Fertilizer Salt Index Calculator
Choose up to four fertilizer materials and enter their weights to estimate the blend salt index, finished grade, and the components that contribute most.
Blend salt index27.29
- Final fertilizer grade
- 7-21-7-0S
- Salt index per nutrient unit
- 0.78
- Component salt-index contributions
- APP 10-34-0: 12.35 (45.3%); UAN 28-0-0: 1.80 (6.6%); Potassium chloride 0-0-62: 13.14 (48.2%)
- Largest salt contributor
- Potassium chloride 0-0-62 (13.14, 48.2% of the blend salt index)
Component contributions
| Component | Weight share | N-P2O5-K2O-S | Salt index contribution | Share of total |
|---|---|---|---|---|
| APP 10-34-0 | 0.62% | 10-34-0-0S | 12.35 | 0.45% |
| UAN 28-0-0 | 0.03% | 28-0-0-0S | 1.8 | 0.07% |
| Potassium chloride 0-0-62 | 0.11% | 0-0-62-0S | 13.14 | 0.48% |
| Water or inert carrier | 0.24% | 0-0-0-0S | 0 | 0% |
Weights may be pounds, kilograms, or batch parts, but liquid volumes must be converted to weight before entry.
Water and inert carrier dilute the finished grade and blend salt index while adding no nutrient units.
How to use this calculator
- Select each fertilizer material used in the blend, or choose Not used for empty lines.
- Enter every component on the same weight basis, such as pounds, kilograms, or batch parts.
- Use water or inert carrier when it is part of the batch weight and should dilute the grade.
- Read the blend salt index, grade, salt index per nutrient unit, and component contribution table.
How the fertilizer salt index is calculated
Salt index is a comparative measure of the osmotic pressure a fertilizer can create in soil solution. This calculator follows the nutrient-unit worksheet method used in extension references for multi-component blends. It does not try to predict crop injury directly; it estimates the salt-index contribution of the materials in the batch.
For each component, the entered weight is first converted to a share of the total batch weight. If a component has percentages of nitrogen, phosphate, and potash listed on the label, the primary nutrient percentage is N + P2O5 + K2O. The component contribution is:
C = (component weight / total batch weight) × (N + P2O5 + K2O) × PSI
PSI is the material's salt index per nutrient unit. For example, potassium chloride has a much larger salt-index value than potassium sulfate on an equal nutrient basis, so it often becomes the largest contributor even when it is not the largest ingredient by weight.
The blend salt index is the sum of every component contribution. The finished fertilizer grade is calculated separately as a weight average of N, P2O5, K2O, and sulfur. Sulfur is shown in the grade because it is part of the product analysis, but it is not added to the nutrient-unit denominator for salt index because the reference method defines nutrient units on the primary plant nutrients.
What moves the result most
The result changes with both analysis and material choice. Adding water or inert carrier lowers the finished grade and the blend salt index because the same fertilizer is spread through more total weight. Switching from potassium chloride to potassium sulfate usually lowers the salt index contribution from the potassium source. Concentrated nitrogen sources can also matter, especially when they are placed close to seed.
What this leaves out
The estimate does not include application rate, band width, soil moisture, soil texture, temperature, crop species, seed distance, ammonia toxicity, or product impurities. It also assumes the entries are weights. Gallons or liters of liquid fertilizer must be converted with the product density before using this worksheet.
Worked example
A 2,000-part blend made from 1,235 parts APP 10-34-0, 57 parts UAN 28-0-0, 226 parts potassium chloride 0-0-62, and 482 parts water calculates to a finished grade of 7-21-7-0S. The component contributions are about 12.35 from APP, 1.80 from UAN, and 13.14 from potassium chloride, for a blend salt index of about 27.29.
The primary nutrient sum in that finished grade is about 35.0 percentage points, so the salt index per nutrient unit is about 0.78. Potassium chloride is the largest contributor in this example even though APP is the largest fertilizer ingredient by weight.
Common questions
What does fertilizer salt index measure?
Salt index compares the osmotic pressure caused by fertilizer materials against a reference material. A higher value means the material can create more salt pressure at the same basis, but it is not a direct injury threshold.
How is a blend salt index calculated?
Each material is weighted by its share of the batch and by its primary nutrient analysis. The calculator multiplies that nutrient share by the material salt index per nutrient unit, then adds the component contributions.
Should liquid fertilizer amounts be entered in gallons or pounds?
Enter weights, not unconverted liquid volumes. If your blend sheet is in gallons or liters, first multiply each liquid volume by the product density so every component is on the same weight basis.
Do water and inert carriers lower the finished blend salt index?
Yes. Water and inert carrier add batch weight without adding nutrient units or salt-index contribution. They dilute both the finished grade and the calculated blend salt index.
Does a lower salt index mean fertilizer cannot burn plants?
No. Salt index is only one part of fertilizer injury risk. Placement, application rate, moisture, soil texture, crop sensitivity, temperature, and ammonia release can all matter as much as the index.