Natural Resources Research

, Volume 23, Issue 3, pp 299–309

Fertilizer Consumption and Energy Input for 16 Crops in the United States

Authors

  • Sheila E. Amenumey
    • University of Minnesota
    • U.S. Geological Survey, University of Minnesota
Article

DOI: 10.1007/s11053-013-9226-4

Cite this article as:
Amenumey, S.E. & Capel, P.D. Nat Resour Res (2014) 23: 299. doi:10.1007/s11053-013-9226-4

Abstract

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.

Keywords

Fertilizerenergy efficiencyenergy inputcrop productivityglobal food security

Introduction

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.

Methods

Production and Productivity Data

Sixteen crops were selected based on the area harvested, importance to the U.S. food supply, and data from previous studies on input of energy for production. Corn, soybean, wheat, sorghum, barley, rice, oats, peanuts, potatoes, and canola were selected as the major grain and oilseed crops. Apples, oranges, tomatoes, cabbage, and spinach were selected as examples of fruits and vegetables. Dry beans were selected as the example legume. Together these crops constitute 80% of the U.S. harvested cropland in 2009. The mass of production, harvested area, mass yield, energy content, protein content, and the application rates of N, P, and K for the 16 crops were obtained from various USDA sources (Table 1). The energy content calculated for each crop was based on the energy content of the crop as it left the farm field (USDA/National Agricultural Library 2013). No processing energy was included in the analysis. The mass yield (Mg/ha) for a crop is calculated from the total harvested mass (Mg) divided by the total area harvested (ha). Based on these data, the following parameters were calculated: the mass yield per mass fertilizer applied (Eq. 1), energy efficiency (Eq. 2), the mass yield per total energy input (Eq. 3), and the mass of protein from crop per mass of crops produced (Eq. 4)
$$ {\text{Mass}}\;{\text{yield}}\;{\text{per}}\;{\text{mass}}\;{\text{of}}\;{\text{fertilizer}}\;{\text{applied}} = {\text{mass}}\;{\text{yield}}\, ( {\text{Mg}}/{\text{ha)}}/{\text{fertilizer}}\;{\text{rate}}\, ( {\text{kg}}/{\text{ha)}} $$
(1)
$$ {\text{Energy}}\;{\text{efficiency}} = {\text{energy}}\;{\text{output}}\, ( {\text{kJ)}}/{\text{energy}}\;{\text{input}}\, ( {\text{kJ)}} $$
(2)
$$ {\text{Mass}}\;{\text{yield}}\;{\text{per}}\;{\text{total}}\;{\text{energy}}\;{\text{input}} = {{{\text{mass}}\;{\text{yield}}\, ( {\text{Mg}}/{\text{ha)}}} \mathord{\left/ {\vphantom {{{\text{mass}}\;{\text{yield}}\, ( {\text{Mg}}/{\text{ha)}}} {{\text{energy}}\,{\text{input}}\,({\text{MJ/ha}})}}} \right. \kern-0pt} {{\text{energy}}\,{\text{input}}\,({\text{MJ/ha}})}} $$
(3)
$$ {\text{Mass}}\;{\text{of}}\;{\text{protein}}\;{\text{per}}\;{\text{mass}}\;{\text{of}}\;{\text{crop}}\;{\text{produced}} = {\text{mass}}\;{\text{of}}\;{\text{protein}}\, ( {\text{kg)}}/{\text{mass}}\;{\text{produced}}\, ( {\text{kg)}} . $$
(4)
Table 1

Agriculture Production (2009) Values, Energy Content and Output, and Fertilizer Application Rates for 16 Crops

Cropa

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

Corn

32,209b

10.3

333,011b

15,270

5,085,077

157.9

155i

53i

56i

Soybeans

30,907b

3.00

91,417b

23,050

1,604,428

51.90

18j

52j

90j

Wheat

20,191b

3.00

60,367b

13,877

828,628

41.00

42i

14i

5i

Sorghum

2,234b

4.40

9,575b

14,180

135,767

60.80

90j

36j

30j

Barley

1,260b

3.90

4,949b

14,730

72,904

57.90

67j

34j

20j

Rice

1,256b

7.90

9,972b

15,480

154,370

122.9

157j

60j

73j

Beans

593b

2.00

1,153b

15,250

17,589

29.70

4i

3i

4i

Oats

558b

2.40

1,351b

16,280

21,995

39.40

62j

39j

56j

Peanuts

437b

3.80

1,675b

23,740

39,753

91.00

38j

54j

84j

Potatoes

421b

46.4

19,564b

2,430

47,541

112.8

188i

127i

151i

Canola

330b

2.00

669b

34,675

22,992

69.70

90 k

29 k

16 k

Oranges

266b

31.2

8,281b

1,970

16,313

61.40

155i

21i

154i

Apples

141c

32.0

4,497c

2,180

9,804

69.70

21i

5i

11i

Tomatoes

44d

34.3

1,508d

630

950

21.60

130i

91i

156i

Cabbage

26d

38.6

1,019d

1,030

1,050

39.70

116i

66i

96i

Spinach

15d

20.9

309d

970

300

20.30

187i

40i

14i

aCrops sorted by area harvested.

bUSDA/National Agricultural Statistics Service (2010a).

cUSDA/National Agricultural Statistics Service (2010b).

dUSDA/National Agricultural Statistics Service (2010c).

eReferences same as “b” if available, if not mass produced is divided area harvested.

fUSDA/National Agricultural Library (2013).

gEnergy output (kg) = mass of crop produced (kg) × energy content of useable crop (kJ/kg).

hExpressed as N.

iUSDA/National Agricultural Statistics Service (2013).

jUSDA/Economic Research Service (2013c).

kCanolaWatch (2011).

lExpressed as P2O5.

mExpressed as K2O.

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

Energy needs for crop growth is a combination of natural and farming activities. The natural energy derived from sunlight (for photosynthesis) is not included in these calculations. The farming activities were divided into eight categories: labor, fuel, machinery, irrigation, fertilizer (N, P, K), pesticides/other chemicals/seeds, electricity, and transportation (Table 2). All the energy inputs for crop production, except fertilizers, were estimated based on data from the literature and summarized in Table 2. Labor energy inputs, calculated from Pimentel (2006) are only for on-farm activities and do not include other labor used in the manufacturing of machinery, fertilizers, or pesticides. Fuel energy inputs are estimated according to the consumption by farm equipment such as harvesters and tractors (Cervinka 1980; Pimentel 2006). Machinery energy inputs, on annual basis, were based on pro-rated energy costs used in fabrication of the machinery plus its maintenance. Irrigated energy input derived from the literature, involves energy required in installing systems, pumping, and delivery of water to crops (Batty and Keller 1980; Pimentel and Pimentel 1996). Because of variation in the use of irrigation and in irrigation system efficiency, these energy inputs vary widely. Transportation energy, included in the energy budget, is energy required to transport farm supplies such as seeds, pesticides, machinery, and fuel, but not fertilizers. Transportation energy inputs are based on assumptions and calculations in Pimentel (1980). Finally, electricity is used for lighting, heating, and cooling barns and other farm structures. Electrical energy input was estimated from the amount of fossil energy used to produce electricity (Pimentel 2006).
Table 2

Distribution of Energy Inputs for the 16 Crops

Cropa

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

Cornd

68,454

208,622

150,836

47,414

444,012

257,073

5,038

25,041

1,206,491

37

37

Soybeanse

37,597

97,567

47,658

 

109,410

164,950

3,839

5,295

466,316

15

23

Wheatf

35,143

101,981

88,969

 

72,551

69,329

4,560

13,679

386,211

19

19

Sorghumg

75

20,660

7,481

8,380

17,992

12,951

5,095

558

73,191

33

25

Barleyh

42

4,478

824

 

7,699

4,313

310

17,665

14

44

Ricei

5,502

17,721

4,200

12,107

17,983

8,648

481

657

67,299

54

27

Beansj

15

2,877

1,194

 

231

3,571

95

126

8,108

14

3

Oatsk

3

1,053

274

 

3,537

957

79

5,903

11

60

Peanutsl

17

3,876

676

 

2,217

14,102

77

162

21,126

48

10

Potatoesm

3,952

9,026

1,155

 

8,016

4,906

272

4,643

31,970

76

25

Canolan

500

1,321

643

 

2,551

1,037

52

46

6,151

19

41

Orangeso

7,042

1,548

361

 

3,875

4,216

43

96

17,181

65

23

Applesp

5,865

6,467

481

 

269

3,488

19

269

16,859

120

2

Tomatoesq

2,200

1,382

241

152

611

304

30

41

4,963

113

12

Cabbager

417

449

150

 

307

73

47

10

1,451

55

21

Spinacht

3

343

55

8

230

75

35

9

758

51

30

Original energy values from the literature were based on plot studies, energy values have been scaled up to cover total crop area harvested in 2009.

aCrops are sorted by areas harvested.

bEnergy Input = energy input from literature (kJ/ha) × area harvested in 2009 × [(yield from the literature)/(yield for 2009)], sources of energy input from literature are shown in footnote (d–s) below.

cFertilizer energy input (kJ) = mass of fertilizer (kg) × fertilizer energy required (kJ/kg)

dPimentel and Patzek (2008)

ePimentel and Patzek (2008).

fPimentel (2006).

gPimentel and Pimentel (2008).

hBukantis and Goodman (1980).

iPimentel (2006).

jPimentel and Pimentel (2008).

kPimentel and Pimentel (2008).

lPimentel and Pimentel (2008).

mPimentel (2006).

nPimentel and Patzek (2008).

oPimentel (2006).

pPimentel (2006).

qPimentel (2006).

rPimentel (2006).

sPimentel and Patzek (2008).

The energy required for fertilizers (N, P, and K) is the sum of the production, packaging, transportation, and application energies (Table 3). The mass of each fertilizer applied to the crop was calculated as the product of the average application rate and the national crop area harvested. To calculate the fertilizer input energy, the energies from Table 2 were multiplied by the total mass of fertilizer applied to each crop in 2009. Results of the energy input distribution and sum of total fertilizer (N + P + K) energy input for the specific crops are shown in Table 2.
Table 3

Fertilizer Energy Requirement for Production, Packaging, Transportation, and Application

Categories

Fertilizer Energy Requirements (kJ/kg)a

Nitrogenb

Phosphorusc

Potassiumd

Production

69,530

7,700

6,400

Packaging

2,600

2,600

1,800

Transportation

4,500

5,700

4,600

Application

1,600

1,500

1,000

Total

78,230

17,500

13,800

aGellings and Parmenter (2002).

bExpressed as N.

cExpressed as P2O5.

dExpressed as K2O.

Results and Discussion

Trends in U.S. Fertilizer Use for All Crops and for Corn from 1964 to 2009

In 2009, crop agriculture used 1.6 × 1010 kg of fertilizers (N, P, and K); N accounted for 65% of this mass. The use of N fertilizer substantially increased from the 1964 to the early 1980s and has leveled off during the past two decades. An analysis of variance (ANOVA) of N fertilizer used for all crops during the past two decades (1990–1999 and 2000–2009) showed no statistically significant difference between the means of the two decades. During the 1960–2009 period, the consumption of P and K fertilizers also increased substantially. However, the increase from the 1960s to the 1980s was less, compared to N fertilizer consumption. A similar ANOVA for the last two decades showed no statistically significant differences in P and K fertilizer consumption for all crops (Fig. 1).
https://static-content.springer.com/image/art%3A10.1007%2Fs11053-013-9226-4/MediaObjects/11053_2013_9226_Fig1_HTML.gif
Figure 1

Use of N, P, and K fertilizers in the U.S. from 1964 to 2009 (USDA/Economic Research Service 2013d)

More fertilizer is used on corn than any other crop in the U.S. In 2009, 44% of total fertilizer use (N, P, and K) was on corn. Figure 2 shows that the use of N on corn increased sharply from the 1960s to the mid-1980s, but has leveled off since then. An ANOVA showed no statistically significant differences between the means of N use on corn during the last two decades (1990–1999 and 2000–2009). The use of both P and K increased in the 1960s and 1970s; since then, use of both P and K has remained relatively constant. An ANOVA for the last two decades of both P and K corn consumption did not show any statistically significant differences. Corn yield had an almost constant, linear increase between 1964 and 2009 at an average rate of 117 kg/ha/year with the exception of a few years (1974, 1983, 1988, and 1993). Since the increase in corn yield outpaced the increased application of fertilizers, it suggests that the increases in corn yield cannot be attributed to fertilizer increases alone, but also to advances in genetics, biotechnology, pesticides, and/or irrigation.
https://static-content.springer.com/image/art%3A10.1007%2Fs11053-013-9226-4/MediaObjects/11053_2013_9226_Fig2_HTML.gif
Figure 2

N, P, and K use as fertilizer on corn, together with corn yields, from 1964 to 2009 (USDA/Economic Research Service 2013d)

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.

The fertilizer application rate multiplied by the harvested area is an estimate the total mass used. The total mass of fertilizers used on fruits and vegetables was small compared to fertilizer use on grains and oilseeds, because the harvested area of vegetables and crops was small relative to the major crops (Fig. 3; Table 1). Even though crops with much larger harvested areas had higher energy inputs, their energy per area values were less compared to fruits and vegetables. For example, the 2009 average N application rate was 42 kg/ha for wheat and 187 kg/ha for spinach, but the total N masses used were 8 × 108 and 2 × 106 kg, respectively. On a per unit area basis, energy input for fruits and vegetable was much higher compared to grains and oilseed crops (Tables 1, 2).
https://static-content.springer.com/image/art%3A10.1007%2Fs11053-013-9226-4/MediaObjects/11053_2013_9226_Fig3_HTML.gif
Figure 3

Total mass N, P, and K fertilizers used on the 16 crops in 2009

Four crops—a grain (corn), an oilseed (soybean), a vegetable (cabbage), and a fruit (orange)—were selected out of the 16 crops for a detailed illustration of the energy inputs (Fig. 4). Inputs of energy for fertilizers for these four crops represent a substantial part of the energy for production. The energy input for fertilizers was between about one-fourth to one-third of the total energy costs of production for all four crops (corn 37%, soybean 23%, cabbage 21%, and orange 22%). However, the energy input of production was much more variable for labor, pesticides, fuel, and machinery among the four crops (Table 2).
https://static-content.springer.com/image/art%3A10.1007%2Fs11053-013-9226-4/MediaObjects/11053_2013_9226_Fig4_HTML.gif
Figure 4

Distribution of energy inputs of corn, soybean, cabbages, and oranges

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).

For the 16 crops, the distribution of the energy inputs for N, P, and K fertilizers per harvested area and for the sum of all non-fertilizers energy inputs is shown in Figure 5. The non-fertilizer input energies per total harvested area varied considerably among the 16 crops. The input energies for P and K per harvested area had a smaller range as compared to N. The grains (corn, barley, wheat, and sorghum) generally had high N energy inputs and relatively low P and K energy inputs. The crop-to-fertilizer mass ratios (Eq. 1) and mass-to-energy ratios (Eq. 3) for vegetables, fruits, and dry beans were greater than most grains and oilseeds (Table 4).
https://static-content.springer.com/image/art%3A10.1007%2Fs11053-013-9226-4/MediaObjects/11053_2013_9226_Fig5_HTML.gif
Figure 5

Percentages of N, P, and K and combined non-fertilizer energy inputs for the 16 crops

Table 4

Mass Yield per Fertilizer Mass, Protein Yield, Energy Efficiency, and Mass Yield per Energy Input for 16 Crops

Crop

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)

Corn

0.28

0.07

0.20

0.18

9.4

1.3

6.3

4.21

Soybeans

0.20

0.17

0.06

0.03

36.5

12.8

60

3.44

Wheat

0.16

0.07

0.22

0.58

13.9

2.1

9.9

2.15

Sorghum

0.13

0.05

0.12

0.14

11.3

1.1

5.4

1.85

Barley

0.28

0.06

0.12

0.20

9.9

1.2

5.8

4.13

Rice

0.15

0.05

0.13

0.11

7.1

0.8

0.1

2.29

Dry Beans

0.14

0.53

0.68

0.52

19.3

21.7

102

2.17

Oats

0.23

0.04

0.06

0.04

16.9

1.4

6.5

3.73

Peanuts

0.08

0.10

0.07

0.05

25.8

5.5

26

1.88

Potatoes

0.61

0.25

0.37

0.31

2.6

1.3

6.3

1.49

Canola

0.11

0.02

0.07

0.13

36.0

1.7

8.2

3.74

Oranges

0.48

0.20

1.49

0.20

0.9

0.4

1.9

0.95

Apples

0.27

1.49

7.01

2.84

0.3

0.8

3.9

0.58

Tomatoes

0.30

0.26

0.38

0.22

1.0

0.5

2.6

0.19

Cabbage

0.70

0.33

0.58

0.40

1.3

0.9

4.2

0.72

Spinach

0.41

0.11

0.52

1.51

2.9

0.7

3.2

0.40

Crops are sorted by areas harvested.

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.

Policy Implications

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.

Conclusions

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.

Acknowledgments

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.

Copyright information

© 2013 International Association for Mathematical Geosciences (outside the USA) 2013