Use the Fertilizer Salt Index Calculator to look up the relative salt index of common fertilizer materials or calculate the salt index of a fertilizer blend from the weights and salt indexes of its individual components.

Preset values use the Wisconsin Extension/Mortvedt reference set. Published values can vary slightly between references and fertilizer formulations.

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Fertilizer Salt Index Formula

Fertilizer salt index is a relative measure of the increase in osmotic pressure caused by a fertilizer compared with the increase produced by the same weight of sodium nitrate. Sodium nitrate is assigned a salt index of 100.

SI = (Δ πF) / (Δ πNaNO₃) × 100
  • SI is the fertilizer salt index
  • ΔπF is the increase in osmotic pressure caused by the fertilizer
  • ΔπNaNO3 is the increase in osmotic pressure caused by the same weight of sodium nitrate

A salt index of 100 means that, on the reference basis used for the index, the fertilizer produces the same relative osmotic effect as the same weight of sodium nitrate. A salt index of 50 represents approximately half of the reference effect, while an index of 120 represents about 1.2 times the sodium nitrate reference.

Fertilizer Blend Salt Index Formula

When the salt indexes and weights of the individual fertilizer materials are known, the salt index of a blend can be calculated from the contribution of each component.

SIB = (Σ(wᵢ SIᵢ)) / (Σ wᵢ)
  • SIB is the salt index of the complete blend
  • wi is the weight of an individual fertilizer component
  • SIi is the salt index of that component

The same calculation can also be expressed using each component’s fraction of the total blend:

SIB = f₁SI₁ + f₂SI₂ + × s + fₙSIₙ

Where each f value is the component’s fraction of the total fertilizer blend by weight.

What Is Fertilizer Salt Index?

Fertilizer salt index is a way to compare the relative effect different fertilizer materials have on the osmotic pressure of the soil solution. Fertilizers dissolve into ions, and those dissolved salts can increase the concentration of the soil solution around roots and seeds.

The salt index scale uses sodium nitrate as the reference material with a value of 100. Fertilizers with lower values generally contribute less osmotic effect per equal weight of material, while fertilizers with higher values contribute more.

Salt index is unitless. It is a comparative index rather than a concentration such as mg/L, an electrical conductivity measurement, or an application rate.

How to Use the Fertilizer Salt Index Calculator

Look Up a Fertilizer

  1. Select Look up a fertilizer salt index.
  2. Select the fertilizer material from the list.
  3. If the fertilizer is not listed, select Custom / other fertilizer and enter a published salt index.
  4. Click Calculate.

The result compares the fertilizer’s index with the sodium nitrate reference value of 100.

Calculate the Salt Index of a Fertilizer Blend

  1. Select Calculate a fertilizer blend salt index.
  2. Select the fertilizer material used for each component of the blend.
  3. Enter the weight of each component using the same unit.
  4. The calculator automatically fills the reference salt index for listed fertilizer materials.
  5. For a custom fertilizer, enter its salt index manually.
  6. Click Calculate to determine the weighted salt index of the complete fertilizer blend.

Common Fertilizer Salt Index Values

The following values are common reference values for fertilizer materials. Published salt indexes can vary somewhat between sources, analyses, and formulations, so use product-specific information when available.

Fertilizer Material Typical Analysis Salt Index
Sodium nitrate Reference 100.0
Ammonia 82-0-0 47.1
Ammonium nitrate 34-0-0 104.0
Ammonium sulfate 21-0-0-24 68.3
Ammonium thiosulfate 12-0-0-26 90.4
Urea 46-0-0 74.4
UAN 28-0-0 63.0
UAN 32-0-0 71.1
Ammonium polyphosphate 10-34-0 20.0
Diammonium phosphate 18-46-0 29.2
Monoammonium phosphate 11-52-0 26.7
Monopotassium phosphate 0-52-35 8.4
Potassium chloride 0-0-62 120.1
Potassium sulfate 0-0-50-18 42.6
Potassium thiosulfate 0-0-25-17 68.0

Fertilizer Salt Index Example

Suppose a 100-pound fertilizer blend contains the following materials:

  • 50 lb of urea with a salt index of 74.4
  • 30 lb of monoammonium phosphate with a salt index of 26.7
  • 20 lb of potassium chloride with a salt index of 120.1

Insert each material and its weight into the blend formula:

SIB = ((50)(74.4) + (30)(26.7) + (20)(120.1)) / (100)

The resulting fertilizer blend salt index is:

SIB = 69.23

The blend therefore has approximately 69.2% of the relative osmotic effect of an equal weight of the sodium nitrate reference.

Why Sodium Nitrate Has a Salt Index of 100

Sodium nitrate was selected as the reference material when the fertilizer salt index system was developed. Its relative osmotic value was assigned a value of 100, and other fertilizer materials are compared against that reference on an equal-weight basis.

This means a fertilizer with an index of 50 is not “50% salt” by weight. Instead, its effect on osmotic pressure is approximately one-half of the sodium nitrate reference under the conditions represented by the salt-index method.

Salt Index Is Not the Same as Fertilizer Concentration

Salt index should not be calculated simply from fertilizer concentration, solubility, solution volume, or fertilizer weight. Those variables can affect the concentration of a fertilizer solution, but they do not define the standard fertilizer salt index.

The standard salt index is based on the fertilizer’s relative effect on osmotic pressure compared with sodium nitrate.

Salt Index vs. Electrical Conductivity

Salt index and electrical conductivity are related to soluble salts but measure different things.

  • Salt index is a relative comparison between fertilizer materials.
  • Electrical conductivity (EC) is a measurable property of a particular soil, water, or fertilizer solution and changes with concentration.

A fertilizer can have a known published salt index, while the EC actually experienced by a crop depends on the quantity applied, available water, soil conditions, and how concentrated the dissolved fertilizer becomes around the roots or seed.

Does a Lower Salt Index Mean a Fertilizer Is Safer?

A lower salt index indicates less relative osmotic effect for the same weight of fertilizer material, but salt index alone does not establish whether an application is safe.

Crop injury from fertilizer salts can also depend on the amount applied, fertilizer placement, distance from the seed, soil texture, soil moisture, crop sensitivity, fertilizer chemistry, and other conditions.

For this reason, the calculator does not assign universal low, moderate, or high injury-risk categories from salt index alone.

Salt Index and Seed-Placed Fertilizer

Salt index is particularly useful when comparing fertilizers that may be placed near or with seed. A concentrated zone of dissolved fertilizer salts can reduce the ability of a germinating seed or young root to take up water.

However, the fertilizer’s salt index is only one part of a seed-placement decision. Application rate, row spacing, fertilizer placement, soil moisture, soil texture, crop, and fertilizer chemistry should also be considered.

Why Fertilizer Blend Composition Matters

Two fertilizers with the same final N-P-K grade can have different salt indexes if they are manufactured from different fertilizer materials.

For example, potassium chloride has a substantially higher salt index than potassium sulfate or monopotassium phosphate. Replacing one potassium source with another can therefore change the salt index of a formulation even if its final nutrient analysis remains similar.

Salt Index per Unit of Plant Nutrient

Salt index may also be expressed relative to the amount of plant nutrient supplied. This can be useful because high-analysis fertilizers require less product to supply the same amount of nutrient.

For a finished fertilizer formulation, a simple comparison can be made by dividing its salt index by the total percentage of nutrients being considered:

SIᵤₙᵢₜ = (SI) / (N + P₂O₅ + K₂O)

This is a different comparison from the equal-weight salt index shown as the calculator’s primary result.

Limitations of Fertilizer Salt Index

Salt index is primarily a comparison tool. It does not directly predict the electrical conductivity that will develop in a particular field or growing medium, and it does not determine a universally safe fertilizer rate.

Actual fertilizer salt injury can depend on factors including:

  • Fertilizer application rate
  • Placement relative to seed and roots
  • Soil moisture
  • Soil texture
  • Crop sensitivity
  • Rainfall or irrigation after application
  • Fertilizer formulation and chemical form
  • Ammonia or other specific chemical effects

FAQ

What is fertilizer salt index?

Fertilizer salt index is a unitless measure comparing the increase in osmotic pressure caused by a fertilizer with that produced by the same weight of sodium nitrate, which is assigned a value of 100.

What fertilizer has a salt index of 100?

Sodium nitrate is the reference fertilizer and is assigned a salt index of 100.

Is a higher fertilizer salt index worse?

A higher salt index means greater relative osmotic effect per equal weight of fertilizer. Whether that creates crop injury depends on application rate, placement, soil moisture, soil type, crop sensitivity, and other conditions.

Can fertilizer salt index be calculated from N-P-K alone?

Not reliably. Two fertilizers with the same N-P-K analysis can contain different source materials and therefore have different salt indexes. The component fertilizer materials or a measured or published product salt index are needed for a better calculation.

How do you calculate the salt index of a fertilizer blend?

Multiply each component’s salt index by its fraction of the total blend weight, then add the contributions together. The calculator performs this weighted calculation automatically.

Does application rate change a fertilizer’s salt index?

No. Salt index is a property used to compare fertilizer materials or formulations on a defined reference basis. Applying more fertilizer increases the total amount of salts introduced, but it does not change the fertilizer’s published salt index.

Is salt index the same as soil salinity?

No. Salt index compares fertilizer materials. Soil salinity is an actual soil condition that is commonly evaluated using measurements such as electrical conductivity.

Can salt index tell me how much starter fertilizer is safe to apply?

Not by itself. Safe starter or seed-placed fertilizer rates depend on additional factors including crop, fertilizer formulation, placement, row spacing, soil texture, and moisture conditions. Follow appropriate local agronomic recommendations when determining an application rate.

References