A yellowing houseplant makes most of us reach for the fertilizer. Hold off for a moment. University of Maryland Extension names overwatering as the number one reason indoor plants yellow. The RHS warns that drought, waterlogging and poor light can all look like a nutrient shortage.

Real deficiencies do happen in pots. NC State notes that the starter fertilizer in most potting mixes lasts only a couple of weeks. Clemson HGIC adds that pot-bound plants are especially prone to running short. The useful skill is reading where the symptom started, then ruling out the more common causes before you feed.

Start with leaf age: mobile vs immobile nutrients

Plants can move some nutrients from old leaves to new growth. NC State’s Extension Gardener Handbook lists nitrogen, phosphorus, potassium and magnesium as mobile in the plant. When they run short, the plant strips them from older leaves, so symptoms show there first.

Iron is relatively immobile, and calcium has limited mobility. The plant cannot pull them back out of old leaves, so new growth shows the shortage first.

Where symptoms startPoints towardNext check
Older, lower leavesNitrogen, phosphorus, potassium or magnesiumLook at colour pattern: uniform, purple, edges or between veins
Newest leavesIron or calciumCheck potting mix pH, water source and drainage
Whole plant at onceUsually not a single nutrientCheck roots and watering first (Clemson)

A few old leaves yellowing on their own is normal ageing. The RHS says the odd lower leaf on a mature plant yellowing and falling is no cause for concern.

How each deficiency looks

These descriptions come from NC State’s nutrient table (Table 1–4) and the RHS nutrient deficiency guide.

NutrientMobilitySymptoms the sources describe
Nitrogen (N)MobileReduced growth; light-green to yellow foliage starting on older growth; fewer side shoots. The RHS adds spindly, stunted plants, sometimes with pink tints
Phosphorus (P)MobileReduced growth; dark-green leaves with purple or red colour in older leaves, especially on the underside along the veins; thin stems; limited root growth
Potassium (K)MobileMargins of older leaves turn yellow and burned; dead spots on older leaves; shortened internodes; wilts readily
Magnesium (Mg)MobileOlder leaves turn yellowish, bronze or reddish while the veins stay green; margins may curl with a puckered look
Iron (Fe)Relatively immobileYellowing between the veins, mainly on young tissue, which may turn almost white
Calcium (Ca)Limited mobilityBud growth stops; young leaves scalloped and abnormally green; leaf tips may stick together; maturing leaves cup; roots can blacken and rot

Two notes from the sources change how you read this table. NC State says magnesium is commonly deficient in foliage plants because it leaches out and is not replaced. It also says calcium is rarely deficient if the correct pH is maintained.

Clemson HGIC sums up the most common indoor pattern: yellowing, browning and death of lower leaves is generally caused by nitrogen or iron deficiency.

For a plant-specific walk-through, see our philodendron nutrient deficiency guide.

Rule out the common look-alikes first

NC State lists the conditions that stop roots taking up nutrients that are present. Waterlogged or compacted mix, temperature extremes, incorrect light and dormancy all limit uptake. It also warns that an apparent shortage of one nutrient can be caused by too much of another.

Roots and watering

Slide the plant out of its pot. Clemson HGIC says the whole plant yellowing is most often caused by overwatering. If most roots are dark, soft and dead, Clemson says it is better to discard the plant. The RHS adds that any root stress, including under- or over-watering, can induce nutrient deficiencies.

Healthy roots are firm and light-coloured, according to Maryland’s diagnosis table. If they are not, fix the root zone first. Our overwatered or underwatered guide walks through the checks.

pH and water source

NC State explains that nutrient availability is tied to pH. Very high or very low pH leaves many nutrients undissolved and out of reach. Iron deficiency can appear even when iron is in the mix if the mix is poorly drained, short of oxygen, or high in pH or phosphorus.

The RHS says interveinal yellowing on acid-loving plants such as gardenia or azalea can be lime-induced chlorosis. Non-ericaceous compost or long-term watering with hard tap water causes it. Our tap water guide covers water quality.

Salts and over-feeding

Too much fertilizer can look like too little. Maryland Extension lists browning of leaf tips and margins, reduced growth, lower leaf drop, dead root tips and sometimes yellowing or stunting. A white crust on the mix or pot is the giveaway.

The RHS adds a specific trap: over-feeding with potassium-rich fertilizer can induce magnesium deficiency.

Light

Low light produces pale, spindly growth. Maryland says plants stretch toward the light and flower poorly. Iowa State warns that more fertilizer in poor light gives spindly, weak growth rather than a fix. Clemson notes that plants in low light have reduced fertilizer needs.

What to do once you have ruled those out

  1. Confirm the plant is growing. NC State says dormant plants absorb few nutrients. Its houseplant chapter advises feeding once a month from March to October, and not from November to February unless deficiency symptoms appear.
  2. Use a complete houseplant fertilizer. Clemson HGIC recommends one containing nitrogen, phosphorus and potassium, such as a balanced 20-20-20 for foliage plants. Our fertilizer schedule covers rates and timing.
  3. Cover micronutrients. Maryland says many indoor plants run short of micronutrients. Use a houseplant fertilizer that contains them, or add a small amount of well-composted, screened compost once a year.
  4. For magnesium, Maryland and NC State both give about 5 ml of Epsom salts per 3.8 litres of water (1 teaspoon per gallon), used twice a year.
  5. For iron chlorosis, Maryland recommends a chelated iron product used to label directions, plus a sulfur product to acidify the mix. Look for one labelled for houseplants.
  6. Repot pot-bound or tired plants. Clemson says regular feeding plus periodic repotting in fresh mix prevents lower-leaf yellowing. Iowa State says to wait 2–4 weeks after repotting before regular feeding.

Judge the result on new growth, not on leaves that were already damaged before you made the change.

When to suspect something else

If the pattern does not fit any row of the table, look wider. The RHS notes that some deficiencies look alike and that leaf tissue analysis is expensive. Maryland lists spider mites, aphids, whiteflies, scale and root rots among the causes of fading, yellowing foliage.

Our yellow leaves decision tree and brown spots guide cover those branches.

References