Fertiliser scorch

What you are looking at

The oldest leaves on the plant, the ones lowest down and furthest from the growing tip, have gone brown and papery around the edges. The brown is dry rather than soft, it has a sharp boundary against the green, and it runs the whole way round the margin rather than sitting in patches.

Push a finger into the compost and it is wet. The plant is wilting anyway.

Look at the surface and there is likely to be a crust, white or yellowish or faintly brown, dusting the compost and rimming the pot. The same deposit gathers around the drainage holes and on the outside of a terracotta pot.

On a lawn the picture is different and easier to read. Yellow or brown stripes lie across the grass in the pattern of a spreader, wider at the turns, with darker green grass in between.

Salt, and why the plant wilts in wet compost

Fertiliser is salt. Every soluble feed is a mixture of mineral salts, and once dissolved they raise the concentration of the water in the compost.

Roots take up water by osmosis, which works only while the water outside the root is less concentrated than the sap inside it. Push enough salt into the compost and that gradient flattens out, and beyond a certain point it reverses and the compost starts drawing water out of the root.

So a badly over-fed plant behaves exactly like a plant in a drought, because from the root’s point of view that is what it is in. The leaves lose water faster than the roots can replace it, and the tissue at the margins, furthest from the veins and last in the queue, dries out and dies. That is the brown edge.

Two other things happen alongside. Ammonium and urea in a strong feed are directly toxic to root tips, killing them outright and shrinking the root system that was struggling already. And the salts left behind as water evaporates from the surface build the crust, which is the visible marker of how much has accumulated.

The four ways people do it

Feeding dry compost accounts for more damage than over-dosing does. A pot that has dried out has no water in it to dilute the feed, so the full concentration arrives at roots that are already short. Watering first and feeding half an hour later removes the problem entirely.

Feeding out of season accounts for most of the rest. A houseplant in a dim room in December is taking up next to nothing, and everything poured in stays there. Four months of winter feeding at the summer rate builds a concentration no plant will tolerate.

Mixing it stronger than the packet says is the one people admit to. The rate on the bottle is not cautious and there is no benefit above it.

And feeding a plant that is already in trouble is the one that feels most like help. A plant with cold-damaged, rotting or freshly disturbed roots cannot use what it has, and adding more makes water harder to reach at the moment it is needed most.

Lawns, manure and the greenhouse border

A lawn scorches in stripes because a spreader delivers granules in a band, and where two passes overlap the grass gets a double dose. Feeding before a dry spell rather than before rain makes it far worse, since nothing washes the granules off the leaf and down into the soil.

Fresh manure burns for a different reason. As it breaks down it releases ammonia, and ammonia damages roots and young stems in contact with it. Stacking manure for a year and spreading it on the surface rather than in the planting hole solves both problems.

Glasshouse borders and growing bags accumulate salts season after season, because they are watered from above, never leached by winter rain, and fed hard all summer. A border that has grown tomatoes for five years without being flushed or replaced will scorch a crop that would be fine anywhere else.

What to do about it

Water the plant, wait, then feed. Never the other way round.

Mix at half the stated strength and feed twice as often.

Stop feeding when the plant stops growing, and leave it alone through the winter.

Leave a struggling plant unfed until it is producing new leaves again.

Check whether the compost already carries a feed charge before adding more.

Flush an affected pot with three or four times its volume of water, twice, a week apart.

Scrape the crusted compost off the surface and replace it.

Repot into fresh compost where the root tips have gone brown.

Soak a scorched lawn or border repeatedly to wash the salt below the roots.

Stack manure for a year, spread it on the surface, and keep poultry pellets away from stems.

What it gets mistaken for

Drought produces identical crisp margins in dry compost. Feel the compost before deciding, because watering a salt-scorched plant helps and feeding a dry one does not.

Hard water leaves a chalky deposit and browns the tips slowly over many months rather than in a week.

Tipburn takes the young leaves in the middle of a lettuce or cabbage and leaves the outer leaves untouched, which is the opposite pattern.

Sun scorch bleaches whichever leaves face the window and spares the shaded side of the same plant.

Root rot softens the roots and sours the compost, while salt damage leaves the roots dry and the compost odourless.

Weedkiller drift distorts and straps the newest growth at the top of the plant without killing the leaf edges below it.