Fertilizer Salt Index & Keep It Seed-Safe
Protects the seed
Enter your fertiliser, rate and placement to get the relative salt load, a seed-safe flag and the threshold— so high-index fertilisers like MOP and urea stay below the rate that scorches germinating seedlings near the row.
Check seedling salt risk
Next: this rate keeps the relative salt load at 75, under the 100 threshold — placement near the seed should be safe.
Salt index compares a fertiliser's osmotic effect to sodium nitrate (=100); high-index products like potassium chloride and urea are riskiest in-furrow on light, dry soils.
Fertiliser salt index — key facts
- Salt index
- vs sodium nitrate = 100
- The risk
- salts pull water from seed
- Highest salt
- MOP, urea, ammonium salts
- Lowest salt
- MAP, K-sulphate, TSP
- Safe fix
- band away (2x2 placement)
- Worst soils
- dry, sandy near seed
- Wider rows
- concentrate the band
- Privacy
- Runs in your browser; nothing uploaded
Feed the crop without burning the seed you just sowed
A little fertiliser by the seed gives crops a fast start — but too much salt too close does the opposite, drawing water out of germinating seedlings and scorching the stand before it establishes. The salt index rates how aggressive each fertiliser is, and high-index products like MOP and urea must be kept below a safe rate near the seed or banded away from the row. A salt-index check settles it with numbers — the relative salt load, the safe threshold, and whether your rate clears it.
This tool gives the relative salt, a seed-safe flag, the threshold and the rate from your fertiliser, placement and conditions. Use it to set safe starter rates, decide when to band fertiliser away from the seed, and lean conservative on dry sandy soils. Pair it with the Broadcast vs Band, Fertilizer (NPK) and Soil Salinity (EC) tools for a full placement plan.
Protect the stand
Keep salt below the seed-safe threshold.
Know the worst offenders
MOP and urea carry the most salt.
Band when needed
Place fertiliser away from the seed row.
Adjust for soil
Go lower on dry, sandy ground.
Frequently Asked Questions
What is the salt index of a fertiliser?+
The salt index rates how much a fertiliser raises the salt concentration of the soil solution around it, relative to sodium nitrate set at 100. Fertilisers with a high salt index draw water out of nearby seeds and roots by osmosis, which can scorch or kill germinating seedlings if too much is placed near the seed.
How does fertiliser scorch seedlings?+
Fertiliser salts dissolve and raise the osmotic concentration of the soil water near the seed. Water moves from where it is less concentrated to more concentrated, so a salty band near a germinating seed pulls water out of it instead of into it — drying, burning and sometimes killing the seedling before it establishes.
Which fertilisers are highest in salt index?+
Potash (MOP / potassium chloride) and urea are among the highest-salt fertilisers per nutrient, along with ammonium nitrate and some ammonium salts. Lower-salt options include MAP, potassium sulphate and triple superphosphate. The calculator uses the fertiliser's salt index to judge how aggressive your chosen product is near the seed.
What is a seed-safe rate?+
It is the most fertiliser you can place with or very near the seed without risking salt injury, given the fertiliser's salt index, the crop's sensitivity, soil moisture, texture and row spacing. The calculator compares your rate against a safe threshold and flags whether you are below it (seed-safe) or above it (risk of scorch).
How do I apply more than the seed-safe rate?+
Band the fertiliser away from the seed — to the side and below (the classic 2x2 placement, about 5 cm beside and below the row) — or broadcast and incorporate it before planting. Moving the salt away from the germinating seed lets you apply the full crop requirement without burning the seedling.
Does soil moisture change the risk?+
Yes. In dry soil there is less water to dilute the salts, so the osmotic pull on the seed is stronger and the safe rate is lower. Moist soil and finer-textured soils with more water-holding buffer the salt and raise the safe rate. Sandy, dry soils are the riskiest place to band fertiliser near seed.
Does row spacing matter?+
Yes — the same amount of fertiliser concentrated in a narrow band near closely spaced seed is far harsher than the same amount spread along wide rows. Wider rows and narrower seed bands concentrate the salt; the safe seed-placed rate is set per length of row, so spacing is part of the calculation.
Does it work for any fertiliser or rate?+
Yes — enter the fertiliser's salt index, your rate and the placement, and it returns the relative salt load, a seed-safe flag, the threshold and the rate. As long as you use the product's salt index, the comparison scales to any fertiliser and any seed-placed or banded rate.
Are the figures precise?+
They're solid planning figures. Real salt injury depends on soil moisture, texture, crop, weather and exactly how the band sits relative to the seed. Use the seed-safe flag as a guard rail, lean conservative on dry sandy soils, and band away from the seed when in doubt — it is about steering, not exact prediction.