Yellowing leaves are one of the most frustrating problems a grower can face — not least because there are so many possible causes. The key to fixing the problem is accurate diagnosis first. Reaching for a nutrient feed when the real issue is overwatering, for example, will make things worse, not better.
This guide walks you through the most common causes of yellowing leaves, how to identify which one you're dealing with, and exactly what to do about it.
Start Here: Which Leaves Are Yellowing?
The location of yellowing on the plant is your first and most important diagnostic clue.
- Lower/older leaves yellowing first → likely a mobile nutrient deficiency (nitrogen, magnesium, potassium) or natural senescence
- Upper/newer leaves yellowing first → likely an immobile nutrient deficiency (calcium, iron, manganese) or root damage
- Yellowing across the whole plant → likely overwatering, root rot, or severe pH lockout
- Yellowing between the leaf veins (interveinal chlorosis) → classic sign of magnesium or iron deficiency
Cause 1: Nitrogen Deficiency
Nitrogen is the most commonly deficient nutrient in garden plants, and the most common cause of yellowing leaves — particularly in tomatoes, brassicas, and leafy crops.
Symptoms: Pale yellow-green colour starting on older lower leaves, progressing upward. Leaves may drop early. Stems can appear thin and weak.
Why it happens: Nitrogen is highly mobile in the plant, so when supplies run low, the plant pulls it from older leaves to support new growth. Sandy or heavily watered soils lose nitrogen quickly.
Fix: Apply a balanced nitrogen-rich base nutrient. Ensure pH is in the correct range (6.0–7.0 in soil) so nitrogen can be absorbed. Browse our plant health range for suitable feeds.
Cause 2: Magnesium Deficiency
Extremely common in tomatoes, peppers, and fruiting plants — especially mid-season when fruiting demand is high.
Symptoms: Interveinal chlorosis on older leaves — the leaf veins stay green while the tissue between them turns yellow. Can progress to brown and crispy edges.
Why it happens: Magnesium is a core component of chlorophyll. High potassium levels can block magnesium uptake (antagonism), which is why it often appears when plants are being heavily fed.
Fix: Apply a cal-mag supplement. We recommend Advanced Nutrients Sensi Cal Mag Xtra, Biobizz CalMag, or Mills CalMag depending on your growing system. A foliar spray of Epsom salts (magnesium sulphate) at 2g per litre can also provide fast relief.
Cause 3: Overwatering
Overwatering is the most common mistake among home growers, and yellowing leaves are one of its earliest signs.
Symptoms: Leaves turn pale yellow and may feel soft or limp rather than crisp. Lower leaves affected first. Soil stays wet for extended periods. Roots may smell unpleasant.
Why it happens: Waterlogged soil starves roots of oxygen, impairing their ability to absorb nutrients — even when nutrients are present in the soil.
Fix: Allow the growing medium to partially dry out between waterings and improve drainage. If root rot has set in, treat with a beneficial bacteria product such as Plant Success Great White or Guard n Aid Healthy Root to help restore root health.
Cause 4: pH Lockout
Even if your nutrient solution is perfectly balanced, plants can't absorb nutrients if the pH is outside the correct range. This is called nutrient lockout, and it's a very common cause of yellowing that's often misdiagnosed as a deficiency.
Symptoms: Multiple deficiency symptoms appearing at once, despite feeding regularly. No improvement after applying nutrients.
Why it happens: Each nutrient has an optimal pH range for absorption. Outside that range, nutrients become chemically unavailable to the plant regardless of how much is present.
Fix: Test your soil or feed water pH using a reliable meter such as the Bluelab Soil pH Pen or the Beginner pH Starter Bundle. For soil, aim for 6.0–7.0. For coco or hydro, aim for 5.5–6.5. Flush the growing medium with pH-corrected water and resume feeding at the correct pH.
Cause 5: Natural Leaf Senescence
Not all yellowing is a problem. As tomatoes and other plants mature, lower leaves naturally yellow and die off as the plant redirects energy to fruiting. This is normal and healthy.
How to tell: Only the very lowest, oldest leaves are affected. The rest of the plant looks healthy and vigorous. No spreading pattern.
Fix: Simply remove the yellowed leaves to improve airflow and reduce disease risk. No feeding changes needed.
Quick Diagnosis Checklist
Before reaching for a product, run through these questions:
- Which leaves are affected — old or new growth?
- Is the yellowing between the veins or across the whole leaf?
- When did you last water, and how wet is the soil?
- What is your feed water/soil pH?
- Are you feeding regularly, or has the plant been unfed?