Tomato Hydroponics: Old Leaves, Young Leaves and a Drifting pH
A polyhouse in Vellayani grows tomatoes in a recirculating nutrient solution. After 30 days, older leaves show interveinal chlorosis, while young leaves are small and uniformly pale. Solution pH has drifted from 5.8 to 7.6; EC is 2.4 dS/m; the recipe supplies 200 mg/L Ca, 50 mg/L Mg and 2 mg/L Fe-EDTA. Root tips are brown. The solution is aerated for only 4 hours a day, and the root-zone temperature is 29°C. A leaf tissue test from the affected older leaves shows 0.11% Mg, while the sufficiency range is 0.3–0.8%; young-leaf Fe is 45 mg/kg, below the 60–200 mg/kg range. The crop is at the flowering stage. The grower adds more Fe-EDTA, but the young leaves worsen and the EC rises to 3.1 dS/m.
1. Distinguish the old-leaf and young-leaf symptoms and name the most likely deficient nutrients. Why are the two patterns different?
Read the worked answer
The old leaves show interveinal chlorosis with green veins, and their tissue Mg is 0.11%, well below the sufficiency range. Magnesium is mobile in the phloem, so when supply is short the plant moves Mg from older leaves to growing points; the older leaves therefore show the deficiency first.
The young leaves are uniformly pale rather than interveinal, and their Fe is 45 mg/kg, below the sufficiency range. Iron is immobile in the plant, so a shortage appears in new growth. The brown root tips and limited aeration add a root-health problem: damaged roots absorb nutrients poorly, which can make both deficiencies worse even if the recipe contains enough Mg and Fe on paper.
2. Explain why adding more Fe-EDTA did not correct the young-leaf chlorosis and may have made it worse.
Read the worked answer
Fe-EDTA is stable only in a limited pH range. At pH 7.6 the chelate breaks down and iron precipitates as insoluble hydroxides, so extra Fe-EDTA does not deliver usable Fe to the roots. The EDTA that is released can bind other cations, and the added salts raise EC.
The rise from 2.4 to 3.1 dS/m increases osmotic stress at the root surface. Combined with the brown roots and poor aeration, the plant cannot take up the iron that remains in solution. Adding more of the same chelate therefore treats the number on the label, not the availability problem.
3. Give a reasoned correction plan, including what you would measure to confirm recovery.
Read the worked answer
First, aerate continuously and reduce the root-zone temperature if possible, because healthy roots are needed for any nutrient correction. Acidify the solution cautiously to pH 5.5–6.0, then prepare a fresh solution at a lower EC. Use an Fe chelate that remains available at the actual pH, such as Fe-EDDHA, or apply a temporary foliar Fe spray. Adjust Mg to bring the solution and tissue levels into range, and do not keep adding Fe-EDTA to a high-pH tank.
Confirm recovery by checking new leaves for green colour and measuring pH and EC daily. Repeat the leaf tissue test after 10–14 days: rising young-leaf Fe and older-leaf Mg, with new growth no longer pale, would show that the correction worked. Fruit set at flowering should be recorded as a practical outcome.