Use of a Soil Hydrometer to
Determine Nitrogen and Phosphorus
in Liquid Swine Manure
Publication 08482
Jun Zhu, Extension Engineer, University of Minnesota
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.
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.
What You Need to Run
the Measurement
The tools involved in the measurement are quite
simple: a soil hydrometer (which can be purchased
from Cole Parmer Company, catalog number: A08292-16, 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).
Hydrometer
Hydrometer
reading
Hydrometer jar
Water in the jar
Figure 1. Demonstration of a hydrometer in the
process condition
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
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.
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
Table 1. The relationship between hydrometer readings and the concentrations of TN in liquid manure
Hydrometer
readings
1.005
1.0055
1.006
1.0065
1.007
1.0075
1.008
1.0085
1.009
1.0095
1.01
1.0105
1.011
1.0115
1.012
1.0125
1.013
1.0135
1.014
1.0145
1.015
1.0155
1.016
1.0165
1.017
1.0175
1.018
1.0185
1.019
1.0195
1.02
1.0205
1.021
1.0215
1.022
1.0225
1.023
1.0235
1.024
1.0245
1.025
1.0255
1.026
1.0265
1.027
1.0275
Finishing
lb/1000 gal
Gestation
lb/1000 gal
Nursery
lb/1000 gal
0.89
1.63
2.36
3.10
3.84
4.57
5.31
6.05
6.79
7.52
8.26
9.00
9.74
10.47
11.21
11.95
12.69
13.42
14.16
14.90
15.64
16.37
17.11
17.85
18.59
19.32
20.06
20.80
21.54
22.27
23.01
23.75
24.48
25.22
25.96
26.70
27.43
28.17
28.91
29.65
30.38
31.12
31.86
32.60
33.33
34.07
2.63
3.05
3.48
3.91
4.33
4.76
5.19
5.61
6.04
6.47
6.89
7.32
7.75
8.17
8.60
9.03
9.45
9.88
10.31
10.73
11.16
11.59
12.01
12.44
12.87
13.29
13.72
14.15
14.57
15.00
15.43
15.85
16.28
16.71
17.13
17.56
17.99
18.41
18.84
19.27
19.69
20.12
20.55
20.97
21.40
21.83
6.10
6.90
7.70
8.50
9.30
10.10
10.90
11.70
12.50
13.30
14.10
14.91
15.71
16.51
17.31
18.11
18.91
19.71
20.51
21.31
22.11
22.91
23.71
24.51
25.31
26.11
26.91
27.71
28.51
29.31
30.11
30.91
31.71
32.52
33.32
34.12
34.92
35.72
36.52
37.32
38.12
38.92
39.72
40.52
41.32
42.12
Hydrometer
readings
(cont’d)
1.028
1.0285
1.029
1.0295
1.03
1.0305
1.031
1.0315
1.032
1.0325
1.033
1.0335
1.034
1.0345
1.035
1.0355
1.036
1.0365
1.037
1.0375
1.038
1.0385
1.039
1.0395
1.04
1.0405
1.041
1.0415
1.042
1.0425
1.043
1.0435
1.044
1.0445
1.045
1.0455
1.046
1.0465
1.047
1.0475
1.048
1.0485
1.049
1.0495
1.05
1.0505
Finishing
lb/1000 gal
Gestation
lb/1000 gal
Nursery
lb/1000 gal
34.81
35.55
36.28
37.02
37.76
38.50
39.23
39.97
40.71
41.45
42.18
42.92
43.66
44.40
45.13
45.87
46.61
47.34
48.08
48.82
49.56
50.29
51.03
51.77
52.51
53.24
53.98
54.72
55.46
56.19
56.93
57.67
58.41
59.14
59.88
60.62
61.36
62.09
62.83
63.57
64.31
65.04
65.78
66.52
67.25
67.99
22.25
22.68
23.11
23.53
23.96
24.39
24.81
25.24
25.67
26.09
26.52
26.95
27.37
27.80
28.23
28.65
29.08
29.51
29.93
30.36
30.79
31.21
31.64
32.07
32.49
32.92
33.35
33.77
34.20
34.63
35.05
35.48
35.91
36.33
36.76
37.19
37.61
38.04
38.47
38.89
39.32
39.75
40.17
40.60
41.03
41.45
42.92
43.72
44.52
45.32
46.12
46.92
47.72
48.52
49.32
50.13
50.93
51.73
52.53
53.33
54.13
54.93
55.73
56.53
57.33
58.13
58.93
59.73
60.53
61.33
62.13
62.93
63.73
64.53
65.33
66.13
66.93
67.74
68.54
69.34
70.14
70.94
71.74
72.54
73.34
74.14
74.94
75.74
76.54
77.34
78.14
78.94
Table 2. The relationship between hydrometer readings and the concentrations of TP in liquid manure
Hydrometer
readings
1.005
1.0055
1.006
1.0065
1.007
1.0075
1.008
1.0085
1.009
1.0095
1.01
1.0105
1.011
1.0115
1.012
1.0125
1.013
1.0135
1.014
1.0145
1.015
1.0155
1.016
1.0165
1.017
1.0175
1.018
1.0185
1.019
1.0195
1.02
1.0205
1.021
1.0215
1.022
1.0225
1.023
1.0235
1.024
1.0245
1.025
1.0255
1.026
1.0265
1.027
1.0275
Finishing
lb/1000 gal
Gestation
lb/1000 gal
Nursery
lb/1000 gal
1.03
1.15
1.28
1.40
1.52
1.64
1.77
1.89
2.01
2.13
2.25
2.38
2.50
2.62
2.74
2.87
2.99
3.11
3.23
3.35
3.48
3.60
3.72
3.84
3.97
4.09
4.21
4.33
4.46
4.58
4.70
4.82
4.94
5.07
5.19
5.31
5.43
5.56
5.68
5.80
5.92
6.04
6.17
6.29
6.41
6.53
1.99
2.39
2.79
3.19
3.58
3.98
4.38
4.78
5.17
5.57
5.97
6.37
6.76
7.16
7.56
7.95
8.35
8.75
9.15
9.54
9.94
10.34
10.74
11.13
11.53
11.93
12.33
12.72
13.12
13.52
13.91
14.31
14.71
15.11
15.50
15.90
16.30
16.70
17.09
17.49
17.89
18.29
18.68
19.08
19.48
19.88
0.98
1.09
1.20
1.31
1.42
1.53
1.64
1.74
1.85
1.96
2.07
2.18
2.29
2.40
2.51
2.62
2.73
2.84
2.95
3.06
3.17
3.28
3.39
3.50
3.61
3.72
3.83
3.93
4.04
4.15
4.26
4.37
4.48
4.59
4.70
4.81
4.92
5.03
5.14
5.25
5.36
5.47
5.58
5.69
5.80
5.91
Hydrometer
readings
(cont’d)
1.028
1.0285
1.029
1.0295
1.03
1.0305
1.031
1.0315
1.032
1.0325
1.033
1.0335
1.034
1.0345
1.035
1.0355
1.036
1.0365
1.037
1.0375
1.038
1.0385
1.039
1.0395
1.04
1.0405
1.041
1.0415
1.042
1.0425
1.043
1.0435
1.044
1.0445
1.045
1.0455
1.046
1.0465
1.047
1.0475
1.048
1.0485
1.049
1.0495
1.05
1.0505
Finishing
lb/1000 gal
Gestation
lb/1000 gal
Nursery
lb/1000 gal
6.66
6.78
6.90
7.02
7.15
7.27
7.39
7.51
7.63
7.76
7.88
8.00
8.12
8.25
8.37
8.49
8.61
8.73
8.86
8.98
9.10
9.22
9.35
9.47
9.59
9.71
9.84
9.96
10.08
10.20
10.32
10.45
10.57
10.69
10.81
10.94
11.06
11.18
11.30
11.42
11.55
11.67
11.79
11.91
12.04
12.16
20.27
20.67
21.07
21.46
21.86
22.26
22.66
23.05
23.45
23.85
24.25
24.64
25.04
25.44
25.84
26.23
26.63
27.03
27.43
27.82
28.22
28.62
29.01
29.41
29.81
30.21
30.60
31.00
31.40
31.80
32.19
32.59
32.99
33.39
33.78
34.18
34.58
34.97
35.37
35.77
36.17
36.56
36.96
37.36
37.76
38.15
6.02
6.12
6.23
6.34
6.45
6.56
6.67
6.78
6.89
7.00
7.11
7.22
7.33
7.44
7.55
7.66
7.77
7.88
7.99
8.10
8.21
8.31
8.42
8.53
8.64
8.75
8.86
8.97
9.08
9.19
9.30
9.41
9.52
9.63
9.74
9.85
9.96
10.07
10.18
10.29
10.39
10.50
10.61
10.72
10.83
10.94
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