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Publication #HS1229

Conversions of parts per million on soil test reports to pounds per acre 1

Guodong Liu, Yuncong Li, and Aparna Gazula2

Soil testing and the resulting fertilization recommendations are critical for appropriate nutrient management in commercial vegetable production. It is just as critical that test results and fertilization recommendations are understood by growers. UF/IFAS soil test reports use parts per million (ppm) to report soil-extractable plant nutrients. The UF/IFAS fertilizer recommendations use pounds of nutrient per acre. Growers also operate with pounds of a particular nutrient per acre to apply fertilizers. If growers don’t understand the soil test reports and fertilization recommendations, however, they may over-fertilize crops. One way to avoid miscommunication is to remember that growers and soil experts sometimes speak different languages.

Nutrients in soil can be classified in several ways (water-soluble nutrients, extractable nutrients, etc.). For fertilization recommendations, soil labs usually use the term, extractable nutrients. The level of extractable nutrients in a soil sample is determined by the extractant used. This means that when we talk about soil nutrient contents, we need to know what extractant was used to obtain the sample because different extractants can give totally different values even for the same soil sample. For example, using a particular extractant (e.g. Mehlich-I) to extract a soil sample can obtain a value of a specific nutrient such as phosphorus, e.g., 50 ppm. This value means that the tested soil contains 50 mg of Mehlich-I-extractable phosphorus per kg of soil. The phosphorus level of a particular soil sample will be greater if the extractant Mehlich-I was used than the phosphorus level in the same sample if the extractant Mehlich-III, Bray-1, or Olsen was used. If no extractant is listed on the soil test report, the reported phosphorus level will be relatively meaningless.

The terms extractant and extractable phosphorus are commonly used and understood in soil labs, but may not be understood as well by growers. To avoid misunderstandings and over- or under-fertilization, we must understand what relationship exists between the numbers of soil test reports, fertilization recommendations, and fertilization practices. How can we compare these numbers? The conversion from parts per million of soil-extractable nutrients on a soil test report to pounds per acre will be helpful for producers. The conversion can, in turn, help to (1) better understand how much extractable nutrient is in the soil; (2) optimize fertilization practices based on these reports; and (3) minimize over-fertilization through optimization of fertilization.

If our soils contain very high levels of phosphorus and calcium but phosphate fertilizers and gypsum are still applied every year, what will happen? The answer is that these applications do not only waste nonrenewable resources but also produce phosphate rock in the fertilized field, particularly when soil pH is greater than 6.5. For example, applying 100 to 120 lb P2O5 per acre is not only unnecessary but also harmful to the environment if a calibrated soil test shows a very high level (e.g., 550 ppm) of phosphorus existing in the soil. Similarly, 1000 lb gypsum (233 lb calcium) per acre every year should not be applied if the soil test shows a very high level of calcium (e.g., 1550 ppm). This article provides a simple conversion method for crop consultants, crop advisors, growers, students, and researchers who are interested in nutrient and water management of crop production.

How to convert parts per million to pounds per acre

If we assume the plow layer or root zone depth is 6 inches, the total weight of soils in the plow layer can be calculated as:

Soil weight per acre = volume x bulk density

= (43560 ft2/acre x 0.5 ft) x 92 lb/ft3 ≈ 2,000,000 lb/acre

Because UF/IFAS soil test reports report extractable nutrients in parts per million, we can readily use a factor of 2 to convert the actual number of ppm to pounds of a particular extractable nutrient per acre. In the above example, the extractable concentrations were reported as 550 ppm of phosphorus and 1550 ppm of calcium. These numbers mean that soil has 1100 pounds of extractable phosphorus per acre and 3100 pounds of calcium per acre. Further, in fertilization of phosphorus and potassium, their oxides are used. Therefore, the pounds of extractable phosphorus pentoxide (P2O5) and potassium oxide (K2O) need to be calculated by additional conversion factors of 2.2913 and 1.2046 for phosphorus and potassium, respectively. The soil in the above example has 550 ppm phosphorus and hence contains 2520 pounds of extractable phosphorus pentoxide per acre. The conversion equations are below. More conversions are available in Table 1.

Extractable nutrient (ppm) × 2 = pounds of the extractable nutrient per acre

Extractable phosphorus (ppm) × 2 × 2.2913 = pounds of the extractable phosphorus pentoxide per acre

Extractable potassium (ppm) × 2 × 1.2046 = pounds of the extractable potassium oxide per acre

Of course, the amount extracted by the soil test is not directly equivalent to fertilizer grade nutrients, and the soil chemistry also influences the plant-available nutrient. Since the soil test is used to predict crop response to added nutrients, at these large soil-test values, we can expect little plant response to application of fertilizers. We must consider other actions and management decisions that can create a confusing and inefficient use of nutrients. The right amount of phosphorus and gypsum (calcium) from fertilizer can help plants grow, but too much will form rock phosphate and related compounds, none of which nutrient sources are available to plants. You will find more information on this and related issues in forthcoming EDIS documents.

How to convert pounds per acre to parts per million:

We can easily use a factor of 0.5 to convert the pounds per acre of a nutrient in its elemental form into ppm. To convert 120 pounds per acre to ppm, just multiply 120 pounds per acre by 0.5, which is to equal 60 ppm. This conversion works for all nutrients reported in these units: macronutrients, such as calcium or magnesium, and micronutrients, such as iron, manganese, zinc, and copper. Again, another factor is needed when converting pounds per acre of phosphorus and potassium in their oxide forms into number of ppm in their elemental forms: 0.4364 and 0.8302, respectively. For example, to convert 100 pounds per acre of phosphate fertilizer (P2O5) to ppm of elemental phosphorus, just multiply 100 by 0.5 and then by 0.4364 to get 21.8 ppm of phosphorus. Similarly, 100 pounds per acre of potash fertilizer (K2O) will increase soil potassium (K) by 41.5 ppm. The conversion equations are below. More conversions are available in Table 2.

Pounds of nutrient applied per acre × 0.5 = increment of the nutrient in soil (ppm)

Pounds of phosphorus pentoxide applied per acre × 0.5 × 0.4364 = increment of phosphorus in soil (ppm)

Pounds of potassium oxide applied per acre × 0.5× 0.8302 = increment of potassium in soil (ppm)

Tables

Table 1. 

Conversion from ppm on soil test report to pounds per acre for fertilization practices

Soil test

Pounds per acre

PPM

P

P2O5

K

K2O

Other nutrients

5

10

22.9

10

12.0

10

10

20

45.8

20

24.1

20

15

30

68.7

30

36.1

30

20

40

91.7

40

48.2

40

25

50

114.6

50

60.2

50

30

60

137.5

60

72.3

60

35

70

160.4

70

84.3

70

40

80

183.3

80

96.4

80

45

90

206.2

90

108.4

90

50

100

229.1

100

120.5

100

55

110

252.0

110

132.5

110

60

120

275.0

120

144.6

120

65

130

297.9

130

156.6

130

70

140

320.8

140

168.6

140

75

150

343.7

150

180.7

150

80

160

366.6

160

192.7

160

85

170

389.5

170

204.8

170

90

180

412.4

180

216.8

180

95

190

435.3

190

228.9

190

100

200

458.3

200

240.9

200

105

210

481.2

210

253.0

210

110

220

504.1

220

265.0

220

115

230

527.0

230

277.1

230

120

240

549.9

240

289.1

240

125

250

572.8

250

301.2

250

130

260

595.7

260

313.2

260

135

270

618.7

270

325.2

270

140

280

641.6

280

337.3

280

145

290

664.5

290

349.3

290

150

300

687.4

300

361.4

300

155

310

710.3

310

373.4

310

160

320

733.2

320

385.5

320

165

330

756.1

330

397.5

330

170

340

779.0

340

409.6

340

175

350

802.0

350

421.6

350

180

360

824.9

360

433.7

360

185

370

847.8

370

445.7

370

190

380

870.7

380

457.7

380

195

390

893.6

390

469.8

390

200

400

916.5

400

481.8

400

250

500

1145.7

500

602.3

500

300

600

1374.8

600

722.8

600

350

700

1603.9

700

843.2

700

400

800

1833.0

800

963.7

800

450

900

2062.2

900

1084.1

900

500

1000

2291.3

1000

1204.6

1000

600

1200

2749.6

1200

1445.5

1200

700

1400

3207.8

1400

1686.4

1400

800

1600

3666.1

1600

1927.4

1600

900

1800

4124.3

1800

2168.3

1800

1000

2000

4582.6

2000

2409.2

2000

Footnote: phosphorus pentoxide (P2O5) contains 43.64% of phosphorus; potassium oxide (K2O) contains 83.02% potassium.

Table 2. 

Conversion from pounds per acre for fertilization practices to ppm on soil test reports

Soil test

PPM

Pounds/acre

P

P2O5

K

K2O

Other nutrients

1

0.5

1.1

0.5

0.6

0.5

2

1.0

2.3

1.0

1.2

1.0

3

1.5

3.4

1.5

1.8

1.5

4

2.0

4.6

2.0

2.4

2.0

5

2.5

5.7

2.5

3.0

2.5

6

3.0

6.9

3.0

3.6

3.0

7

3.5

8.0

3.5

4.2

3.5

8

4.0

9.2

4.0

4.8

4.0

9

4.5

10.3

4.5

5.4

4.5

10

5.0

11.5

5.0

6.0

5.0

11

5.5

12.6

5.5

6.6

5.5

12

6.0

13.7

6.0

7.2

6.0

13

6.5

14.9

6.5

7.8

6.5

14

7.0

16.0

7.0

8.4

7.0

15

7.5

17.2

7.5

9.0

7.5

16

8.0

18.3

8.0

9.6

8.0

17

8.5

19.5

8.5

10.2

8.5

18

9.0

20.6

9.0

10.8

9.0

19

9.5

21.8

9.5

11.4

9.5

20

10.0

22.9

10.0

12.0

10.0

21

10.5

24.1

10.5

12.6

10.5

22

11.0

25.2

11.0

13.2

11.0

23

11.5

26.4

11.5

13.9

11.5

24

12.0

27.5

12.0

14.5

12.0

25

12.5

28.6

12.5

15.1

12.5

26

13.0

29.8

13.0

15.7

13.0

27

13.5

30.9

13.5

16.3

13.5

28

14.0

32.1

14.0

16.9

14.0

29

14.5

33.2

14.5

17.5

14.5

30

15.0

34.4

15.0

18.1

15.0

31

15.5

35.5

15.5

18.7

15.5

32

16.0

36.7

16.0

19.3

16.0

33

16.5

37.8

16.5

19.9

16.5

34

17.0

39.0

17.0

20.5

17.0

35

17.5

40.1

17.5

21.1

17.5

36

18.0

41.2

18.0

21.7

18.0

37

18.5

42.4

18.5

22.3

18.5

38

19.0

43.5

19.0

22.9

19.0

39

19.5

44.7

19.5

23.5

19.5

40

20.0

45.8

20.0

24.1

20.0

41

20.5

47.0

20.5

24.7

20.5

42

21.0

48.1

21.0

25.3

21.0

43

21.5

49.3

21.5

25.9

21.5

44

22.0

50.4

22.0

26.5

22.0

45

22.5

51.6

22.5

27.1

22.5

46

23.0

52.7

23.0

27.7

23.0

47

23.5

53.8

23.5

28.3

23.5

48

24.0

55.0

24.0

28.9

24.0

49

24.5

56.1

24.5

29.5

24.5

50

25.0

57.3

25.0

30.1

25.0

51

25.5

58.4

25.5

30.7

25.5

52

26.0

59.6

26.0

31.3

26.0

53

26.5

60.7

26.5

31.9

26.5

54

27.0

61.9

27.0

32.5

27.0

55

27.5

63.0

27.5

33.1

27.5

56

28.0

64.2

28.0

33.7

28.0

57

28.5

65.3

28.5

34.3

28.5

58

29.0

66.5

29.0

34.9

29.0

59

29.5

67.6

29.5

35.5

29.5

60

30.0

68.7

30.0

36.1

30.0

61

30.5

69.9

30.5

36.7

30.5

62

31.0

71.0

31.0

37.3

31.0

63

31.5

72.2

31.5

37.9

31.5

64

32.0

73.3

32.0

38.5

32.0

65

32.5

74.5

32.5

39.1

32.5

66

33.0

75.6

33.0

39.7

33.0

67

33.5

76.8

33.5

40.4

33.5

68

34.0

77.9

34.0

41.0

34.0

69

34.5

79.1

34.5

41.6

34.5

70

35.0

80.2

35.0

42.2

35.0

71

35.5

81.3

35.5

42.8

35.5

72

36.0

82.5

36.0

43.4

36.0

73

36.5

83.6

36.5

44.0

36.5

74

37.0

84.8

37.0

44.6

37.0

75

37.5

85.9

37.5

45.2

37.5

76

38.0

87.1

38.0

45.8

38.0

77

38.5

88.2

38.5

46.4

38.5

78

39.0

89.4

39.0

47.0

39.0

79

39.5

90.5

39.5

47.6

39.5

80

40.0

91.7

40.0

48.2

40.0

81

40.5

92.8

40.5

48.8

40.5

82

41.0

94.0

41.0

49.4

41.0

83

41.5

95.1

41.5

50.0

41.5

84

42.0

96.2

42.0

50.6

42.0

85

42.5

97.4

42.5

51.2

42.5

86

43.0

98.5

43.0

51.8

43.0

87

43.5

99.7

43.5

52.4

43.5

88

44.0

100.8

44.0

53.0

44.0

89

44.5

102.0

44.5

53.6

44.5

90

45.0

103.1

45.0

54.2

45.0

91

45.5

104.3

45.5

54.8

45.5

92

46.0

105.4

46.0

55.4

46.0

93

46.5

106.6

46.5

56.0

46.5

94

47.0

107.7

47.0

56.6

47.0

95

47.5

108.8

47.5

57.2

47.5

96

48.0

110.0

48.0

57.8

48.0

97

48.5

111.1

48.5

58.4

48.5

98

49.0

112.3

49.0

59.0

49.0

99

49.5

113.4

49.5

59.6

49.5

100

50.0

114.6

50.0

60.2

50.0

101

50.5

115.7

50.5

60.8

50.5

102

51.0

116.9

51.0

61.4

51.0

103

51.5

118.0

51.5

62.0

51.5

104

52.0

119.2

52.0

62.6

52.0

105

52.5

120.3

52.5

63.2

52.5

106

53.0

121.4

53.0

63.8

53.0

107

53.5

122.6

53.5

64.4

53.5

108

54.0

123.7

54.0

65.0

54.0

109

54.5

124.9

54.5

65.6

54.5

110

55.0

126.0

55.0

66.2

55.0

111

55.5

127.2

55.5

66.9

55.5

112

56.0

128.3

56.0

67.5

56.0

113

56.5

129.5

56.5

68.1

56.5

114

57.0

130.6

57.0

68.7

57.0

115

57.5

131.8

57.5

69.3

57.5

116

58.0

132.9

58.0

69.9

58.0

117

58.5

134.1

58.5

70.5

58.5

118

59.0

135.2

59.0

71.1

59.0

119

59.5

136.3

59.5

71.7

59.5

120

60.0

137.5

60.0

72.3

60.0

121

60.5

138.6

60.5

72.9

60.5

122

61.0

139.8

61.0

73.5

61.0

123

61.5

140.9

61.5

74.1

61.5

124

62.0

142.1

62.0

74.7

62.0

125

62.5

143.2

62.5

75.3

62.5

126

63.0

144.4

63.0

75.9

63.0

127

63.5

145.5

63.5

76.5

63.5

128

64.0

146.7

64.0

77.1

64.0

129

64.5

147.8

64.5

77.7

64.5

130

65.0

148.9

65.0

78.3

65.0

131

65.5

150.1

65.5

78.9

65.5

132

66.0

151.2

66.0

79.5

66.0

133

66.5

152.4

66.5

80.1

66.5

134

67.0

153.5

67.0

80.7

67.0

135

67.5

154.7

67.5

81.3

67.5

136

68.0

155.8

68.0

81.9

68.0

137

68.5

157.0

68.5

82.5

68.5

138

69.0

158.1

69.0

83.1

69.0

139

69.5

159.3

69.5

83.7

69.5

140

70.0

160.4

70.0

84.3

70.0

141

70.5

161.5

70.5

84.9

70.5

142

71.0

162.7

71.0

85.5

71.0

143

71.5

163.8

71.5

86.1

71.5

144

72.0

165.0

72.0

86.7

72.0

145

72.5

166.1

72.5

87.3

72.5

146

73.0

167.3

73.0

87.9

73.0

147

73.5

168.4

73.5

88.5

73.5

148

74.0

169.6

74.0

89.1

74.0

149

74.5

170.7

74.5

89.7

74.5

150

75.0

171.9

75.0

90.3

75.0

Footnotes

1.

This document is HS1229, one of a series of the Horticultural Sciences Department, UF/IFAS Extension. Original publication date August 2013. Visit the EDIS website at http://edis.ifas.ufl.edu.

2.

Guodong Liu, assistant professor, Horticultural Sciences Department, UF/IFAS, 1253 Fifield Hall, Gainesville, FL 32611; Yuncong Li, professor, Soil and Water Science Department, Tropical Research and Education Center, UF/IFAS, 18905 SW 280th Street, Homestead, FL 33031; Aparna Gazula, UF/IFAS Alachua County Extension, 2800 NW 39 Ave, Gainesville, FL 32609.


The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county's UF/IFAS Extension office.

U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.