Buxus Block Turning Yellow After Liming
Six weeks after potting, a block of 2,000 Buxus sempervirens 'Suffruticosa' plugs (potted into 3-gal containers) shows pale yellow new growth with distinctly green veins. The crop went into a 70:30 aged pine bark:peat mix amended with 8 lb/yd³ dolomitic lime and 1.5 lb/yd³ micronutrient mix, and is fertigated weekly with 20-10-20 at 150 ppm N. Pour-through results are pH 7.4 and EC 0.9 mS/cm. Irrigation water is municipal well water at pH 8.1 with 190 mg/L CaCO3 alkalinity. A second block potted the same day from the same bark batch but without the lime looks normal. Tissue analysis of recently expanded leaves returns 28 ppm Fe and 22 ppm Mn. Media temperature is 24°C, air 17-19°C at night. The grower reads the pale colour as nitrogen deficiency and proposes doubling the fertilizer rate to 300 ppm N.
1. Identify the most probable cause of the symptom pattern and explain how the evidence excludes nitrogen, magnesium and root-rot explanations.
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Most probable cause: iron (and probably manganese) deficiency induced by high substrate pH and bicarbonate in the irrigation water - an induced, not a true, deficiency.
Reasoning chain:
- Where the symptom appears. Chlorosis is on the newest growth with veins staying green. Immobile nutrients (Fe, Mn, Zn, Cu, B) fail in new leaves because they cannot be redistributed from older tissue. Nitrogen and magnesium are mobile, so they show on older or whole-plant foliage first.
- The paired-block comparison. Same bark batch, same irrigation, same fertilizer - the only difference is dolomitic lime. The no-lime block is normal. That single contrast implicates pH, not fertility or pathogens.
- The numbers support it. Pour-through pH 7.4 is well above the 5.5-6.3 range where Fe and Mn stay plant-available; 190 mg/L alkalinity and 8 lb/yd³ dolomite both push pH up; well water at pH 8.1 adds bicarbonate with every irrigation.
- Why not N. N deficiency is uniform pale green including veins, and typically starts low on the plant. EC 0.9 with weekly 150 ppm N is not a starving crop, and doubling N would raise EC, not fix a pH-driven micronutrient lockout.
- Why not Mg. Mg chlorosis is interveinal but appears on older leaves first and is favoured by low pH, the opposite of this situation.
- Why not root rot. Root disease gives patchy, non-uniform decline, wilt, and soft brown roots; here the pattern follows the lime amendment uniformly across 2,000 plants and EC is normal.
The tissue values are only corroborating - the experimental contrast is the strongest evidence.
2. Give the immediate corrective treatment and the changes needed to stop it recurring in the next crop, with specific targets and rates.
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Immediate (this crop):
- Acidify the irrigation water so the alkalinity is neutralised. Target roughly 50-80 mg/L CaCO3 residual and a solution pH of about 5.8-6.2. Injecting sulfuric or citric acid at a rate calculated from the 190 mg/L alkalinity is the standard approach; re-test after injection rather than assuming.
- Deliver Fe in a form that works at high pH. Fe-EDDHA chelate is effective up to about pH 9; Fe-DTPA largely fails above pH 6.5, so DTPA would waste money here. Apply as a drench to the substrate, not only as foliar, because the roots need to take it up.
- A foliar spray of iron sulfate or chelate at about 0.5-1.0 g/L gives a fast cosmetic greening in 7-10 days but is temporary; use it to buy time while the root-zone pH comes down, not as the fix.
- Check Mn too; a manganese sulfate drench may be needed since Mn behaves similarly at high pH.
Preventive (next crop):
- Drop the dolomitic lime or cut it drastically; if pH correction is needed use calcitic lime and only after a mix test. Dolomite in a bark mix with alkaline water is a double alkalinisation.
- Re-run a pour-through or saturated media extract on every new bark lot before potting; bark pH varies by supplier and age.
- Switch part of the N programme to ammonium/urea-based acidifying formulations, keeping the nitrate share adequate for cool media.
- Monitor pour-through pH monthly and after any fertilizer change, targeting roughly 5.8-6.3.
Do not double the N rate: it adds salt, does not address the lockout, and can push soft growth that masks the diagnosis.
3. Describe how you would verify the diagnosis and confirm the correction worked, using tests the nursery can actually run.
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Verification should combine a substrate test, a plant test and a visual audit, with a defined timeline.
- Substrate pH/EC by pour-through. Irrigate to container capacity, wait 30-60 minutes, then collect leachate from a sample of 10-15 containers spread across the block, not just the worst plants. Record pH and EC. Repeat on the no-lime block as a control. A pH difference of 1.0-1.5 units between blocks confirms the lime is the driver.
- Saturated media extract (SME) or 1:2 dilution on a composite sample of the mix itself, to separate what the mix contributes from what the water and fertilizer contribute. Do this before potting and again mid-crop.
- Tissue sampling. Sample the most recently fully expanded leaves (not the chlorotic tip growth and not old leaves) from 20+ plants, treatment and control separately. Re-sample 3-4 weeks after acidification - new growth should show higher Fe/Mn and the visual greening should appear on leaves that emerge after treatment, not on the already-yellow ones.
- A replicated mini-trial. Treat 50 plants with acidified water plus EDDHA and leave 50 untreated; if the treated plants green up on new growth while untreated ones do not, the diagnosis is confirmed.
- Visual scoring. Tag 20 plants and score 0-5 for chlorosis weekly; also photograph a fixed frame so progress is comparable.
Timeline: re-test pour-through at 7 and 21 days, tissue at 30 days, and a full block audit before the next potting date. If pH falls but greening does not follow, look at Mn, root health, or herbicide carryover in the bark rather than adding more iron.