6 Signs You Are Over Fertilizing Your Plants

A plant that has just been fed often answers with a flush of dark green growth, and the natural response is to feed again sooner, or with a stronger mix. That is how most fertilizer injury happens. Garden fertilizers are soluble salts, and the line between enough and too much is narrower than the bag suggests.

Why too much fertilizer hurts

Water moves into a root because the sap inside is more concentrated than the soil water outside. Dissolve enough fertilizer in that soil water and the difference shrinks or reverses, so roots struggle to take up water even when the soil is wet. The old name for the damage, fertilizer burn, describes the result well: leaves scorched as if by drought in a pot that was watered the day before.

Heat makes it worse. As soil dries in hot weather, the salts left behind become more concentrated, and a plant that is transpiring hard draws more of them into its leaves. Roots also handle a steady, modest supply of nutrients better than an occasional heavy dose. A big feed followed by weeks of nothing swings the salt level around the roots up and down, and the roots take the stress each time.

The six signs

Yellow, wilting lower leaves

The oldest leaves at the base yellow and droop first. Because the roots cannot take up water normally, the plant wilts even in moist soil.

Brown leaf tips and margins

Salts carried up in the water stream collect where water leaves the leaf. Browning usually starts at the tip and works inward along the edge.

Dark or rotting roots

Slide a pot plant out and look. Healthy roots are firm and pale. Roots injured by salts turn brown or black, die back from the tips and invite rot.

Slow or stalled growth

A plant fed to grow faster grows less, or stops, and new leaves come in small.

Leaf drop

Lower leaves fall early, sometimes while still partly green.

A crust on the soil

As water evaporates it leaves fertilizer salts behind as a white or tan crust on potting mix, and sometimes as deposits on the outside of clay pots.

University of Maryland Extension lists the same group of symptoms for container plants, and names the usual causes: too much fertilizer, applications too close together, and concentrates mixed at the wrong strength.

Most of these signs also appear when a plant is overwatered, and several appear when it is too dry. The clue is timing. Fertilizer injury tends to show within days of a feed, often after an extra dose or a mix made “a bit stronger to help.”

Saving an over-fertilized plant

The remedy is to wash the salts out. Stand a container where it can drain freely and run clear water through it until it pours from the drainage holes, then repeat several times, each time with at least the pot’s own volume of water. Lift off any crust gently, taking as little mix with it as possible. A small pot of badly salted mix is easier to repot in fresh mix than to flush. In the open garden, one long, slow soaking carries soluble salts below the root zone, provided the soil drains. On a lawn, a spilled bag or a pass where the spreader overlapped shows up a few days later as brown stripes or patches. Sweep up what can be recovered and soak the spot heavily at once to dilute the rest and carry it down.

Cut away leaves that have turned fully brown, since they will not recover. Stop feeding until new growth looks normal, and then start again at the label rate or below it.

Feeding without burning

Read the label and measure rather than pour. A soil test every few years shows what a bed actually lacks, and the answer is often less than expected. Slow-release and organic sources give up their nutrients over weeks, and an inch or two of compost worked in each year builds fertility gently. Water granular fertilizer in after spreading it and brush any granules off the leaves. Never feed a plant that is already wilting from dry soil; water it first and feed later.

Handle concentrated fertilizers with gloves, keep them sealed and out of reach of children and pets, and sweep stray granules off paths and driveways back onto the soil. What washes off hard surfaces goes down storm drains, and excess nitrogen and phosphorus from many gardens and lawns adds up in groundwater, streams and lakes, where it feeds algae.