Hydroponic Tomato Leaves Turning Yellow: Main Causes
Hydroponic tomato leaves turning yellow can point to low magnesium, early blight on older leaves, pH drift or roots short on oxygen. UF/IFAS notes that excess potassium often brings on magnesium deficiency in lower leaves. Look at where the yellow starts, then check the solution plus roots.
The short answer
Note which leaves yellow plus how. Lower leaves that yellow under excess potassium point to low magnesium, according to UF/IFAS. Yellow tissue around brown spots with rings points to early blight. University of Missouri Extension notes a pH outside the required range can block nutrient uptake, so check pH plus solution EC too.
Is lower leaf yellowing normal?
University of Tennessee Extension notes that leaves below the most mature cluster are typically removed. They offer little benefit to developing fruit once that fruit starts to ripen. Some yellowing low on an older vine may simply mean those leaves are due to come off. Yellow that climbs fast or shows spots deserves a closer look.
| Pattern | Likely cause | Action |
|---|---|---|
| Old leaves below ripening fruit | Leaves past their use | Remove them |
| Lower leaves yellow with excess potassium | Low magnesium | Check the solution balance |
| Yellow around brown ringed spots | Early blight | Remove spotted leaves |
| Yellowing across the plant | pH outside range | Check pH with a meter |
| Yellowing with brown roots | Low oxygen at the roots | Aerate plus cool the water |
Source: UF/IFAS Extension, University of Minnesota Extension plus University of Missouri Extension.
What does magnesium deficiency look like?
UF/IFAS notes that plants exposed to excess potassium often develop magnesium deficiency on the lower leaves. The fruit often develops blossom end rot too, mainly early in the season. University of Tennessee Extension notes not to use softened water, since softeners remove calcium plus magnesium and replace them with sodium.
How do root problems cause yellowing?
University of Missouri Extension notes water temperature caps how much oxygen the solution can hold, and warmer water holds less. It notes a pH outside the required range may stop the plant from using its nutrients. University of Minnesota Extension notes early blight starts as small dark spots on older leaves, with yellow tissue around them.
How to: step by step
- Note whether yellowing is on old or new leaves.
- Look for brown spots with rings on lower leaves.
- Examine roots for color plus smell.
- Check solution EC plus pH with a meter.
- Remove old yellow leaves below ripening fruit.
- Adjust the solution only by the maker’s directions.
Troubleshooting
Lowest leaves yellow below ripe fruit: leaves past their use. Remove them.
Lower leaves yellow with a high potassium feed: low magnesium. Check the solution balance.
Yellow around ringed spots: early blight. Strip spotted leaves.
For related reading see how to spot 8 common cannabis plant deficiencies by leaf symptoms, caring for your hydroponic system plus understanding ph and how it affects cannabis plants. Kind Seed Co also stocks cannabis seeds.
Sources
- Tomatoes, peppers and cucumbers in small scale soilless and hydroponic systems (W 844 C), University of Tennessee Extension
- Nutrient solution formulation for hydroponic tomatoes in Florida (HS796), UF/IFAS Extension
- Hydroponic nutrient solutions (G6984), University of Missouri Extension
- Early blight in tomato and potato, University of Minnesota Extension
FAQ
Should I remove yellow tomato leaves?
Often, yes. University of Tennessee Extension notes leaves below the most mature cluster are typically removed once fruit starts to ripen, since they add little. University of Minnesota Extension advises removing leaves with leaf spots from early blight. Clear them away from the system so they do not rot in the water.
Can pH cause yellow leaves?
It can. University of Missouri Extension notes a solution with pH outside the required range may stop the plant from using its nutrients. Nutrients can be present in the reservoir yet unavailable to the roots. Check pH with a meter before adding anything, since a pH fix may solve the yellowing.
How often should solution be changed?
The listed sources do not set one schedule for home tomato systems. University of Missouri Extension advises monitoring pH, EC, dissolved oxygen, temperature plus alkalinity to hold roots in their comfort zone. Change or adjust the solution whenever readings drift, and follow the maker's directions for the nutrient you use.
Do tomatoes need extra magnesium?
Not always. UF/IFAS notes that plants exposed to excess potassium often develop magnesium deficiency on lower leaves. It notes some water sources supply magnesium while others need it added. Have your water checked for pH, calcium plus magnesium, then match the nutrient mix to what the water lacks.
Can I use softened water for hydroponic tomatoes?
University of Tennessee Extension advises against it. Water softeners remove the minerals that cause hardness, such as calcium plus magnesium, and replace them with sodium. That leaves less of both nutrients for the plant. Use water that has not gone through a softener, and have it checked first.
