Fertilizer Salt Index & Keep It Seed-Safe

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.

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.

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.

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).

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.

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.

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.

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.

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.