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Fertilizer Technology
Bulletin Spring 2005
Growth Products Research Division For Agricultural Fertilizers
Page 1 of 2
An analysis of salt indices of comparative liquid fertilizer
solutions and their relative percent (%) of slow release
nitrogen.
have only nitrogen in their formulations. This provides
a direct relationship between their respective salts
indices and free urea content. (See chart 1)
Salt content of liquid fertilizers and their relative
percentages of slow release nitrogen is one of the most
important characteristics of liquids. It is well
documented that all fertilizers contain salts. These
salts will effect both the salt concentration of the soil
but also increase the osmotic potential of a solution.
An index of all fertilizer materials and soil
amendments has long been established, giving the
grower the ability not only to judge the burn potential
to various crops, but also classify the fertilizer material
relative to each other and show what most likely will
cause injury. i
Table 1. Nitrogen Sources
Material
The salt index of any mixed fertilizer containing N, P
and K is the sum of the partial salt index per unit of
plant nutrients times the number of units due to each
component in the formulation.
Nitrogen Sources
N-Sure® (28-0-0)
Urea nitrogen and slowly
available nitrogen from
Triazone®
CORON® (28-0-0)
Urea nitrogen and slowly
available nitrogen from
methylene urea and
polymethylol ureas
Nitro-30® (30-0-0)
Urea nitrogen and slowly
available nitrogen from
methylene urea
The liquid slow release fertilizers evaluated in Table 1
are compared on the basis of salt index since each
product uses as its base raw material urea nitrogen and
Chart 1
Salt Index Based on 1.62 per Unit of Urea Nitrogen
50
Urea 46.6% N
Salt Index of 75.4
45
35
30
25
CORON (28-0-0) Salt
Index of 36
20
N-Sure (28-0-0)
Salt Index of 35
15
10
5
Nitro-30 (3-0-0)
Salt Index of 4
Salt Index
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75
73
70
66
63
60
57
53
50
47
44
41
37
34
31
28
24
21
18
15
8
11
5
3
0
0
Units of Free Urea Nitrogen
40
Bulletin Spring 2005
Fertilizer Technology
Growth Products Research Division For Agricultural Fertilizers
Page 2 of 2
The slow release nitrogen forms contained in the
reviewed products, are methylene ureas, polymethylol
ureas and triazone ureas. They are long chain nitrogen
molecules and have relatively undetectable salt indices.
Therefore, the salt index values must be attributed to
the free urea portion of the solution. Each product
was evaluated for its salt index at an independent
laboratory using “Soil Chemistry Analysis”, Jackson p.
245. The results of these analyses are indicated below.
Table 2. Salt Index
Material
Index
N-Sure® (28-0-0)
35 ii
CORON® (28-0-0)
36 iii
Nitro-30® (30-0-0)
4 iv
21.6 units). The calculation can then be made of the
remaining slow release nitrogen contained in each
product. (28 – 21.6 = 6.4 units of SRN) (See Chart 2)
Conclusion:
There is no universal analytical methodology for
determining free vs. combined nitrogen in slow release
nitrogen products of varying chemical structure,
molecular weight and technology that has proven to
be unreliable. Unlike other methods of determining
these percentages in liquid solutions, salt index values
provide a universal standard to compare products and
their potential for both salt effects and slow release
nitrogen content.
Salt Index of Fertilizers, 1986, Pm-1274d, Iowa State
University
Western Fertilizer Handbook, 1985, The Interstate
Publishers & Printers
ii Midwest Laboratories, Inc. Report # 00-070-2136
iii Midwest Laboratories, Inc. Report # 00-070-2136
iv Midwest Laboratories, Inc. Report # 00-070-2136
i
Based on these results listed in Table 2, the free urea
units and percentage of Slow Release Nitrogen vs. free
urea nitrogen can be calculated on the partial salt
index of 1.618 per unit of free urea nitrogen. For
example the N-sure product has a salt index of 35,
divided by a partial salt index of urea will give the
number of units of free urea nitrogen (35 ÷ 1.62 =
Chart 2
Total Units of Free Urea/Slow Relase Nitrogen Based on Salt Index
50
45
0
Units SRN
40
Units Free Urea
Units of Nitrogen
35
30
25
46.6
6.40
5.78
21.60
22.22
N-Sure (28-0-0)
CoRon (28-0-0)
20
27.53
15
10
5
2.47
0
Urea (46.6% N)
Nitro 30 (30-0-0)
Nitrogen Formulations
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