Yellow leaves on the new flush, white crust on the bags
You are in charge of a 5,000-polybag nursery attached to a range office. Irrigation comes from a borewell with pH 8.4, high bicarbonate and electrical conductivity 0.9 dS/m. Watering is by overhead rose twice a day, at 07:00 and again at 18:00. Three weeks after pricking out, about 600 Acacia nilotica seedlings show yellow new leaves with veins still green, while the oldest leaves remain dark green and the plants are otherwise erect. In a low corner near the boundary wall, 250 bags show a white crust on the outside of the polythene, stunted and wilting seedlings, brown mushy roots, and a white mould at the collar. Two days ago 20 mm of rain fell and that corner held standing water for about six hours. You have Rs 5,000, no soil or water testing laboratory within 60 km, and the Divisional Officer wants a written explanation by evening.
1. Diagnose the two problems separately, giving the reasoning, and name the quick field checks you can run without a laboratory.
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Two different problems, not one.
Problem 1 - the 600 yellow plants. Yellowing of the new flush with veins still green is interveinal chlorosis of an immobile nutrient, which points to iron. The cause is the water, not a shortage of iron in the soil: at pH 8.4 with high bicarbonate, iron is converted to insoluble oxides and the plant cannot absorb it. The old leaves stay dark green precisely because iron is not re-translocated - this is what separates it from nitrogen deficiency, where the old leaves yellow first. The 18:00 overhead watering keeps the medium constantly wet and raises bicarbonate at the root zone.
Problem 2 - the 250 bags in the corner. White crust outside the bag, brown mushy roots, collar mould and wilting after six hours of standing water add up to waterlogging and salt accumulation, with a root-rot organism (Pythium, Rhizoctonia or Fusarium) at the collar. The crust is salt drawn up and deposited as the water evaporates, aggravated by the boundary wall's efflorescence.
Field checks. Compare pH of borewell water and of leachate squeezed from a bag; dig up two affected and two healthy plants and smell and feel the roots; check whether symptoms follow the low corner or the whole bed; and note whether watering was skipped after the rain.
2. Give your priority actions for the next 48 hours and state what must NOT be done.
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Contain the rot first, because it kills; correct the chlorosis second, because it only slows growth.
Next 48 hours.
- Stop all irrigation in the low corner; cut a shallow drain to the road side and shift the 250 bags to raised benches or a higher bed.
- Uproot and destroy the worst 40–50 bags away from the nursery; do not stack them near the beds.
- Drench the remaining affected bags with a labelled fungicide such as carbendazim plus mancozeb, or with Trichoderma viride if available, and repeat as per label.
- Spray the 600 chlorotic plants with 0.5 per cent ferrous sulphate solution (5 g per litre) with a pinch of citric acid, in the evening, on the leaves, not the soil; repeat after a week.
- Shift overhead watering to early morning only and reduce frequency; water until the bag is wet, then let it dry slightly.
Do not. Do not add lime or wood ash, which will raise pH further and worsen the iron problem. Do not increase urea, which pushes soft growth that rots faster. Do not re-use the corner soil or the mouldy bags. Do not drench a waterlogged bag with more water to 'wash out the salt' - it drives oxygen out of the root zone.
3. The Divisional Officer asks for a permanent fix within Rs 5,000. What will you propose, and how do you explain the white crust to the nursery staff?
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Spend on drainage and water discipline; the chemistry follows.
Proposal within Rs 5,000. Raise the low corner with 2–3 trolley loads of soil and sand, or build low wooden/brick benches so no bag stands in water; add coarse sand and well-rotted compost to the potting mixture to open it up (roughly one part sand to three parts soil plus one part compost); buy 2 kg ferrous sulphate, 1 kg citric acid, 500 g carbendazim plus mancozeb, and a cheap pH strip book; and mulch the bags to slow evaporation.
Explanation for staff. The white crust is not fungus and not lime from the wall - it is salt. Borewell water carries dissolved salts; when the water evaporates from the bag and from the bag surface, the salts stay behind and build up as a white layer. Over months this makes the medium salty, roots lose water to it, and the plant wilts even though the bag is wet. The cure is not more water but better drainage, less frequent deep watering in the morning, and occasional leaching of the bags with clean water so the salts wash out below the bag.
The written reply must state the diagnosis, the immediate action, the cost, and one monitoring line: pH of water and leachate recorded fortnightly, and a count of affected bags, so the next report shows whether the fix worked.