Over-fertilizing (Fertilizer Burn)
Fertilizer burn happens when fertilizer salts build up in the small volume of bonsai soil and draw water out of the roots (and from the leaf margins)…
Over-fertilizing (Fertilizer Burn) at a glance
| Also known as | Fertilizer burn, salt burn, nutrient burn |
|---|---|
| Affected parts | Roots, leaf margins |
| Most at risk | Recently repotted or weak trees, and small pots fed at full strength |
| Main cause | Too much fertilizer / feeding a stressed or dormant tree |
| Severity | Moderate; can burn roots and set the tree back |
| Reversible? | Yes if flushed promptly |
What to look for
- Leaves: Brown, dry, scorched leaf tips and margins; sometimes sudden leaf drop; growth checks or distorts.
- Soil: A white or yellowish crusty salt deposit on the soil surface or pot rim.
- Roots: Burned roots are darkened/dry at the tips; in bad cases similar to drought damage.
- Whole tree: Overall scorched, stalled appearance soon after a heavy feed.
Overview
Fertilizer burn happens when fertilizer salts build up in the small volume of bonsai soil and draw water out of the roots (and from the leaf margins), scorching tissue. It typically follows over-feeding, feeding at too high a concentration, or feeding a tree that is freshly repotted, root-damaged, sick, or dormant — all situations where the roots can't cope.
Causes
Feeding too often or too strong; using full-strength chemical feed on a small pot; fertilising a freshly repotted, weak, or dormant tree; or letting salts accumulate in soil that is never flushed.
Commonly confused with
Leaf scorch from sun looks similar but isn't tied to feeding and spares shaded leaves. Underwatering wilts with dry soil. The salt crust plus a recent heavy feed points to fertilizer burn.
Which species are affected
All species; greatest risk in small pots, with strong chemical fertilisers, and on trees that are recovering, sick, or dormant.
Treatment
Flush the soil thoroughly with plenty of plain water (water heavily several times) to leach out excess salts; remove any surface crust. Stop feeding until the tree recovers and is growing. Resume at half strength and less often.
Prevention
Feed at recommended (or lower) rates; never feed freshly repotted, weak, or dormant trees; prefer gentler organic feeds for small pots; flush soil periodically; follow the species' feeding schedule.