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Extension > Agriculture > Nutrient Management > Fertilizing alfalfa in Minnesota

Use of a soil hydrometer to determine nitrogen and phosphorus in liquid swine manure

Jun Zhu, Extension Engineer, University of Minnesota

Copyright © 2013 Regents of the University of Minnesota. All rights reserved.

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Introduction

An important component of on-farm nutrient management is estimation of nutrient contents in liquid manure prior to land application. However, to date, there have been few techniques amenable to use by animal producers to quickly test the nutrient contents in liquid manure, such as total nitrogen (TN) and phosphorus (TP). Generally speaking, it is difficult, and usually inaccurate, to measure the concentrations of TN and TP in bulk liquid in storage facilities even if agitation is used. A better alternative is to conduct real time analysis during application. In a recent study, UM researchers have developed an improved method using a soil hydrometer to measure the specific gravity of liquid swine manure and then relate it to the nutrients concentrations in the manure. After tested on 6 gestation, 6 finishing, and 5 nursery farms, the method is believed to be able to present reliable results in measuring TN and TP in liquid manure from pigs at different growth stages.

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Why This Method Works

The basis for this measurement is that there is a statistically significant correlation between the total solids contents and nutrient elements in liquid swine manure and there is also a correlation between solids contents and the specific gravity of the manure. Specific gravity is a measure of the density of a liquid, and can be easily measured with a simple piece of lab equipment known as a hydrometer. With these two correlations established, a direct relationship between the nutrient contents (N and P) and the manure’s specific gravity can be constructed to determine the concentrations of N and P using a hydrometer. Although the concept of using a hydrometer to measure manure nutrients is not new, previous workers have all attempted to develop a simple, one-size-fits-all type of equation at the sacrifice of estimation accuracy. The equations did not take into account the differences among manures produced by pigs at different growth stages, such as gestation, nursery, and finish. This is why the method has not been adopted widely by swine producers. Our work has alleviated this problem by developing separate equations to address different manures. The equations were evaluated by rigorous field experiments and found to produce accurate nutrient estimations.

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What You Need to Run the Measurement

Hydrometer demonstration


Figure 1. Demonstration of a hydrometer in the process condition.

The tools involved in the measurement are quite simple: a soil hydrometer (which can be purchased from Cole Parmer Company, catalog number: A-08292-16, with precision at 0.0005 divisions; cost: $25.50), a hydrometer jar also from Cole Parmer at $16.05 (catalog number A-06135-65, 500 mL), a brush of any type as long as it fits into the jar for agitation without spilling, and the tables showing the relationships between the hydrometer readings and the TN and TP concentrations in the manure, which are attached at the end of this factsheet. Figure 1 presents an illustration of a hydrometer in the jar and demonstrates how the readings should be taken in measuring the specific gravity of a particular liquid (water here).

The reading on the hydrometer can go up to 4 digits after the decimal point. When a number is read, e.g., 1.0215, the N and P contents in the manures for finishing, gestation, and nursery pigs will be 25.22 and 5.07 lb/1000 gal, 16.71 and 15.11 lb/1000 gal, and 32.52 and 4.59 lb/1000 gal, respectively (from Table 1 and 2). As can be seen from the tables, there are huge differences in terms of the N and P contents for manures from pigs at different growing stages, using one equation to represent them all will obviously result in erroneous application rates.

Steps To Do the Measurement

  • Fill the hydrometer jar with the manure sample for which N and P are to be determined
  • Use the brush to thoroughly mix the sample in the jar
  • Place the hydrometer in the jar immediately after mixing
  • Take a reading at the liquid surface on the hydrometer within 10 seconds after placing it in the jar
  • Refer to the tables for TN and TP concentrations based on the hydrometer readings
  • Calculate the TN and TP loadings based on the total amount of manure applied

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What the Error Ranges Are by Using This Method

Based on our analysis, the error ranges for nutrients estimation could be ±17% for TN and ±12% for TP for finishing manure, ±10% for TN and ±11% for TP for gestation manure, and ±13% for both TN and TP for nursery manure.

Possible Ways to Reduce Errors In Estimation

  • Make sure the collected sample is representative, which means the sample is well mixed and thus uniform
  • Make sure the sample in the hydrometer jar is thoroughly agitated using the brush to suspend all the solids before placing the hydrometer
  • Make sure that the readings are taken within 10 seconds after mixing to avoid the settlement of solids
  • The procedure can be repeated two or three times to get replicate readings and averages

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Table 1. The relationship between hydrometer readings and the concentrations of TN in liquid manure

Hydrometer readings
Finishing lb/1000 gal
Gestation lb/1000 gal
Nursery lb/1000 gal
1.005
0.89
2.63
6.10
1.0055
1.63
3.05
6.90
1.006
2.36
3.48
7.70
1.0065
3.10
3.91
8.50
1.007
3.84
4.33
9.30
1.0075
4.57
4.76
10.10
1.008
5.31
5.19
10.90
1.0085
6.05
5.61
11.70
1.009
6.79
6.04
12.50
1.0095
7.52
6.47
13.30
1.01
8.26
6.89
14.10
1.0105
9.00
7.32
14.91
1.011
9.74
7.75
15.71
1.0115
10.47
8.17
16.51
1.012
11.21
8.60
17.31
1.0125
11.95
9.03
18.11
1.013
12.69
9.45
18.91
1.0135
13.42
9.88
19.71
1.014
14.16
10.31
20.51
1.0145
14.90
10.73
21.31
1.015
15.64
11.16
22.11
1.0155
16.37
11.59
22.91
1.016
17.11
12.01
23.71
1.0165
17.85
12.44
24.51
1.017
18.59
12.87
25.31
1.0175
19.32
13.29
26.11
1.018
20.06
13.72
26.91
1.0185
20.80
14.15
27.71
1.019
21.54
14.57
28.51
1.0195
22.27
15.00
29.31
1.02
23.01
15.43
30.11
1.0205
23.75
15.85
30.91
1.021
24.48
16.28
31.71
1.0215
25.22
16.71
32.52
1.022
25.96
17.13
33.32
1.0225
26.70
17.56
34.12
1.023
27.43
17.99
34.92
1.0235
28.17
18.41
35.72
1.024
28.91
18.84
36.52
1.0245
29.65
19.27
37.32
1.025
30.38
19.69
38.12
1.0255
31.12
20.12
38.92
1.026
31.86
20.55
39.72
1.0265
32.60
20.97
40.52
1.027
33.33
21.40
41.32
1.0275
34.07
21.83
42.12
1.028
34.81
22.25
42.92
1.0285
35.55
22.68
43.72
1.029
36.28
23.11
44.52
1.0295
37.02
23.53
45.32
1.03
37.76
23.96
46.12
1.0305
38.50
24.39
46.92
1.031
39.23
24.81
47.72
1.0315
39.97
25.24
48.52
1.032
40.71
25.67
49.32
1.0325
41.25
26.09
50.13
1.033
42.18
26.52
50.93
1.0335
42.92
26.95
51.73
1.034
43.66
27.37
52.53
1.0345
44.40
27.80
53.33
1.035
45.13
28.23
54.13
1.0355
45.87
28.65
54.93
1.036
46.61
29.08
55.73
1.0365
47.34
29.51
56.53
1.037
48.08
29.93
57.33
1.0375
48.82
30.36
58.13
1.038
49.56
30.79
58.93
1.0385
50.29
31.21
59.73
1.039
51.03
31.64
60.53
1.0395
51.77
32.07
61.33
1.04
52.51
32.49
62.13
1.0405
53.24
32.92
62.93
1.041
53.98
33.35
63.73
1.0415
54.72
33.77
64.53
1.042
55.46
34.20
65.33
1.0425
46.19
34.63
66.13
1.043
56.93
35.05
66.93
1.0435
57.67
35.48
67.74
1.044
58.41
35.91
68.54
1.0445
59.14
36.33
69.34
1.045
59.88
36.76
70.14
1.0455
60.62
37.19
70.94
1.046
61.36
37.61
71.74
1.0465
62.09
38.04
72.54
1.047
62.83
38.47
73.34
1.0475
63.57
38.89
74.14
1.048
64.31
39.32
74.94
1.0485
65.04
39.75
75.74
1.049
65.78
40.17
76.54
1.0495
66.52
40.60
77.34
1.05
67.25
41.03
78.14
1.0505
67.99
41.45
78.94

Table 2. The relationship between hydrometer readings and the concentrations of TP in liquid manure

Hydrometer readings
Finishing lb/1000 gal
Gestation lb/1000 gal
Nursery lb/1000 gal
1.005
1.03
1.99
0.98
1.0055
1.15
2.39
1.09
1.006
1.28
2.79
1.20
1.0065
1.40
3.19
1.31
1.007
1.52
3.58
1.42
1.0075
1.64
3.98
1.53
1.008
1.77
4.38
1.64
1.0085
1.89
4.78
1.74
1.009
2.01
5.17
1.85
1.0095
2.13
5.57
1.96
1.01
2.25
5.97
2.07
1.0105
2.38
6.37
2.18
1.011
2.50
6.76
2.29
1.0115
2.62
7.16
2.40
1.012
2.74
7.56
2.51
1.0125
2.87
7.95
2.62
1.013
2.99
8.35
2.73
1.0135
3.11
8.75
2.84
1.014
3.23
9.15
2.95
1.0145
3.35
9.54
3.06
1.015
3.48
9.94
3.17
1.0155
3.60
10.34
3.28
1.016
3.72
10.74
3.39
1.0165
3.84
11.13
3.50
1.017
3.97
11.53
3.61
1.0175
4.09
11.93
3.72
1.018
4.21
12.33
3.83
1.0185
4.33
12.72
3.93
1.019
4.46
13.12
4.04
1.0195
4.58
13.52
4.15
1.02
4.70
13.91
4.26
1.0205
4.82
14.31
4.37
1.021
4.94
14.71
4.48
1.0215
5.07
15.11
4.59
1.022
5.19
15.50
4.70
1.0225
5.31
15.90
4.81
1.023
5.43
16.30
4.92
1.0235
5.56
16.70
5.03
1.024
5.68
17.09
5.14
1.0245
5.80
17.49
5.25
1.025
5.92
17.89
5.36
1.0255
6.04
18.29
5.47
1.026
6.17
18.68
5.58
1.0265
6.29
19.08
5.69
1.027
6.41
19.48
5.80
1.0275
6.53
19.88
5.91
1.028
6.66
20.27
6.02
1.0285
6.78
20.67
6.12
1.029
6.90
21.07
6.23
1.0295
7.02
21.46
6.34
1.03
7.15
21.86
6.45
1.0305
7.27
22.26
6.56
1.031
7.39
22.66
6.67
1.0315
7.51
23.05
6.78
1.032
7.63
23.45
6.89
1.0325
7.76
23.85
7.00
1.033
7.88
24.25
7.11
1.0335
8.00
24.64
7.22
1.034
8.12
25.04
7.33
1.0345
8.25
25.44
7.44
1.035
8.37
25.84
7.55
1.0355
8.49
26.23
7.66
1.036
8.61
26.63
7.77
1.0365
8.73
27.03
7.88
1.037
8.86
27.43
7.99
1.0375
8.98
27.82
8.10
1.038
9.10
28.22
8.21
1.0385
9.22
28.62
8.31
1.039
9.35
29.01
8.42
1.0395
9.47
29.41
8.53
1.04
9.59
29.81
8.64
1.0405
9.71
30.21
8.75
1.041
9.84
30.60
8.86
1.0415
9.96
31.00
8.97
1.042
10.08
31.40
9.08
1.0425
10.20
31.80
9.19
1.043
10.32
32.19
9.30
1.0435
10.45
32.59
9.41
1.044
10.57
32.99
9.52
1.0445
10.69
33.39
9.63
1.045
10.81
33.78
9.74
1.0455
10.94
34.18
9.85
1.046
11.06
34.58
9.96
1.0465
11.18
34.97
10.07
1.047
11.30
35.37
10.18
1.0475
11.42
35.77
10.29
1.048
11.55
36.17
10.39
1.0485
11.67
36.56
10.50
1.049
11.79
36.96
10.61
1.0495
11.91
37.36
10.72
1.05
12.04
37.76
10.83
1.0505
12.16
38.15
10.94

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