Yellowing leaves: working out which deficiency
Yellow leaves can mean six different things. Where the yellow appears, and whether the veins stay green, narrows it down fast.
What you see
There are three patterns and they point in different directions:
- Older leaves at the bottom, uniformly pale — mobile nutrient deficiency, usually nitrogen. The plant is moving nutrients to new growth.
- Older leaves, yellow between green veins — magnesium. Very common in small reservoirs.
- New leaves at the top, yellow or distorted — immobile nutrient problem, usually iron or calcium. Often a pH problem rather than a shortage.
Why it happens
pH out of range
The most common real cause. Iron, manganese and zinc become unavailable above pH 6.5 even when present in the solution. Yellowing new growth with a pH of 7.0 is a pH problem, not a deficiency.
Magnesium deficiency
Yellow between the veins of older leaves, veins staying green. Common in small reservoirs where the plant has exhausted the magnesium first. A solution change fixes it faster than an additive.
Nitrogen deficiency
Uniform pale yellowing from the bottom up, with slow growth. Genuine in a reservoir that has not been changed, or in beans and peas that were never fed.
Overwatering and poor oxygen
Yellowing with wilting and drooping, not just colour change. Check the roots — see the root rot entry.
Cold water
Below about 15 °C roots stop taking up nutrients and the plant yellows regardless of what is in the solution.
What to do
Test pH first. Always. It is the cheapest check and the most frequent cause. Aim for 5.8–6.2 for most crops.
- Change the reservoir rather than adding supplements. In a small system this resolves most deficiencies outright, because you restore the intended balance rather than guessing which element is short.
- Check the water temperature — 18–20 °C for most crops.
- Check the roots for brown or slimy growth.
- Only then consider a specific additive, and only if the pattern clearly matches magnesium or iron.
Yellow between the veins on older basil leaves — magnesium or pH?
Third reservoir in a row with the same pattern.
All problems
Six things go wrong in a small reservoir. The index covers each.