My romaine lettuce outer leaves are turning yellow from the tips inward. EC is 1.4, pH is 6.1. Is this a nitrogen deficiency, a pH issue blocking absorption, or something else? How do I diagnose it?

Your EC and pH readings are actually in reasonable range, which narrows the diagnosis. Here's how to work through it systematically.

The direction of yellowing is the key diagnostic clue

In hydroponics, the pattern of symptom progression tells you whether you're dealing with a mobile or immobile nutrient:

• Outer/older leaves affected first: Mobile nutrient deficiency (nitrogen, phosphorus, magnesium, potassium). The plant moves these nutrients from old growth to new growth when supply is limited.
• Inner/newer growth affected first: Immobile nutrient problem (calcium, iron, boron, manganese). The plant cannot relocate these once deposited in tissue.

Your description - yellowing from tips inward on outer leaves - strongly points to a mobile nutrient issue, most likely nitrogen or magnesium.

Ruling out pH lockout

At pH 6.1, you're in the optimal range for lettuce (5.8–6.5). True nutrient lockout from pH alone is unlikely here. You'd need to be above 7.0 or below 5.0 for severe iron or manganese lockout. Cross pH off your suspect list.

Assessing EC 1.4

EC 1.4 is technically adequate for lettuce but sits at the lower end for mature romaine, which has a larger root system and higher transpiration rate than loose-leaf varieties. In the final 2–3 weeks of a romaine grow, pushing EC to 1.6–1.8 is common practice. The outer-leaf yellowing you're seeing at week 4+ of a romaine grow may simply reflect the plant's increased demand outpacing supply.

First intervention: Raise EC from 1.4 to 1.6–1.7 and observe over 3–4 days. Do not change pH.

Nitrogen source check

If you're using a general-purpose hydroponic nutrient formula, check whether your nitrogen is predominantly nitrate (NO3⁻) or ammonium (NH4⁺). An NH4:NO3 ratio above 1:4 can cause competitive inhibition at the root membrane, effectively reducing nitrogen uptake even when EC appears adequate. Most quality hydroponic nutrients are formulated with predominantly nitrate nitrogen, but some cheaper 2-part formulas skew ammonium-heavy.

Magnesium deficiency check

Interveinal chlorosis (yellowing between leaf veins while veins stay green) on outer leaves points to magnesium. If the pattern shows green veins with yellow areas between them, add Cal-Mag at 1–2 mL/gallon and observe. If the whole outer leaf yellows uniformly (no distinct vein pattern), nitrogen is more likely.

Action sequence

1. Confirm the yellowing pattern: tip-inward with green veins = Mg; uniform yellowing = N
2. Raise EC to 1.6–1.7 with your standard nutrient formula
3. If Mg suspected, add Cal-Mag supplement at 1 mL/gallon
4. Check again in 48–72 hours - nitrogen improvement is fast in hydroponics; new growth should show darker green within 3–5 days

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This answer is reviewed against published horticultural research. It is not a substitute for advice from a certified horticulturalist.