Short answer: look at which leaves are yellow. Yellow older leaves usually mean the plant is short of a mobile nutrient such as nitrogen. Pale new leaves usually mean an immobile nutrient such as iron is not being absorbed, often because pH is too high.
Older, lower leaves yellow first
Nitrogen, phosphorus, potassium and magnesium can move within the plant. When they run short, the plant moves them from old leaves to new growth, so older leaves show symptoms first (Greenhouse Grower, with Neil Mattson).
- Even yellowing of older leaves: most often nitrogen running low. Near harvest in a Kratky container this is common as the solution is used up. Early in the crop it suggests the solution was mixed too weak.
- Yellowing between the veins of older leaves: points to magnesium.
Newest leaves pale or yellow
Iron and calcium cannot move once deposited, so shortages show on new growth. Pale new leaves with green veins usually point to iron, and iron shortage is often caused by pH running too high rather than a lack of iron in the solution (Greenhouse Grower, with Neil Mattson).
Yellowing with wilting
If yellow leaves come with wilting and brown, slimy roots, the problem is root rot rather than nutrition. Damaged roots cannot take up nutrients, whatever is in the solution.
What to check, in order
- pH of the solution
- EC of the solution
- Solution level, and whether roots still reach it
- Root color and smell
What not to do
Do not add more nutrient before measuring. If the cause is high pH or root damage, extra nutrient raises the strength without fixing anything. Symptoms overlap, so a measurement is worth more than a guess.
Related guide: Kratky nutrient solution problems.
Sources
- Greenhouse Grower, with Neil Mattson (Cornell University). Tips to Avoid Nutrient Issues in Greenhouse Lettuce.
