The short version: Fertiliser burn is salt damage. Too much feed pulls water out of the roots, so the plant wilts in wet soil and the leaf tips and edges turn brown and crisp within days of feeding. Stop feeding, scrape off visible granules, soak the root zone with plain water three times, letting it drain between soakings, and shade the plant while it recovers. Pots hold their salts, so flush them or repot. Next time, water first, use the label rate or less, and keep the feed off stems and leaves.
You fed the lettuce on Saturday because it looked a bit pale. By Tuesday it looks far worse: the bed is damp, the plants are limp, and the leaf edges have turned the colour of toast. The plant looks worse the day after you were kind to it, which is one of gardening's small cruelties.
What fertiliser burn is
Fertilisers are salts. Roots take up water by osmosis, with water moving from the weaker solution in the soil into the stronger one inside the root. Tip too much fertiliser into the root zone and the soil water becomes the stronger solution. The New South Wales primary industries department describes what happens next in its guide to salt in irrigation water: osmosis runs in reverse, water leaves the roots, and the plant shows the same stress as a drought, with leaf tips dying back and margins yellowing, scorching and turning brown or black. That is why a burnt plant wilts in wet soil. The edges go first because they are the tissue furthest from the major veins, and the damage is spread evenly over the plant rather than in the spots and rings that leaf diseases leave.
How it happens
Nearly every case comes down to concentration: too much salt, in too little water, too close to the roots.
- Too much, or too hot. Doubling the label rate doubles the salt, not the growth, and a quick-release feed put on in the heat and not watered in is a common cause on its own.
- Dry soil. The same dose in dry soil sits in far less water, so the solution around the roots is stronger. Feeding a plant already wilting from heat or drought is the classic mistake.
- A heap in one spot. Pellets piled against a stem, a spill where the bag tipped, or a fertiliser spike give one patch of root a concentrated dose.
- Fresh manure. Fresh poultry manure burns roots, young plants most of all, which is why Gardening Australia's advice is to age it in the compost heap first.
- Spraying leaves in the sun. A foliar feed that dries on a hot leaf leaves its salts behind. Spray weak, in the early morning or evening, and not in hot, dry or windy weather.
Is it burn, or something that looks like it?
Crisp brown edges have several causes, and the fix for one can make another worse.
- Fertiliser burn: you fed within the last few days, the soil is moist, and the plant is wilting anyway. A pot often carries a white or tan crust on the mix.
- Heat and wind scorch: the same edges after hot, windy days with the soil drying out and no feeding. Even a shaded plant can scorch in a heatwave when leaves lose water faster than roots replace it.
- Waterlogging and root rot: days of rain, sour-smelling soil and a plant yellowing from the bottom, because roots without air die back, or rot, and stop supplying the leaves. Our guide to waterlogged soil has the fixes.
- Salty water: salty bore or dam water gives the same symptoms slowly, in every bed it reaches, and sprayed over leaves it scorches them even at low salinity.
- Potassium deficiency: the margins of the oldest leaves go yellow then brown and cup downward while new growth looks fine. It comes on gradually, and it is a shortage rather than an overdose: hungry soil, or roots too damaged or waterlogged to take potassium up.
If you fed in the last few days and the soil is moist, not sodden and sour, treat it as burn.
The fix
- Stop feeding until the plant has made new, healthy leaves, then start again at half strength into moist soil.
- Remove what you can see. Scrape granules, pellets or crusted mix off the surface and away from the stem, bin them, and rinse any fertiliser off the leaves.
- Flush the root zone. Water slowly with plain water until the soil is wet well below the roots, carrying the salts down past them, then repeat the next day and again a few days later. Extra water to leach salts below the root zone is the department's own remedy for salty soil. Let the bed drain between soakings, and do not soak again while it is still sodden.
- Pots: pour on at least the pot's own volume of water, so it runs freely from the drainage holes, let it drain, and do this three times in all. Empty the saucer each time, or the salts are drawn straight back up. Flushing washes out most of a liquid or quick-release feed; slow-release pellets are picked off by hand, and a small pot, or one with pellets mixed through, is repotted into fresh mix. Fresh potting mix can harbour Legionella longbeachae bacteria, so put the mask and gloves on first, open the bag with your face clear of it, dampen the mix and wash your hands after.
- Shade it for a few days, so the damaged roots have less to supply.
- Leave the scorched leaves on until new ones come through. They still make food and shade the new growth, and the brown parts will not turn green again.
Fresh growth means the plant has enough root left. No new growth, continued wilting and a stem softening at the base mean it has not, or that rot has set in: pull it, check the spot drains, flush it and resow.
Not doing it again
- Measure the rate on the packet or the cap, and if in doubt use less.
- Half strength for seedlings. ABC Organic Gardener recommends feeding home-raised seedlings with fish emulsion at about half the recommended concentration so their fine roots are not burnt.
- Water before and after. Feed into moist soil, water the feed in, and hold off in a heatwave or when a plant is wilting. Little and often beats one big dose, especially in pots and sandy soils.
- Keep it off stems and leaves. Spread granules in a ring under the outer leaves and pour liquids onto the soil. Our guide to side dressing a growing crop shows the spacing.
- Compost manure first. Fresh manure ages in the heap for three to six months, which deals with the burn, not the germs. Bagged manure composted to Australian Standard AS 4454 is the ready-to-use kind; a bag marked only aged still counts as untreated for food safety.
- Prefer the gentler salt, and mulch. The department rates potassium chloride at a salt index of 114 against 46 for potassium sulphate, so where a plant needs potassium, sulphate of potash is the kinder source. Mulch slows the surface evaporation that concentrates salts near the top.
Burn risk across Australia's climate zones
Burn follows heat, dry soil and small volumes of mix rather than the calendar, so take these as the pattern to watch for.
- Tropical (Darwin and the Top End in the Northern Territory, Townsville and Cairns in Queensland): wet-season downpours wash soluble feed past the roots; the burn risk comes in the dry and the build-up, when a feed goes onto hot, drying soil.
- Subtropical (Brisbane and the south-east of Queensland, the northern rivers of New South Wales): summer rain leaches feed below the roots and the hot spells between showers concentrate what is left: feed lightly into moist soil in the morning, and skip it before a hot week.
- Arid and semi-arid (Broken Hill in New South Wales, Alice Springs in the Northern Territory, Mildura in Victoria, Kalgoorlie in Western Australia): little rain to wash salts through and fierce evaporation to concentrate them, so this is burn country. Feed lightly, flush pots on purpose, and know how salty your bore water is.
- Temperate (Melbourne in Victoria, Sydney and the coast of New South Wales): the risk peaks in the summer heat runs, so feed summer crops lightly into moist soil and let a heat run pass before the next dose.
- Mediterranean (Adelaide in South Australia, Perth and the south of Western Australia): fast-draining sands lose feed quickly, which tempts heavier doses, and a long dry summer concentrates whatever is there. Light, frequent feeds into watered soil suit the sands.
- Cool and alpine (Canberra in the ACT, Tasmania, Stanthorpe in Queensland, Orange and Armidale on the tablelands of New South Wales, Ballarat in Victoria): the heat windows are short, but a hot dry spell still concentrates a feed, and a pot on a sunny verandah does not know which zone it is in.
These zones are broad groupings. If you sit between two, go by what the soil and the forecast are doing on the day you reach for the bag.
A note on manure and food safety
Burn is a plant injury, not a contamination: a scorched leaf has been dried out by salt. The food safety question is about raw manure, which can carry germs that get onto crops from the soil, from splash and from leaf contact. The NSW Food Authority's guidance on raw manure is to use only commercially treated products, never to put untreated manure on short-term crops such as leafy vegetables and herbs, and otherwise to leave 45 to 180 days between application and harvest, the period read from its decision trees for the crop and the local temperature. If fresh manure caused the burn, follow that guidance before you pick from the bed. Wear gloves, and wash your hands and your produce.
Frequently asked questions
Can I still eat vegetables that have fertiliser burn?
Yes, if the feed was a fertiliser sold for edible crops or a commercially composted product. The scorched parts are dead, dry tissue, so trim them off and use the sound leaves and fruit, washed as usual. A product not sold for edible crops goes by its label. If the feed was fresh manure, do not eat leafy greens or herbs from that planting, and wait out the Food Authority's exclusion period above on longer crops.
Can organic fertilisers burn plants?
Yes. Fresh poultry manure is a well-known cause of burnt seedlings, and pelletised manure, blood and bone and fish emulsion all burn when overdone. Organic products generally release more slowly, which lowers the risk without removing it.
Why did my seedlings die after I added pelletised chicken manure?
Most likely a dose meant for a bed went into a punnet, or the pellets sat against the stems, and a seedling has very little root to lose. Mix composted material through the whole bed before planting, keep pellets to the label rate and clear of stems, and feed seedlings with a half-strength liquid instead.
Can seaweed solution burn plants?
Plain seaweed solution is a tonic rather than a feed, low in the main nutrients, so at the label rate it is unlikely to burn roots. Any liquid can scorch leaves if it is mixed too strong and sprayed on in the heat, so keep to the label dilution and spray in the cool of the day.
Water first, measure, and feed the soil rather than the plant, and the bag in the shed goes back to being a help rather than a hazard.
Related growing guides
- What fertiliser does and when a vegetable garden needs it
- Side dressing: feeding vegetables once they are growing
- Watering pots through an Australian summer
- Getting a vegetable garden through a heatwave
Replacing a seedling that did not make it? Browse the vegetable seed range and the herb seed range.