Gardening glossary
Chlorosis
Chlorosis is a symptom, not a disease, and reading the yellowing pattern is the key to diagnosis. Healthy leaves are green because chlorophyll absorbs red and blue light and reflects green. When chlorophyll synthesis is disrupted or existing chlorophyll degrades faster than it is replaced, the green fades and underlying yellow carotenoids become visible.
Pattern is everything. Interveinal chlorosis on the youngest leaves, where veins stay green and the tissue between them turns yellow, typically indicates iron deficiency — often triggered by alkaline soil (pH above 7.0) that locks iron out of solution even when present. Interveinal chlorosis on older leaves usually points to magnesium deficiency, since magnesium is mobile and the plant relocates it to new growth first. Uniform yellowing of older leaves while new growth stays green is the classic signature of nitrogen deficiency. Yellowing with brown leaf margins suggests potassium deficiency or salt buildup from fertilizer. Yellowing across the whole plant, especially with soft drooping growth and damp soil, indicates root rot or chronic overwatering — the roots cannot take up nutrients even when the soil contains them.
Non-nutritional causes include viral infection (mosaic patterns), herbicide drift, sudden cold, transplant shock, and severely compacted or waterlogged root zones. Houseplants moved into low light often develop a pale washed-out chlorosis as the plant downsizes its chlorophyll investment to match available light.
Treatment depends on the pattern. Soil-test before adding nutrients to avoid over-correction. For iron chlorosis on alkaline soil, foliar chelated iron sprays give the fastest response while longer-term soil acidification (elemental sulfur, organic mulch) addresses the underlying pH. Nitrogen and magnesium respond quickly to dilute feeding. If overwatering is the cause, fix drainage before adding fertilizer — wet soil with extra salts only makes the situation worse.