---
title: "Fertilizer Salt Index Calculator: Detect Seed-Burn Risk Before the Planter Rolls"
canonical: "https://theyieldgrid.com/fertilizer-salt-index-calculator/"
model_id: "tyg-640"
model_version: "1.0.0"
last_reviewed: "2026-08-20T11:00:10"
reviewed_by: "Umer Hayiat"
---

# Fertilizer Salt Index Calculator: Detect Seed-Burn Risk Before the Planter Rolls

> Canonical calculator: [https://theyieldgrid.com/fertilizer-salt-index-calculator/](https://theyieldgrid.com/fertilizer-salt-index-calculator/)

## What this calculator does

Home - Free Gardening Calculators & Tools - Fertilizer Salt Index Calculator: Detect Seed-Burn Risk Before the Planter Rolls Seedling death from in-furrow fertilizer rarely looks like fertilizer damage. The stand thins out unevenly, emergence is patchy, and the soil looks fine. What happened underground is a chemistry problem: the fertilizer solution surrounding the seed created an osmotic environment more concentrated than the seed’s own cellular fluid. Water moved out of the germinating seed rather than into it. That process, plasmolysis, is irreversible once it starts and it is entirely preventable if the salt load is calculated before application.

## Inputs

| Input | ID | Type | Unit | Range or choices | Required |
|---|---|---|---|---|---|
| Crop Type * | `fsicalc-crop` | select |  | — Select Crop — = ``; Corn = `corn`; Soybeans = `soybean`; Wheat = `wheat`; Cotton = `cotton`; Sorghum = `sorghum` | Yes |
| Soil Texture * | `fsicalc-soil` | select |  | — Select Texture — = ``; Sandy = `sandy`; Loam = `loam`; Clay = `clay` | Yes |
| Nitrogen % (N) * | `fsicalc-n` | number | 0–46% | 0 to 46 | Yes |
| Potassium % (K₂O) * | `fsicalc-k` | number | e.g., KCl = 62% K₂O, MOP | 0 to 62 | Yes |
| Phosphorus % (P₂O₅) | `fsicalc-p` | number |  | 0 to 54 | No |
| Application Rate (gal/acre) * | `fsicalc-rate` | number | gal/acre | 0.1 to 200 | Yes |

## Outputs

| Output ID | Default state |
|---|---|
| `fsicalc-crop-err` | Required |
| `fsicalc-soil-err` | Required |
| `fsicalc-n-err` | Enter a number between 0 and 46 |
| `fsicalc-k-err` | Enter a number between 0 and 62 |
| `fsicalc-p-err` | Enter a number between 0 and 54 |
| `fsicalc-rate-err` | Must be between 0.1 and 200 |
| `fsicalc-results` | Fill in the fields above and click Calculate to see your fertilizer salt index, plasmolysis risk, and safe rate analysis. — Salt Index Score Relative osmotic pressure rating In-Furrow Safety Gauge (N+K Rate vs. Max Safe Rate) 0% 25% 50% Safe Zone 75% Caution 100%+ DANGER Applied N+K — lbs/acre Max Safe Rate — lbs N+K/acre Risk Ratio — applied ÷ max safe Fertilizer Salt Index Reference Table Fertilizer Analysis Salt Index Risk Class UAN Solution 28-0-0 47.3 Moderate–High Anhydrous Ammonia 82-0-0 |
| `fsicalcout_primary` | — |
| `fsicalc-interp` |  |
| `fsicalcout_nk` | — |
| `fsicalcout_max` | — |
| `fsicalcout_ratio` | — |
| `fsicalc-warnings` |  |
| `fsicalc-warnings-title` |  |
| `fsicalc-warnings-list` |  |

## Formula and method

The Salt Index quantifies the osmotic pressure a fertilizer creates relative to sodium nitrate. High salt index materials placed in direct seed contact can desiccate germinating seeds via plasmolysis — the seed loses cellular water to the high-concentration solution. Our model calculates the exact osmotic pressure exerted on germinating seeds based on nutrient density and soil texture. Show the calculation steps Step 1: Compute Applied N+K Pounds per Acre The calculator converts fertilizer percentages to applied nutrient mass using density-adjusted coefficients: Applied_N_K = Rate (gal/ac) x [ (N% x 0.1084) + (K% x 0.1112) ] The coefficients 0.1084 and 0.1112 represent pounds of N and K₂O, respectively, delivered per gallon per percentage point of nutrient content. These are derived from representative liquid solution densities: UAN 28-0-0 at approximately 10.6 lb/gal and 10-34-0 at approximately 11.7 lb/gal, averaged for the nutrient fractions. Step 2: Determine Adjusted Maximum Safe Rate Max_Safe = Base_Soil_Max x Crop_Sensitivity_Factor Sandy = 5.0 lbs N+K/acre | Loam = 6.5 | Clay = 8.0 Corn = 1.00 | Soybeans = 0.80 | Wheat = 0.85 | Cotton = 0.85 | Sorghum = 1.10 Step 3: Compute Salt Index Score Salt_Index = (N% x 1.69) + (K% x 1.57) These are relative salt index coefficients based on Rader (1943), subsequently updated by IPNI and land-grant university extension references. The index is a unitless score representing osmotic activity relative to sodium nitrate. Step 4: Risk Classification Risk_Ratio = Applied_N_K / Max_Safe Safe: Risk_Ratio 1.0 Rounding Rules Applied N+K and max safe rate are displayed to two decimal places. Risk ratio is displayed as a whole number percentage. Salt index score is displayed to one decimal place. Assumptions and Limits The model assumes in-furrow placement with direct seed contact. Actual risk for 2×2 placement (2 inches to the side, 2 inches below the seed) is substantially lower and would allow considerably higher nutrient loads. Density coefficients are representative averages. Products that deviate significantly from typical UAN or standard starter densities will produce slightly different applied lb/gal values. Check product-specific density if precision is required. Soil moisture at the time of application is not a model input. Dry soils concentrate the applied solution in less soil water, increasing effective osmotic potential beyond what the calculator predicts. In drought conditions, apply a conservative buffer by targeting the lower end of the safe zone. The crop sensitivity factors used are based on published extension research for typical cultivars. Treated seeds, seed coatings, or unusually small seed lots may alter effective tolerance. Soil texture is entered as a single category. Fields with spatial variability across texture classes should be run at the sandiest texture present to avoid under-predicting risk in sensitive zones. The salt index formula does not account for interactions between multiple nutrient sources in the same solution. Tank mixing high-index materials may produce additive osmotic effects not fully captured in the separate per-nutrient coefficients. Results are for educational and planning purposes. Final agronomic decisions should incorporate a current soil test, product-specific technical data sheets, and consultation with a certified crop adviser where required by local regulations. To understand how soil physical properties affect how nutrients move and concentrate near the seed zone, the soil bulk density calculator provides a useful reference point for evaluating your soil structure before planning starter applications. Some growers decide on a gallon-per-acre rate first based on equipment settings and then try to find a product that “scores safe.” The more defensible approach is to determine the safe rate ceiling from the formula first, then calibrate equipment to that rate. Working backward from a desired rate often results in rationalization of borderline inputs. Fix: Use the calculator to establish the maximum allowable applied N+K load, then determine what rate of your specific product delivers that load.

## Verified worked examples

### Scenario 1: Corn on Loam Soil with UAN 28-0-0 at 2 gal/acre

Crop: Corn (sensitivity factor 1.00) Soil Texture: Loam (base max = 6.5 lbs N+K/acre) Fertilizer: 28-0-0 (N = 28%, K₂O = 0%) Application Rate: 2 gal/acre Applied N+K = 2 x (28 x 0.1084 + 0 x 0.1112) = 2 x 3.035 = 6.07 lbs/acre Max Safe Rate = 6.5 x 1.00 = 6.5 lbs/acre Salt Index Score = 28 x 1.69 + 0 x 1.57 = 47.3 Risk Ratio = 6.07 / 6.5 = 93.4% Result: CAUTION. Applied load is approaching the safe limit. Any increase in rate, any drier-than-expected soil, or any field variability toward sandier texture would push this into the Danger zone. This rate would require downward adjustment to stay clearly safe for sensitive establishment conditions.

### Scenario 2: Soybeans on Sandy Soil with 10-34-0 at 5 gal/acre

Crop: Soybeans (sensitivity factor 0.80) Soil Texture: Sandy (base max = 5.0 lbs N+K/acre) Fertilizer: 10-34-0 (N = 10%, K₂O = 0%) Application Rate: 5 gal/acre Applied N+K = 5 x (10 x 0.1084 + 0 x 0.1112) = 5 x 1.084 = 5.42 lbs/acre Max Safe Rate = 5.0 x 0.80 = 4.0 lbs/acre Salt Index Score = 10 x 1.69 + 0 x 1.57 = 16.9 Risk Ratio = 5.42 / 4.0 = 135.5% Result: DANGER. This scenario is a textbook example of why a low salt index score does not automatically indicate safety. 10-34-0 has a low score, but at 5 gal/acre on sandy soil with soybeans, the applied load is 35% above the adjusted safe limit. Reducing to 3.7 gal/acre or moving to 2×2 placement resolves the risk.

### Scenario 3: Corn on Clay Soil with 10-34-0 at 3 gal/acre

Crop: Corn (sensitivity factor 1.00) Soil Texture: Clay (base max = 8.0 lbs N+K/acre) Fertilizer: 10-34-0 (N = 10%, K₂O = 0%) Application Rate: 3 gal/acre Applied N+K = 3 x (10 x 0.1084 + 0 x 0.1112) = 3 x 1.084 = 3.25 lbs/acre Max Safe Rate = 8.0 x 1.00 = 8.0 lbs/acre Salt Index Score = 10 x 1.69 + 0 x 1.57 = 16.9 Risk Ratio = 3.25 / 8.0 = 40.6% Result: SAFE. Clay soil’s higher buffering capacity, combined with a low-salt starter formula and a conservative rate, produces a comfortable margin. This combination is a reliable baseline for corn growers concerned about phytotoxicity risk on heavier ground.

## Assumptions

Model assumes in-furrow application directly at seed. 2×2 placement significantly reduces risk. Crop sensitivity modifiers based on seed size and hull protection (Corn most tolerant; Soybeans most sensitive). Soil moisture at application affects actual osmotic risk — dry soils concentrate salts further. Results are estimates. Always consult local extension agronomist guidelines for final recommendations. Phosphorus (P₂O₅) has a relative salt index of ~0.10 and is excluded from primary N+K risk calculation. This calculator assumes in-furrow placement — fertilizer in direct seed contact. Risk thresholds increase by ~30–40% for 2×2 placement (2 inches to the side and 2 inches below the seed). Our model calculates the exact osmotic pressure exerted on germinating seeds based on nutrient density and soil texture. Show the calculation steps Step 1: Compute Applied N+K Pounds per Acre The calculator converts fertilizer percentages to applied nutrient mass using density-adjusted coefficients: Applied_N_K = Rate (gal/ac) x [ (N% x 0.1084) + (K% x 0.1112) ] The coefficients 0.1084 and 0.1112 represent pounds of N and K₂O, respectively, delivered per gallon per percentage point of nutrient content. These are derived from representative liquid solution densities: UAN 28-0-0 at approximately 10.6 lb/gal and 10-34-0 at approximately 11.7 lb/gal, averaged for the nutrient fractions. Step 2: Determine Adjusted Maximum Safe Rate Max_Safe = Base_Soil_Max x Crop_Sensitivity_Factor Sandy = 5.0 lbs N+K/acre | Loam = 6.5 | Clay = 8.0 Corn = 1.00 | Soybeans = 0.80 | Wheat = 0.85 | Cotton = 0.85 | Sorghum = 1.10 Step 3: Compute Salt Index Score Salt_Index = (N% x 1.69) + (K% x 1.57) These are relative salt index coefficients based on Rader (1943), subsequently updated by IPNI and land-grant university extension references. The index is a unitless score representing osmotic activity relative to sodium nitrate. Step 4: Risk Classification Risk_Ratio = Applied_N_K / Max_Safe Safe: Risk_Ratio 1.0 Rounding Rules Applied N+K and max safe rate are displayed to two decimal places. Risk ratio is displayed as a whole number percentage. Salt index score is displayed to one decimal place. Assumptions and Limits The model assumes in-furrow placement with direct seed contact. Actual risk for 2×2 placement (2 inches to the side, 2 inches below the seed) is substantially lower and would allow considerably higher nutrient loads. Density coefficients are representative averages. Products that deviate significantly from typical UAN or standard starter densities will produce slightly different applied lb/gal values. Check product-specific density if precision is required. Soil moisture at the time of application is not a model input. Dry soils concentrate the applied solution in less soil water, increasing effective osmotic potential beyond what the calculator predicts. In drought conditions, apply a conservative buffer by targeting the lower end of the safe zone. The crop sensitivity factors used are based on published extension research for typical cultivars. Treated seeds, seed coatings, or unusually small seed lots may alter effective tolerance. Soil texture is entered as a single category. Fields with spatial variability across texture classes should be run at the sandiest texture present to avoid under-predicting risk in sensitive zones. The salt index formula does not account for interactions between multiple nutrient sources in the same solution. Tank mixing high-index materials may produce additive osmotic effects not fully captured in the separate per-nutrient coefficients. Results are for educational and planning purposes. Final agronomic decisions should incorporate a current soil test, product-specific technical data sheets, and consultation with a certified crop adviser where required by local regulations. To understand how soil physical properties affect how nutrients move and concentrate near the seed zone, the soil bulk density calculator provides a useful reference point for evaluating your soil structure before planning starter applications. The model assumes in-furrow placement with direct seed contact. Actual risk for 2×2 placement (2 inches to the side, 2 inches below the seed) is substantially lower and would allow considerably higher nutrient loads. Density coefficients are representative averages. Products that deviate significantly from typical UAN or standard starter densities will produce slightly different applied lb/gal values. Check product-specific density if precision is required. Soil moisture at the time of application is not a model input. Dry soils concentrate the applied solution in less soil water, increasing effective osmotic potential beyond what the calculator predicts. In drought conditions, apply a conservative buffer by targeting the lower end of the safe zone. The crop sensitivity factors used are based on published extension research for typical cultivars. Treated seeds, seed coatings, or unusually small seed lots may alter effective tolerance. Soil texture is entered as a single category. Fields with spatial variability across texture classes should be run at the sandiest texture present to avoid under-predicting risk in sensitive zones. The salt index formula does not account for interactions between multiple nutrient sources in the same solution. Tank mixing high-index materials may produce additive osmotic effects not fully captured in the separate per-nutrient coefficients. Results are for educational and planning purposes. Final agronomic decisions should incorporate a current soil test, product-specific technical data sheets, and consultation with a certified crop adviser where required by local regulations. To understand how soil physical properties affect how nutrients move and concentrate near the seed zone, the soil bulk density calculator provides a useful reference point for evaluating your soil structure before planning starter applications. Critical Warnings The Plasmolysis Threshold Is Not Forgiving: Once osmotic potential in the seed zone exceeds cellular concentration in the germinating seed, water efflux begins within hours. There is no field intervention that reverses this. The only corrective measure is prevention: calculate before application, not after stand establishment fails. Muriate of Potash (KCl) Is a Special Hazard: KCl carries a relative salt index exceeding 116. At even 1 gal/acre in-furrow, a 0-0-60 source delivers enough K₂O to exceed safe thresholds on sandy soils for most crops. Sulfate of Potash (SOP), at a salt index around 46, is a substantially lower-risk potassium source for in-furrow use. Low Salt Index Score Does Not Mean Low Risk at High Rates: 10-34-0 has one of the lowest salt index scores among common starters, but Scenario 2 above demonstrates that at 5 gal/acre on sandy soil with soybeans, it still exceeds the safe rate by over 35%. Rate and soil context determine actual phytotoxicity risk; the score alone is only half the picture. Soil Moisture Amplification: The thresholds in this model assume adequate soil moisture at planting. Dry seedbeds concentrate dissolved salts in a smaller volume of soil solution. Under drought-stressed soil conditions, the effective safe rate may be substantially lower than the model output suggests. Minimum Standards for In-Furrow Application Applied N+K load should remain at or below 75% of the calculated Max Safe Rate for comfortable clearance on in-furrow placement. For soybeans, the adjusted max safe rate on sandy soil is 4.0 lbs N+K/acre. This figure should be treated as a firm ceiling, not a guideline, given soybean sensitivity during early germination. Any fertilizer blend containing KCl should be flagged for alternative sourcing or moved to 2×2 placement rather than in-furrow contact, regardless of the rate. Applications on sandy soils during dry springs should use the Clay threshold as a conservative proxy, or reduce the calculated safe rate by at least 20%. Competitor Trap: Many “fertilizer burn calculators” online focus exclusively on salt index score as the decision variable. They present a number, compare it against a generic threshold, and call it safe or unsafe. This misses the second half of the problem entirely. A product with a low index score applied at a high rate on sandy soil with a sensitive crop can easily exceed the plasmolysis threshold. The actual risk is a function of applied pounds per acre delivered, not of the index score in isolation. Any tool that omits rate, soil texture, or crop sensitivity from its analysis is incomplete. When evaluating nitrogen sources for in-furrow programs, the manure nitrogen availability calculator is useful for growers who incorporate organic N sources and need to account for mineralization timing before adding synthetic supplements. Understanding total available N prevents overcalculation when multiple sources are in the system. Applied N+K load should remain at or below 75% of the calculated Max Safe Rate for comfortable clearance on in-furrow placement. For soybeans, the adjusted max safe rate on sandy soil is 4.0 lbs N+K/acre. This figure should be treated as a firm ceiling, not a guideline, given soybean sensitivity during early germination. Any fertilizer blend containing KCl should be flagged for alternative sourcing or moved to 2×2 placement rather than in-furrow contact, regardless of the rate. Applications on sandy soils during dry springs should use the Clay threshold as a conservative proxy, or reduce the calculated safe rate by at least 20%. Competitor Trap: Many “fertilizer burn calculators” online focus exclusively on salt index score as the decision variable. They present a number, compare it against a generic threshold, and call it safe or unsafe. This misses the second half of the problem entirely. A product with a low index score applied at a high rate on sandy soil with a sensitive crop can easily exceed the plasmolysis threshold. The actual risk is a function of applied pounds per acre delivered, not of the index score in isolation. Any tool that omits rate, soil texture, or crop sensitivity from its analysis is incomplete. When evaluating nitrogen sources for in-furrow programs, the manure nitrogen availability calculator is useful for growers who incorporate organic N sources and need to account for mineralization timing before adding synthetic supplements. Understanding total available N prevents overcalculation when multiple sources are in the system. A field classified as Loam by average texture analysis may contain significant sandy knoll areas that behave more like Sandy soil in terms of buffering capacity. Applying a rate calculated to be safe for Loam to a planter pass through sandy variability zones will exceed the threshold in those areas without triggering the calculator’s warning. Fix: If the field has texture variability, run the calculator at the Sandy texture setting and use that rate uniformly, or use variable-rate planting technology to adjust application in real time.

## Limitations and safety

Model assumes in-furrow application directly at seed. 2×2 placement significantly reduces risk. Crop sensitivity modifiers based on seed size and hull protection (Corn most tolerant; Soybeans most sensitive). Soil moisture at application affects actual osmotic risk — dry soils concentrate salts further. Results are estimates. Always consult local extension agronomist guidelines for final recommendations. Phosphorus (P₂O₅) has a relative salt index of ~0.10 and is excluded from primary N+K risk calculation. The model assumes in-furrow placement with direct seed contact. Actual risk for 2×2 placement (2 inches to the side, 2 inches below the seed) is substantially lower and would allow considerably higher nutrient loads. Density coefficients are representative averages. Products that deviate significantly from typical UAN or standard starter densities will produce slightly different applied lb/gal values. Check product-specific density if precision is required. Soil moisture at the time of application is not a model input. Dry soils concentrate the applied solution in less soil water, increasing effective osmotic potential beyond what the calculator predicts. In drought conditions, apply a conservative buffer by targeting the lower end of the safe zone. The crop sensitivity factors used are based on published extension research for typical cultivars. Treated seeds, seed coatings, or unusually small seed lots may alter effective tolerance. Soil texture is entered as a single category. Fields with spatial variability across texture classes should be run at the sandiest texture present to avoid under-predicting risk in sensitive zones. The salt index formula does not account for interactions between multiple nutrient sources in the same solution. Tank mixing high-index materials may produce additive osmotic effects not fully captured in the separate per-nutrient coefficients. Results are for educational and planning purposes. Final agronomic decisions should incorporate a current soil test, product-specific technical data sheets, and consultation with a certified crop adviser where required by local regulations. To understand how soil physical properties affect how nutrients move and concentrate near the seed zone, the soil bulk density calculator provides a useful reference point for evaluating your soil structure before planning starter applications. Critical Warnings The Plasmolysis Threshold Is Not Forgiving: Once osmotic potential in the seed zone exceeds cellular concentration in the germinating seed, water efflux begins within hours. There is no field intervention that reverses this. The only corrective measure is prevention: calculate before application, not after stand establishment fails. Muriate of Potash (KCl) Is a Special Hazard: KCl carries a relative salt index exceeding 116. At even 1 gal/acre in-furrow, a 0-0-60 source delivers enough K₂O to exceed safe thresholds on sandy soils for most crops. Sulfate of Potash (SOP), at a salt index around 46, is a substantially lower-risk potassium source for in-furrow use. Low Salt Index Score Does Not Mean Low Risk at High Rates: 10-34-0 has one of the lowest salt index scores among common starters, but Scenario 2 above demonstrates that at 5 gal/acre on sandy soil with soybeans, it still exceeds the safe rate by over 35%. Rate and soil context determine actual phytotoxicity risk; the score alone is only half the picture. Soil Moisture Amplification: The thresholds in this model assume adequate soil moisture at planting. Dry seedbeds concentrate dissolved salts in a smaller volume of soil solution. Under drought-stressed soil conditions, the effective safe rate may be substantially lower than the model output suggests. Minimum Standards for In-Furrow Application Applied N+K load should remain at or below 75% of the calculated Max Safe Rate for comfortable clearance on in-furrow placement. For soybeans, the adjusted max safe rate on sandy soil is 4.0 lbs N+K/acre. This figure should be treated as a firm ceiling, not a guideline, given soybean sensitivity during early germination. Any fertilizer blend containing KCl should be flagged for alternative sourcing or moved to 2×2 placement rather than in-furrow contact, regardless of the rate. Applications on sandy soils during dry springs should use the Clay threshold as a conservative proxy, or reduce the calculated safe rate by at least 20%. Competitor Trap: Many “fertilizer burn calculators” online focus exclusively on salt index score as the decision variable. They present a number, compare it against a generic threshold, and call it safe or unsafe. This misses the second half of the problem entirely. A product with a low index score applied at a high rate on sandy soil with a sensitive crop can easily exceed the plasmolysis threshold. The actual risk is a function of applied pounds per acre delivered, not of the index score in isolation. Any tool that omits rate, soil texture, or crop sensitivity from its analysis is incomplete. When evaluating nitrogen sources for in-furrow programs, the manure nitrogen availability calculator is useful for growers who incorporate organic N sources and need to account for mineralization timing before adding synthetic supplements. Understanding total available N prevents overcalculation when multiple sources are in the system.

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## Provenance

- Model ID: `tyg-640`
- Model version: `1.0.0`
- Reviewed by: Umer Hayiat
- Page modified: 2026-08-20T11:00:10
- Runtime SHA-256: `d36e78a1ed79826240efbfcd8a786449df3b071fcf90d34861cc966dc8d438ad`

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