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Managing Phosphorus Saturated Soils

Introduction

Truck in field applying poultry litter

Five of the eight eastern panhandle counties in West Virginia’s portion of the Chesapeake Bay watershed include 98% of the state’s poultry industry. In 2018, reported broiler production was 83 million birds and turkey production was 3 million birds in the Potomac Headwaters region, resulting in over 100,000 tons of poultry litter being produced (NASS 2020 Annual Statistical Bulletin). During the last 50 years, most of the poultry litter produced was applied within the watershed. These repeated litter applications have resulted in an accumulation of phosphorus (P) in agricultural soils, increasing the potential of nutrient losses to waterways. 

West Virginia’s eastern panhandle has been identified as a high phosphorus loading area of the Chesapeake Bay. West Virginia’s Phase II Chesapeake Bay Watershed Implementation Plan (WIP) noted that the agricultural sector is responsible for 49% of the total delivered nitrogen load and 62% of the total delivered phosphorus load. A portion of this nutrient loading is attributed to the repeated land application of poultry litter as a soil amendment, resulting in high levels of phosphorus accumulation and saturation in pasture, hay and cropland soils.

At the beginning of September 2018, the WVU Soil Testing Lab changed from the Mehlich 1 to the Mehlich 3 soil test extraction method. The Mehlich 3 method provides new information to customers who are using the lab to make soil fertility decisions. Included with the nutrient analysis is a phosphorus saturation ratio (PSR) or Psat. Phosphorus saturation ratio is defined as the ratio between the amount of phosphorus present in the soil and the total capacity of that soil to retain phosphorus. The ability of phosphorus to be bound in the soil is primary a function of iron (Fe) and aluminum (Al) content in that soil. Mehlich 3 extraction allows iron and aluminum levels to be determined and then, a ratio of phosphorus to the sum of iron plus aluminum results in a percentage being reported. This tells the farmer how much iron and aluminum are currently holding phosphorus and how much is left to accept additional applied phosphorus. 

Research has shown that there is a phosphorus saturation threshold level where a soil changes from a sink for phosphorus to be a source for phosphorus loss to the environment. Results from these studies included a group of Mid-Atlantic soils. The threshold level was determined using the relationship between water soluble phosphorus and a soil's PSR (Figure 1).  The soil phosphorus saturation threshold where soils switch from a phosphorus sink to a phosphorus source was determined to be .1 or 10%. The water-soluble phosphorus concentration increased significantly once the threshold PSR was exceeded (Dari et al, 2018).

Chart showing the relationship between water soluble phosphorus and the phosphorus saturation ratio for all soil samples in this study.

Figure 1. Relationship between water soluble phosphorus (WSP) and phosphorus saturation ratio (PSR) for all soil samples in this study. Threshold PSR is 0.10; 95% confidence interval = 0.05 to 015; p,0.0001. Group 1: Mid-Atlantic United States; Group 2: Ozark Plateau Group; Group 3: Southeastern Piedmont; Group 4: Southern-most Atlantic Coast and Gulf Coast Plain; Group 5: Various regions (Dari et al 2018).

Results

To help farmers in the eastern panhandle better understand the environmental risks of phosphorus saturated soils, a WVU student was hired in the fall of 2019 to collect soil samples from crop, hay and pasture fields. Our survey area included Grant, Hardy, Mineral, Hampshire, Pendleton and Jefferson counties. A total of 92 fields had samples collected. The 38 pastures and 26 hayfields had a sampling depth of 0 to 2 inches, and the 28 crop field samples were collected at the 0- to 6-inch depth. The WVU Soil Testing Lab recommendation system ranks soils as low, medium, optimum and excess (Table 1).

Table 1. Fertility rating break points in WVU fertilizer recommendation system.

Fertility Rating

Phosphorus Level (ppm)

Low

0 – 14

Medium

15 – 29

Optimum

30 – 59

Excess

> 60

As the student collected soil samples, field histories were requested, and each field was categorized as high, medium and low for historic litter applications to the fields. Results from the crop fields showed heavy historic use of litter with a corresponding excessive level of fertility across all fields. The degree of phosphorus saturation was ten percent or above for all fields except for one (Table 2). Crop field phosphorus levels ranged from 60 ppm to over 700 ppm, an excess fertility rating for all sampled fields except one (Table 2). The threshold PSR of 10% was exceeded on all but one of these crop fields. Twenty-six hay fields were sampled with 40% having soil test phosphorus levels in the excess range and above the PSR Threshold level of 10% (Table 3). Of 38 pastures, 52% had phosphorus levels in the excessive range and above the 10% phosphorus threshold level (Table 4).

Table 2. Row crop fields with their location, crop, soil test phosphorus, and phosphorus saturation percentage ranked in order of their soil test phosphorus concentration.

Count

Acres

Crop

Litter Use

Phosphorus (ppm)

Tillage

County Farm #

Sample Depth in Inches

% Phosphorus Saturation

Hardy

9.5

Corn

High

700

Conventional

Hardy 14

6

74

Pendleton

17.5

Corn

High

360

Conventional

Pendleton 10

6

37

Pendleton

22

Corn

High

350

Conventional

Pendleton 1

6

50

Pendleton

34

Corn

High

350

Conventional

Pendleton 11

6

38

Pendleton

26.5

Corn

High

340

Conventional

Pendleton 2

6

47

Pendleton

20.6

Corn

High

310

Conventional

Pendleton 5

6

30

Pendleton

7.2

Corn

High

280

Conventional

Pendleton 6

6

33

Hardy

19.13

Corn

High

270

Conventional

Hardy 11

6

29

Hardy

19.13

Corn

High

270

Conventional

Hardy 15

6

34

Pendleton

20.5

Sorghum

High

220

No-Till

Pendleton 7

6

32

Pendleton

7.23

Corn

High

160

Conventional

Pendleton 3

6

21

Hardy

8.51

Corn

High

150

Conventional

Hardy 10

6

16

Jefferson

40

Corn

High

150

No-Till

Jefferson 1

6

19

Jefferson

35

Corn

High

150

No-Till

Jefferson 10

6

21

Jefferson

20

Corn

High

140

No-Till

Jefferson 2

6

16

Hardy

22.82

Corn

High

120

No-Till

Hardy 2

6

15

Jefferson

90

Corn

High

120

No-Till

Jefferson 9

6

21

Jefferson

33

Soybeans

High

120

No-Till

Jefferson 10

6

15

Hardy

10.2

Barley

High

110

No-Till

Hardy 6

6

15

Jefferson

67

Corn

High

110

No-Till

Jefferson 6

2

16

Hardy

10.2

Barley

High

110

No-Till

Hardy 6

6

15

Jefferson

51

Corn

High

100

No-Till

Jefferson 3

6

12

Jefferson

24

Corn

High

98

No-Till

Jefferson 2

6

13

Jefferson

76

Corn

High

93

No-Till

Jefferson 3

6

18

Hardy

8.08

Sorghum

High

92

No-Till

Hardy 7

6

12

Pendleton

6.67

Corn

High

63

Conventional

Pendleton 4

6

10

Jefferson

20

Soybeans

High

62

No-Till

Jefferson 8

6

10

Jefferson

11

Soybeans

High

29

No-Till

Jefferson 9

6

7

 

Table 3. Hay fields with their location, soil test phosphorus, and phosphorus saturation percentage ranked in order of their soil test phosphorus concentration.

County

Acres

Crop

Litter Use

Phosphorus (ppm)

Tillage

County Field #

Sample Depth in Inches

% Phosphorus Saturation

Grant

3.8

Hay

High

120

No-Till

Grant 16

2

19

Grant

3.87

Hay

High

90

No-Till

Grant 11

2

18

Jefferson

35

Hay

High

84

No-Till

Jefferson 8

2

15

Grant

4.6

Hay

High

81

No-Till

Grant 6

2

13

Grant

7.47

Hay

High

78

No-Till

Grant 15

2

14

Grant

12.6

Hay

Low

77

No-Till

Grant 30

2

14

Jefferson

22

Hay

Low

74

No-Till

Jefferson 6

2

12

Grant

6.85

Hay

High

67

No-Till

Grant 13

2

10

Mineral

11.5

Hay

Low

66

No-Till

Mineral 4

2

8

Grant

11.4

Hay

Low

64

No-Till

Grant 29

2

15

Grant

7.2

Hay

Low

57

No-Till

Grant 28

2

6

Mineral

7.64

Hay

Low

51

No-Till

Mineral 1

2

13

Grant

30.76

Hay

Low

44

No-Till

Grant 31

2

8

Grant

3.68

Hay

High

41

No-Till

Grant 4

2

11

Hardy

31.21

Hay

High

38

No-Till

Hardy 1

2

6

Grant

33.75

Hay

Low

37

No-Till

Grant 19

2

7

Grant

3.05

Hay

High

30

No-Till

Grant 5

2

5

Grant

16.7

Hay

Low

29

No-Till

Grant 32

2

5

Mineral

18.1

Hay

Low

28

No-Till

Mineral 3

2

3

Jefferson

15

Hay

Low

28

No-Till

Jefferson 4

2

4

Jefferson

10

Hay

Low

24

No-Till

Jefferson 5

2

4

Grant

8.75

Hay

Low

21

No-Till

Grant 24

2

4

Jefferson

15

Hay

Low

19

No-Till

Jefferson 4

2

4

Jefferson

23

Hay

Low

19

No-Till

Jefferson 5

2

3

Grant

4.76

Hay

Low

15

No-Till

Grant 18

2

15

Mineral

11.6

Hay

Low

15

No-Till

Mineral 5

2

2


Table 4. Pastures with their location, soil test phosphorus, and phosphorus saturation ranked in order of their soil test phosphorus concentration.

County

Acres

Crop

Litter Use

Phosphorus (ppm)

Tillage

County Field #

Sample Depth in Inches

% Phosphorus Saturation

Hardy

6.18

Pasture

High

220

No-Till

Hardy 5

2

28

Hardy

6.74

Pasture

High

190

No-Till

Hardy 3

2

21

Hardy

3

Pasture

Low

150

No-Till

Hardy 23

2

17

Hardy

5

Pasture

Low

140

No-Till

Hardy 20

2

17

Hardy

4

Pasture

Low

140

No-Till

Hardy 21

2

18

Hardy

4

Pasture

Low

120

No-Till

Hardy 22

2

16

Jefferson

29

Pasture

Medium

110

No-Till

Jefferson 7

2

15

Hardy

19.1

Pasture

High

100

No-Till

Hardy 4

2

13

Grant

4.81

Pasture

High

100

No-Till

Grant 9

2

19

Hardy

7

Pasture

Low

100

No-Till

Hardy 24

2

12

Pendleton

98

Pasture

Low

96

No-Till

Pendleton 9

2

12

Grant

3.72

Pasture

High

93

No-Till

Grant 2

2

14

Pendleton

72

Pasture

Low

87

No-Till

Pendleton 8

2

12

Jefferson

14.3

Pasture

Low

87

No-Till

Jefferson 7

2

13

Grant

1.3

Pasture

High

82

No-Till

Grant 17

2

12

Hardy

4

Pasture

Low

79

No-Till

Hardy 16

2

9

Hardy

16

Pasture

Low

75

No-Till

Hardy 19

2

11

Grant

5.81

Pasture

High

72

No-Till

Grant 3

2

6

Hardy

9

Pasture

Low

68

No-Till

Hardy 17

2

8

Grant

4.03

Pasture

High

63

No-Till

Grant 14

2

10

Grant

30.49

Pasture

High

60

No-Till

Grant 7

2

11

Grant

1

Pasture

High

57

No-Till

Grant 10

2

11

Hardy

28.38

Pasture

High

55

No-Till

Hardy 12

2

7

Grant

3.32

Pasture

High

51

No-Till

Grant 12

2

8

Hardy

34.98

Pasture

High

44

No-Till

Hardy 13

2

6

Grant

3

Pasture

Low

43

No-Till

Grant 25

2

8

Hardy

20

Pasture

Low

43

No-Till

Hardy 18

2

7

Grant

0.75

Pasture

High

42

No-Till

Grant 8

2

7

Grant

6.7

Pasture

Low

38

No-Till

Grant 21

2

6

Grant

10.85

Pasture

Low

35

No-Till

Grant 27

2

14

Hardy

25.04

Pasture

High

33

No-Till

Hardy 9

2

4

Grant

4.5

Pasture

Low

33

No-Till

Grant 26

2

5

Grant

4.8

Pasture

Low

27

No-Till

Grant 22

2

6

Grant

4.6

Pasture

Low

25

No-Till

Grant 23

2

5

Mineral

13.5

Pasture

Low

24

No-Till

Mineral 2

2

3

Grant

16.24

Pasture

High

22

No-Till

Grant 1

2

3

Hardy

11.01

Pasture

High

19

No-Till

Hardy 8

2

3

Grant

17

Pasture

Low

11

No-Till

Grant 20

2

2

 

Table 5. WVU Soil Testing Lab’s degree of phosphorus saturation recommendations.

Rating

Phosphorus Saturation

Soil Test Report Note

Low

0-10%

Your soil phosphorus concentration is not an environmental concern. Follow the phosphorus fertilization recommendations provided in your soil test report.

Medium

11-25%

Your soil phosphorus concentration is near the level to be of environmental concern. There is unlikely to be a benefit to more P fertilizer than will be removed by your crop.

High

26-100%

Your soil phosphorus concentrations are well above the level for environmental concern. Do not apply additional P fertilizer and implement strategies to reduce your soil test P concentration.

Conclusions

Having farmers better understand phosphorus soil chemistry is key to reducing or eliminating applications of poultry litter to phosphorus-saturated fields. The Mehlich 3 soil test enables famers to identify fields that are vulnerable to phosphorus losses and then, take steps to manage the fields to minimizes losses. The WVU Soil Testing Lab includes phosphorus saturation guidance for low, medium and high levels of phosphorus saturation (Table 5). These recommendations should be included in all nutrient management plan narratives for each field. These recommendations are in alignment with current phosphorus research in the Mid-Atlantic region (Sims et al 2002, Dari et al 2018). The Mehlich 3 soil test method will allow the addition of a field’s PSR as a factor into the WV Phosphorus Site Index.        

References

Dari, B., V. Nair, A. Sharpley, P. Kleinman, D. Franklin and W. Harris. 2018. Consistency of Threshold Phosphorus Saturation Ratio across a Wide Geographic Range of Acid Soils. Agrosystems, Geosciences & Environment. Agrosyst. Geosci. Environ. 1:180028 (2018) doi:10.2134/age2018.08.0028, https://acsess.onlinelibrary.wiley.com/doi/pdf/10.2134/age2018.08.0028

Sims, J. T., R. O. Maguire, A. B. Leytem, K. L. Gartley, and M. C. Pautler. 2002 Evaluation of Mehlich 3 as an Agri-Environmental Soil Phosphorus Test for the Mid-Atlantic United States of America. Journal of the Soil Science Society of America

USDA, 2020 Annual Statistical Bulletin Number 51,  https://www.nass.usda.gov/Statistics_by_State/West_Virginia/Publications/Annual_Statistical_Bulletin/index.php

WV watershed Implementation Plan 2, Appendix C. Sources of N and P in WV Potomac Counties http://www.wvchesapeakebay.us/WIP/files/phase2/Appendix_C_Sources_of_N&P_in_WV_Potomac_Counties.pdf


Authors:  Tom Basden, WVU Extension Specialist – Nutrient Management; Ed Rayburn, WVU Extension Specialist – Forage Agronomy; Louis McDonald, WVU Professor of Environmental Soil Chemistry and Soil Fertility; and Emily Morrow, WVU Extension Agent – Jefferson County

Project Participants: Paden Rightsell, WVU student, and WVU Extension agents Brad Smith, Alex Smith and David Seymour

Last Reviewed: October 2021