Yellow between the veins on older leaves, veins stay green

Likely causes, most likely first

Ranked for this sign on its own. Your stage, medium and readings change the order, so run the checker for your plant.

1. PossibleMagnesium deficiency

   NutrientLow urgencyRead: Magnesium (Mg) in cannabis: interveinal yellowing, targets and the K-Ca competition

   Fix: Check root-zone pH (Mg uptake falls at low pH) and whether heavy K or Ca is crowding it out. If short, bring Mg to about 40-70 mg/L with magnesium sulfate (roughly 0.4-0.5 g/L of the heptahydrate adds ~40-50 mg/L Mg) or magnesium nitrate. A 1-2 g/L magnesium sulfate foliar spray greens veg leaves faster; never spray buds.

   Recovery: Interveinal yellowing stops climbing in 5-7 days; leaves above the damage stay green.

   How to confirm it

   • Measure input pH and root-zone (runoff or reservoir) pH. Calibrate the meter with fresh pH 7 and pH 4 buffers, then measure the mixed feed and the runoff (containers) or reservoir (hydro). Soil and living soil should sit at 6.0-7.0, coco, rockwool and peat mixes at 5.5-6.5, hydro at 5.5-6.2. Track 3+ readings; one reading is not a trend.
   • Get a water report (or test your water). Ask your water utility for the annual report or test well water: alkalinity (mg/L CaCO3), pH, EC, Ca, Mg, Na, Cl, B, and whether it uses chlorine or chloramine. Alkalinity above ~100-150 mg/L as CaCO3 pushes root-zone pH up; sodium above ~50 mg/L and chloride above ~70 mg/L are a problem; softened water is high in sodium.
   • Leaf tissue analysis. Send 20-30 most-recently-matured leaves (no petioles) from affected plants, and separately from healthy ones, to an agricultural lab. Compare with cannabis and hemp sufficiency ranges (for example N 3.3-4.8%, Mg 0.40-0.81%, K 1.8-2.9% dry weight in recent mature leaves). Results take 1-2 weeks; use it to confirm a chronic problem, not an emergency.
   • Watch new growth for 5-10 days after one change. Make one correction at a time, mark the top node with a twist tie and photograph the same leaves every 2-3 days. Damaged leaves never recover; judge only new growth and whether the symptom stops spreading. Mobile-nutrient symptoms stop climbing within 5-7 days; immobile ones show in new leaves within 7-10 days.

2. PossibleLow root-zone pH starving calcium and magnesium

   pHDeal with it this weekRead: pH and nutrient availability: what pH really controls, by medium

   Fix: Raise root-zone pH to range: coco 5.8-6.2, hydro 5.6-6.0, soil 6.2-6.8. Use pH up (potassium hydroxide or carbonate) in small steps; in coco, feed at 6.0-6.2 for a few waterings with Ca and Mg on board. A fast fall in hydro points to dying roots or microbes; check them.

   Recovery: pH holds in range for a week; new leaves spot-free within 7-10 days.

   How to confirm it

   • Measure input pH and root-zone (runoff or reservoir) pH. Calibrate the meter with fresh pH 7 and pH 4 buffers, then measure the mixed feed and the runoff (containers) or reservoir (hydro). Soil and living soil should sit at 6.0-7.0, coco, rockwool and peat mixes at 5.5-6.5, hydro at 5.5-6.2. Track 3+ readings; one reading is not a trend.
   • Check the meters against fresh standards. Put the pH probe in fresh pH 7.0 and 4.0 buffer and the EC probe in a 1.413 mS/cm standard. If pH reads more than 0.1-0.2 off, recalibrate; if it can't be calibrated or the slope is below ~90-95%, replace the probe. Many 'lockouts' are a drifting meter.
   • Pour-through extraction for container pH and EC. Water normally, wait 30 minutes to 2 hours, pour enough pH-adjusted water on top to collect about 50 mL of leachate (roughly 75 mL for a 15 cm pot, 150 mL for a 4 L pot, 350 mL for 11 L), and measure pH and EC of that 50 mL. Less disruptive than full runoff and more repeatable.

3. PossibleUnbuffered or new coco pulling calcium and magnesium

   Root zoneDeal with it this weekRead: Buffering coco coir: why it matters and how to do it

   Fix: Feed with Ca and Mg on board: calcium nitrate and magnesium sulfate, or a Cal-Mag, bringing the feed to about 150-200 mg/L Ca and 40-60 mg/L Mg, at pH 5.8-6.2. For the next batch, rinse and buffer coco by soaking it in a calcium (and magnesium) nitrate solution before planting.

   Recovery: New leaves clean within 7-10 days; runoff Ca/Mg symptoms stop spreading.

   How to confirm it

   • Measure runoff EC against input EC. Water to 10-20% runoff, collect the runoff in a clean cup and measure EC of both with a calibrated meter (1.413 mS/cm standard). Runoff more than ~0.5 mS/cm above input means salt build-up; well below input means the plant is hungry or you are over-leaching. In hydro, compare reservoir EC with the EC you mixed.
   • Measure input pH and root-zone (runoff or reservoir) pH. Calibrate the meter with fresh pH 7 and pH 4 buffers, then measure the mixed feed and the runoff (containers) or reservoir (hydro). Soil and living soil should sit at 6.0-7.0, coco, rockwool and peat mixes at 5.5-6.5, hydro at 5.5-6.2. Track 3+ readings; one reading is not a trend.
   • Get a water report (or test your water). Ask your water utility for the annual report or test well water: alkalinity (mg/L CaCO3), pH, EC, Ca, Mg, Na, Cl, B, and whether it uses chlorine or chloramine. Alkalinity above ~100-150 mg/L as CaCO3 pushes root-zone pH up; sodium above ~50 mg/L and chloride above ~70 mg/L are a problem; softened water is high in sodium.

4. Less likelyCal-Mag overuse (excess calcium and nitrogen)

   Too much of somethingLow urgencyRead: Cal-Mag: what it is, when you need it and when you don't

   Fix: On hard tap water with a complete base feed, stop the Cal-Mag supplement; it adds N and pushes K and Mg out of balance. Use it only for RO or soft water and unbuffered coco. LEDs do not create a Cal-Mag need.

   Recovery: K and Mg symptoms stop spreading within 7-10 days.

   How to confirm it

   • Get a water report (or test your water). Ask your water utility for the annual report or test well water: alkalinity (mg/L CaCO3), pH, EC, Ca, Mg, Na, Cl, B, and whether it uses chlorine or chloramine. Alkalinity above ~100-150 mg/L as CaCO3 pushes root-zone pH up; sodium above ~50 mg/L and chloride above ~70 mg/L are a problem; softened water is high in sodium.
   • Measure runoff EC against input EC. Water to 10-20% runoff, collect the runoff in a clean cup and measure EC of both with a calibrated meter (1.413 mS/cm standard). Runoff more than ~0.5 mS/cm above input means salt build-up; well below input means the plant is hungry or you are over-leaching. In hydro, compare reservoir EC with the EC you mixed.

5. Less likelyExcess potassium crowding out magnesium and calcium

   Too much of somethingLow urgencyRead: Nutrient antagonism and Mulder's chart: what the evidence actually says

   Fix: Stop K boosters and keep K near 175-250 mg/L; cation antagonism (K vs Mg and Ca) is the interaction that reliably matters in practice.

   Recovery: Interveinal yellowing stops climbing in 7 days.

   How to confirm it

   • Measure runoff EC against input EC. Water to 10-20% runoff, collect the runoff in a clean cup and measure EC of both with a calibrated meter (1.413 mS/cm standard). Runoff more than ~0.5 mS/cm above input means salt build-up; well below input means the plant is hungry or you are over-leaching. In hydro, compare reservoir EC with the EC you mixed.
   • Leaf tissue analysis. Send 20-30 most-recently-matured leaves (no petioles) from affected plants, and separately from healthy ones, to an agricultural lab. Compare with cannabis and hemp sufficiency ranges (for example N 3.3-4.8%, Mg 0.40-0.81%, K 1.8-2.9% dry weight in recent mature leaves). Results take 1-2 weeks; use it to confirm a chronic problem, not an emergency.

6. Less likelySulfur deficiency

   NutrientLow urgencyRead: Sulfur (S) in cannabis: role, deficiency, targets and sulfate sources

   Fix: Rare unless a feed is built only from nitrates and chlorides. Supply about 50-120 mg/L S as sulfate, e.g. via magnesium sulfate or potassium sulfate at normal rates.

   Recovery: New leaves emerge green within 7-10 days.

   How to confirm it

   • Leaf tissue analysis. Send 20-30 most-recently-matured leaves (no petioles) from affected plants, and separately from healthy ones, to an agricultural lab. Compare with cannabis and hemp sufficiency ranges (for example N 3.3-4.8%, Mg 0.40-0.81%, K 1.8-2.9% dry weight in recent mature leaves). Results take 1-2 weeks; use it to confirm a chronic problem, not an emergency.
   • Measure PPFD at the canopy top. Use a quantum (PAR) meter, or a calibrated phone app as a rough guide, at the top leaves in a 5-9 point grid. Compare with the stage target: seedlings 100-300, veg 300-700, flower 800-1,200 µmol/m²/s at ambient CO2. Bleaching and taco at the top above ~1,100 point to light; stretch below ~300 in veg points to too little.
   • Watch new growth for 5-10 days after one change. Make one correction at a time, mark the top node with a twist tie and photograph the same leaves every 2-3 days. Damaged leaves never recover; judge only new growth and whether the symptom stops spreading. Mobile-nutrient symptoms stop climbing within 5-7 days; immobile ones show in new leaves within 7-10 days.

Look for

Christmas-tree pattern: green midrib and veins on a yellow blade, on lower fan leaves; margins may stay green..