported >50 and >85% of the N and K, respectively, it used for agriculture (USDA/Economic Research Service 2013a). Economically, it is no longer profitable for most industries to produce ammonia for fertilizer within the U.S. The ammonia imported to the U.S. comes primarily from Trinidad and Tobago, Canada, Russia, and Ukraine. Most of the K imported to the U.S. comes from Canada (Huang 2007). In contrast, the U.S. is a major producer of P, exporting 44% of the total P produced (Huang 2007; USDA/Economic Research Service 2013a). Over the past decade, the US Department of Agriculture (USDA) reported an increase in the import of K (USDA/Economic Research Service 2013b). This dependence on imported N and K could be a future concern in meeting the growing demands on U.S. crop agriculture.
The concern about the future availability of fossil fuels in the U.S. has driven the development of renewable biofuels (ethanol from corn, biodiesel from soybeans) (Hill et al. 2006; Cruse et al. 2010). Over the last decade, the U.S. has increased its ethanol production by about tenfold (USDOE 2012). Ultimately, there are concerns that increased demand for crops used for biofuels will lead to increases in the area planted with crops and increase the need for fertilizers (FAO 2008). As a consequence, the U.S. may depend more on imported N and K fertilizers to meet the higher demand that has resulted from increased biofuel production (Huang et al. 2009).
Fertilizer (N, P, and K) production, transport, and application are a substantial fraction of the total energy that is required of modern agriculture. This paper estimates the total energy inputs and the energy of fertilizer inputs for 16 selected crops grown in the U.S. (major grains and oilseeds, selected fruits, and vegetables). Three parameters are defined to help put the mass and energy of applied fertilizers into context: (a) mass yield of harvested crop per mass of fertilizer applied, (b) mass yield of harvested crop per total input energy, and (c) energy efficiency, which is the energy available from harvested crop per total input energy. Nitrogen fertilizer inputs were also compared to the protein content of the 16 crops. The purpose of this analysis was to determine the overall energy input associated with fertilizers for the various crops. This information will be valuable for societal and policy discussions on alternative sources for fertilizers and on the efficient use of fertilizers to meet increasing future demands on agriculture.
Production and Productivity Data
Agriculture Production (2009) Values, Energy Content and Output, and Fertilizer Application Rates for 16 Crops
Area Harvested ha × 103
Mass Yielde Mg/ha
Mass of Crop Produced kg × 106
Energy Contentf kJ/kg
Energy Outputg kJ × 109
Energy Output per Hectareg kJ/ha × 106
Nitrogen Application Rateh kg/ha
Phosphorous Applicationl Rate kg/ha
Potassium Application Ratem kg/ha
The N content of N fertilizer is expressed as N, whereas P and K fertilizers are expressed as phosphate (P2O5) and potash K2O, respectively. For consistency and simplicity, the terms N, P, and K are used to describe these masses throughout this paper.
Energy Inputs Calculations
Distribution of Energy Inputs for the 16 Crops
Laborb kJ × 109
Fuelb kJ × 109
Machineryb kJ × 109
Irrigationb kJ × 109
Fertilizersc kJ × 109
Pesticides, Chemicals and Seedsb kJ × 109
Electricityb kJ × 109
Trans-portationb kJ × 109
Total Energy Input kJ × 109
Energy Input per Area (kJ/ha) x106
Percent of fertilizer Energy Input
Fertilizer Energy Requirement for Production, Packaging, Transportation, and Application
Fertilizer Energy Requirements (kJ/kg)a
Results and Discussion
Trends in U.S. Fertilizer Use for All Crops and for Corn from 1964 to 2009
Distribution of Fertilizer Use and Energy Inputs for the Sixteen Crops
The average fertilizer application rate varied greatly amongst the 16 crops (Table 1). In general, vegetables and fruits had higher fertilizer application rates compared to many grains and oilseeds. The N application rate for soybeans and dry beans were relatively small since these crops are able to fix nitrogen from the atmosphere.
Fertilizer Energy, Energy Efficiency, Mass Yield, and Energy Yield for the 16 Crops
The energy input of fertilizers as percent of the total energy input had a wide range for the 16 crops (2% for apples to 60% for oats). The harvested-area weighted-average of energy cost of fertilizers was 28% of the total energy input. This percentage is comparable to the estimate of 30% made by Pimentel and Pimentel (2008).
Mass Yield per Fertilizer Mass, Protein Yield, Energy Efficiency, and Mass Yield per Energy Input for 16 Crops
Mass Yield per Total Energy Input (kg/MJ)
Mass Yield per Mass N (Mg/kg N)
Mass Yield per Mass P (Mg/kg P)
Mass Yield per Mass K (Mg/kg K)
Mass of Protein per Mass produced (%)
Ratio of Protein Energy to N Energy (kJ/kJ)
Ratio of Mass of Protein to Mass of N (kg/kg)
Energy Efficiency (kJ/kJ)
Excluding solar energy inputs, energy efficiency ratios (Eq. 2) for the 16 crops are in the range of 0.2–4.2 with an un-weighted mean of 2.1 (Table 4). That is, the energy that is available from the crops after harvest is 20–420% of the energy expended to grow the crops. Tables 1 and 4 show that the crops with the highest fertilizer application rates (fruits and vegetables) have the lowest energy efficiency ratios. However, the energy return of crops cannot be the only criteria used to assess their value. Many crops are grown not only for their energy content, but also for their other values such as vitamins, fiber, protein, minerals, and taste. In particular, vegetables and fruits are important sources of many vitamins. These data also show that the energy efficiency ratios of grain crops such as oats (3.73) and barley (4.13) are comparable to corn (4.21, Table 4). The efficiency values of oats and barley agree with previous propositions that their feedstock could serve as potential alternate crops for the production of biofuels (Kim and Dale 2004).
One of the important uses of N by crops is in their production of protein. Table 4 compares the protein contents of the 16 crops. Grains, vegetables, and fruits have lower protein content (mass of protein per mass produced, 7.1–9.4% for grains and 0.3–2.9% for fruits and vegetables) compared to oilseeds (28.5–36.5%) and dry beans (19.3%). Protein mass to N mass ratios were 8, 26, and 60 for canola, soybeans, and peanuts, respectively. A similar pattern was observed for protein energy to N energy ratios. The protein mass to N mass and protein energy to N energy ratios for dry beans were 102 and 21.7, respectively (see Table 4). Even though dry beans had the lowest fertilizer energy inputs (2.3 × 1011 kJ, 3% of total energy input), the protein content, protein mass to N mass, and protein energy to N energy ratios were much higher compared to all other crops. With such remarkably high protein content and energy return, the substitution of dry beans, a protein-rich food, for exported corn and wheat could increase the supply of dietary protein needed in many countries without increasing the N fertilizer use in the U.S.
The agricultural systems in the U.S. rely heavily on fossil fuels, but the emerging uncertainties about the fuel supply necessitate a critical look at fertilizer management practices. Recent high fertilizer energy costs have been attributed to the price of natural gas and difficulty in mining of fertilizers (Cassman and Liska 2007; Huang et al. 2009; Cordell et al. 2009). Non-renewable natural gas accounts for about 72–90% of the cost of manufacturing N fertilizers. Phosphorus also requires fossil fuels in the mining, extracting, processing, and transportation of P. Mining of P has become energy intensive and expensive as newly found deposits are more difficult to extract. Estimation of the currently known, easily mined P reserves suggest that the world P reserves will likely be depleted in the next 50–100 years (Cordell et al. 2009). The world has enough K to last a long time, but extraction and mining of K is limited to just a few countries. Most countries, including the U.S., import almost all of the K fertilizers consumed.
The actual energy required to obtain fertilizers will continue to increase as they become even more critical to agricultural production. The U.S. and world agriculture faces the challenge of producing more food, feed, fuel, and fiber for the growing population (Prasad 2013). The populations of the U.S. and worldwide are projected to rise by 26 and 34%, respectively, by 2050 (United Nation 2013; US Census Bureau 2013). To feed this many people without substantially increasing arable lands could be accomplished by intensification of production, which includes a dependence on fertilization (Bruinsma 2009). In order to increase food production amidst these challenges, farmers will have to employ management strategies to use fertilizers effectively and efficiently.
The focus of societal and policy discussions has generally been on fertilizer management on the farm to increase yields and food quality without adversely affecting the environment. This approach, however, does not address the finiteness of P reserves, fossil fuel depletion, the enormous amounts of energy used in the fertilizer manufacturing process, and the consequences on global food security for the growing population. In view of these challenges, it is critical that an integrated approach is considered by society and policy-makers to address fertilizer management and future scarcity issues (Prasad 2009). Cordell et al. (2009) suggest that societal efforts should be made to change consumption, recovery, and recycling of P. In addition, discussions on other alternate fertilizer sources such as crop residues, animal and human waste, N-fixing legumes, and planting of cover crops could be re-visited to help protect future generations from food security crises.
Fertilizers (N, P, and K) have played and continue to play an important role in sustaining agricultural output for food, feed, fuel, and fiber in the U.S. Increases in crop yields, such as with corn, have been linked to increases in fertilizer consumption. However, emerging uncertainties in P reserves and increases in the cost of fossil fuels used in manufacturing of N fertilizers indicate the need to better understand agricultural energy inputs in relation to fertilizer energy inputs. For 16 crops examined here, the fertilizer input energy was 2–60% of the total input energy used to grow the selected crops. The fertilizer input energy as a percent of total input energy was 19–60% for seven grains, 10–41% for two oilseeds, 25% for potatoes, 12–30% for three vegetables, 2–23% for two fruits, and 3% for dry beans. The harvested-area weighted-average of fertilizer input energy for the 16 crops was 28% of the total energy input.
Generally, crops that were grown and harvested over large areas had much higher fertilizer energy inputs as well as total energy inputs. Aside from higher fertilizer energy inputs, crops that were harvested over large areas also had much higher energy input to fuel, machinery, and pesticides.
The energy efficiency ratios for the 16 crops, excluding solar energy, were in the range of 0.2 (tomatoes) to 4.2 (corn). Higher energy efficiency ratios were observed for grains and oilseed crops than for fruits and vegetables crops. However, fruits and vegetables are not grown only for their energy content, but are also important sources of vitamins, fiber, and minerals.
Comparison of protein content of the 16 crops revealed that dry beans had high protein mass to N mass (102) and protein energy to N energy (21.7) ratios, yet fertilizer input energy was one of the lowest (3% of the total energy input) among the crops. The possibility of supplying alternate crops with high protein content to low-income populations without increasing N fertilizer energy inputs is promising.
Despite the uncertainties in future sources of fertilizers, society and policy discussions have been focused on the management of fertilizer applications on the farm to increase yields and food quality without adversely affecting the environment. These approaches do not address the enormous energy and fossil fuel resources required for manufacturing N fertilizer or the enormous energy required to mine the finite supply of phosphate. To prevent food shortages in the future, it is critical that an integrated approach is considered to simultaneously address both environmental impacts and future scarcity issues of fertilizers. In addition, alternative sources of fertilizers, more efficient use of fertilizers, and energy efficient measures in the manufacturing of fertilizers could be employed to alleviate potential future problems.
We would like to acknowledge the support of the U.S. Geologic Survey National Water-Quality Assessment Program. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.