# Fertilizer Formulation and Calculation - PDF

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```					  Dr. John J. Mortvedt

Calculating Salt Index
Salt content is one of the most critical characteristics of fertilizers that
should be considered when fertilizers are applied, especially with
seed-row or “in furrow”placement.

Summary: Salt index (SI) of a fertilizer          Banded P tends to be more efficient      lbs/A of N + K2O in direct seed contact
is a measure of the salt concentration         on very acid soils, highly calcareous       with corn and sorghum. These applied
that fertilizer induces in the soil            soils, and those soils with very low        to formulations using KCl as the K
solution. SI does not predict the exact        levels of available soil P. Band            source and would not be accurate if
amount of a fertilizer material or             applications also are usually more          potassium phosphate was used as the
formulation that could produce crop            efficient when low P application rates      source of K instead of KCl. This is
injury on a particular soil, but it does       are used.                                   because of the lower SI value of
allow comparisons of fluid formulations           Early planting dates, large amounts of   potassium phosphate compared with
regarding their potential salt effects. As     crop residues on the soil surface, and      KCl (Table 1).
we all know, placement of some                 soil compaction may subject plants to         Crop tolerance to increased osmotic
formulations in or near the seed may           more stress. Banded nutrients are           pressures (salt content) of the soil
decrease seed germination or result in         usually more effective for crops under      solution in the vicinity of the seed varies
seedling injury.                               these stress conditions. Vegetables         considerably. For example, wheat is
Fluid fertilizers containing potassium      respond well to banded fertilizers          more tolerant of high salt conditions
phosphate as the source of K have lower        because they require a relatively large     than is grain sorghum, while corn is
SI values than those containing KCl.           percentage of their total nutrients early   intermediate. Tolerance of most oil-seed
When applied near the seed, fertilizers        in their growth period, and their rooting   crops (soybeans and cotton) to seed-row
with lower SI values generally cause           volume in the soil usually is restricted.   application of nutrients is very low, and
fewer problems in seed germination or             As extra equipment has been installed    seed-row application of fertilizer for
seedling injury. SI of any fluid               on planters over the years, it has become   these crops should be viewed with
formulation can be calculated using the        more difficult to have enough room to       caution.
SI values of the most common fertilizer        include the coulters required to open         Fluid fertilizers may produce a lower
sources. Dealers or growers then can           the soil for fertilizer placement below     osmotic pressure in the soil solution
select those formulations with lower SI        and to the side of the seed row. Some       than granular products of a similar grade.
values that best fit their needs.              growers have quit applying starters         Fewer problems generally are
because of this limitation and also         encountered using fluids as seed-row
because of the weight of openers for        fertilizers when compared to granular,

B
much attention over the years.         very large planters. Others have applied    since less soil water is required and salts
Usually, the fertilizer is placed at   starters directly to the seed furrow,       are mainly dissolved in fluid
a depth greater than that of the seed to       which does not require extra openers.       formulations.
allow root interception of the fertilizer         Other considerations. Banding away                  Seed-row application
band as roots grow outward and                 from the seed row is recommended over          This method refers to placement of
downward in the soil.                          seed-row application under most             relatively lower rates of nutrients in
Band vs broadcast                   conditions when applying higher             direct seed contact, usually for row
nutrient rates, especially N, K, and S.     crops. It also has been called “pop-up”
Regions showing the greatest
Plants can efficiently use nutrients        or “in-furrow” application, but “seed-
improvement in efficiency from banding
banded away from the seed row without       row” is more descriptive. Seed-row
over broadcasting lie in the northern
adversely affecting seed germination or     placement increases the possibility of
U.S. and Canada, where colder soil
seedling emergence.                         early root interception by nutrients.
conditions are experienced during
spring seeding of row crops and small             Recommendations for fertilizer                Problems. Major concern of this
grains. Higher P rates are generally           placement in direct seed contact vary       practice is decreased seed germination or
recommended if growers broadcast               with crop. For many years maximum           seedling injury caused by high salt
instead of band their fertilizers.             recommendations ranged from 10-20           concentrations in the soil solutions
around germinating seeds.

Spring 2001                                                        Fluid Journal                                                    1
Also, some starter components such         particular soil. However, it does                            To calculate SI of any formulation:
as urea, UAN, or ammonium thiosulfate           compare one fertilizer formulation with                      1. List the material, grade, and weight
can produce free ammonia (NH3) under            others regarding the osmotic (salt)                        for each component in columns 1-3.
certain soil conditions. Direct seed            effects. It also shows which fertilizers
2. Determine nutrient units in columns
contact by NH3 could result in poor             (those with a higher SI) will be most
4-6 by multiplying the weight of each
germination or seedling death.                  likely to cause injury to germinating
component by its nutrient content and
Selection of the proper starter fertilizer is   seeds or seedlings if placed close to or in
dividing each result by 20.
the way to minimize this occurrence.            the seed row.
3. List SI per plant nutrient unit in
Fertilizers best suited for seed-row                 Calculating salt index
each component in column 7.
application have 1) low salt index, 2)             The SI of a mixed fertilizer (NPKS) is
high water solubility, 3) contain N, P, K                                                                    4. Determine the SI due each
the sum of the SI of each component per
and S, with relatively high P content, 4)                                                                  component by multiplying the sum of
unit of plant nutrient times the number
contain both urea and ammonium-                                                                            the nutrient units in columns 4-6 times
of units in that component. See Table 2
nitrogen, 5) minimize content of                                                                           the corresponding SI value in column 7.
for SI calculations of 7-21-7.
compounds that liberate NH3, and 6) use
potassium phosphate instead of KCl as
Table 1. Salt index values of fertilizer materials.
the K source.
—————Salt Index————
Salt index
Per equal       Per unit of
Salt content is one of the most
wts of materials nutrients*
critical characteristics of fertilizers used    Material and analysis
for row-seed placement. The SI is a             Nitrogen/Sulfur
measure of the salt concentration that           Ammonia, 82%N                                                        47.1             0.572
fertilizer induces in the soil solution.         Ammonium nitrate, 34%N                                              104.0             3.059
Ammonium sulfate, 21%N, 24%S                                         68.3             3.252
The SI of a material is expressed as        Ammonium thiosulfate, 12%N, 26%S                                     90.4             7.533
the ratio of the increase in osmotic             Urea, 46%N                                                           74.4             1.618
pressure of the salt solution produced by        UAN, 28%N (39% a. nitrate, 31% urea)                                 63.0             2.250
a specific fertilizer to the osmotic                32%N (44% a. nitrate, 35% urea)                                   71.1             2.221
pressure of the same weight of NaNO3,           Phosphorus
which is based on a relative value of               APP, 10%N, 34%P2O5                                                20.0             0.455
100. Sodium nitrate was chosen as the                  ,
DAP 18%N, 46%P2O5                                                 29.2             0.456
standard because it was 100 percent
MAP, 11%N, 52%P2O 5                                               26.7             0.405
water soluble and it was a commonly
used nitrogen fertilizer when the SI                Phosphoric acid, 54%P 2O5                                                          1.613a
concept was first proposed in 1943.                                     72%P2O5                                                        1.754a
Higher analysis fertilizers usually have a      Potassium
lower SI because fewer ions of salts are            Monopotassium phosphate, 52%P2O5, 35%K2O                           8.4             0.097
placed in the soil solution per unit of             Potassium chloride, 62%K 2O                                      120.1             1.936
plant nutrient when they dissolve.
Potassium sulfate, 50%K 2O, 18%S                                  42.6             0.852
Note that the N and K materials of
Potassium thiosulfate, 25%K2O, 17%S                               68.0             2.720
commonly used fertilizers (Table 1) have
a
higher SI values than those of P                    Salt index per 100 lbs of H3PO4         *One unit equals 20 lb.
materials. The SI of a mixed formulation
containing N, P, and/or K is the sum of
the SI values of its components.                Table 2. Calculating salt index of 7-21-7.
Although the total SI for a high-analysis                                                                                        —Salt index—
NPK mixture may be greater than that                                   %                                   Nutrient units      per unit     in for-
for a low-analysis NPK mixture, the SI           Material            Nutrient         lbs/ton       N        P2O 5    K 2O      (20 lb)a  mulation
per unit of plant nutrients may be lower              (1)                (2)              (3)        (4)       (5)       (6)     (7)             (8)
in the high-analyses product. Therefore,
the lower fertilizer rate needed to supply       10-34-010%N, 34%P 2O5                  1,235        6.2       21.0      —       0.455          12.4
the same amount of plant nutrients               UAN 28%N                                  57        0.8       —         —       2.250          1.8
subjects the germinating seeds to less           KCl62%K 2O                               226        —         —         7.0     1.936          13.6
potentially adverse salt effects.                Water                                    482        —         —         —       —              —
____         __        __        __      _____          ___
It should be noted that the SI does not
2,000        7.0       21.0      7.0                    27.8 b
predict the exact amount of fertilizer
a
material or a fertilizer formulation that           Salt index per unit (20 lb) of plant nutrients, listed Table 1, also called the partial salt index.
b
could produce crop injury on a                      0.79 SI/unit plant nutrient

Spring 2001                                                              Fluid Journal                                                                    2
5. Total individual SI values of all              Other points to consider are:              phosphate solutions are prepared. SI per
components in column 8.                              When K2SO4 is used instead of KCl,         unit of N due to the ammoniating
SI values for a 6-24-6 formulation             the SI is somewhat lower. However, the        solution is not included because its
containing potassium phosphate are                solubility of K2SO4 is lower than that of     contribution has already been accounted
shown in Table 3. Phosphoric acid is              KCl, so this must be considered in            for in the SI per 100 lbs of H3PO4, since
first ammoniated to a 1-3-0 ratio, which          producing formulations relatively high        it has been converted to ammonium
results in an approximate 50-50 mixture           in K2O.                                       phosphate. The same method is used for
of MAP and DAP at about pH 6.8.                                                                 calculating the SI of ammoniated H2SO4
SI values of acids are given as values
Potassium phosphate is then added to                                                            formulations.
per 100 lbs of acid rather than a unit of
provide all of the K and remainder of the         20 lbs. Also, the SI of H3PO4 varies with        SI values and SI per plant nutrient of
P. Resulting SI of this grade and SI per          P concentration of the acid.                  some commonly used liquid
unit of plant nutrients are much lower                                                          formulations are listed in Table 4. Note
SI values calculated differ for
than those for 7-21-7, which contained                                                          that all formulations containing
formulations when ammoniated
KCl.                                                                                            potassium phosphate have relatively
low SI values. The two formulations
Table 3. Calculating salt index of 6-24-6.                                                   containing KCl (7-21-7 and 4-10-4)
have much higher SI values and are not
—Salt index—          suggested for use in seed-row placement.
%                            Nutrient units          per unit     in for-     These results show that SI of fluid
Material     Nutrient       lbs/ton       N    P2O5        K2O       (20 lb)a   mulation     fertilizers varies significantly,
(1)          (2)         (3)         (4)       (5)      (6)        (7)           (8)     depending on the grade and components
in the formulation.
NH3            82%N         146        6.0         —        —          —b            —
H3PO4        54%P2O5        666        —           18.0     —          1.613         10.7    Dr. Mortvedt, previously with the
Potassium 22%K2O,                                                                            National Fertilizer and Environmental
phosphate 22%P2O5           546        —           6.0      6.0        0.097         1.2     Research Center, TVA, Muscle Shoals,
Water                       642        —           —        —          —             —
AL, and Colorado State University, is a
consultant with U.S. Borax.
___        __          __       __         _____         ___
2,000      6.0         24.0     6.0                      11.9c
a Salt index per unit (20 lb) of plant nutrients, listed in Table 1,
also called the partial salt index.
b Ammoniation of phosphoric acid to a 1-3-0 ratio forms a mixture of MAP and DAP.

c 0.32 SI/unit plant nutrient.

Table 4. Salt index of some common liquid formulations.
Salt index per
unit of plant
Formulation                Salt index                  nutrient (20 lb)
a
2-20-20                        7.2                      0.17
3-18-18a                       8.5                      0.22
6-24-6a                       11.5                      0.32
6-30-10a                      13.8                      0.30
9-18-9a                       16.7                      0.48

10-34-0b                      20.0                      0.45
7-21-7c                       27.8                      0.79
4-10-10c                      27.5                      1.18
28%UANc                       63.0                      2.25
a
These grades are formulated using potassium phosphate as the K source.
b
Use in seed-row placement with caution.
c
Not suggested for use in seed-row placement.

Spring 2001                                                           Fluid Journal                                                    3

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