Reading a Soil Test and Rescuing a Yellowing Rice Crop
Ramesh Karki farms 0.6 ha of transplanted rice followed by wheat in the Jhapa terai. A soil sample analysed at the district soil laboratory returned pH 5.4, organic matter 1.8%, total nitrogen 0.09%, available P2O5 12 kg/ha and exchangeable K2O 95 kg/ha, with a silt loam texture. The district agriculture office's blanket recommendation for transplanted rice is 100 kg N, 30 kg P2O5 and 30 kg K2O per hectare, to be supplied through urea, DAP and muriate of potash. Last season Karki incorporated 5 t/ha of farmyard manure and applied the full blanket dose, yet harvested only 3.9 t/ha against 5.2 t/ha on a neighbouring field. This season, 25 days after transplanting, the crop is pale yellow-green across the entire field, and Karki wants to top-dress an extra 30 kg N/ha at once. He also asks whether lime is worth buying from the cooperative store 12 km away, since his father never used it.
1. Interpret the soil test values and convert the blanket dose into a season fertilizer schedule for this field. Give the quantity of urea, DAP and MOP to be bought per hectare and show your arithmetic.
Read the worked answer
Interpretation
- pH 5.4 is moderately acid. Phosphate is fixed by Al/Fe, molybdenum availability falls, and the following wheat crop will suffer more than rice. The pH is a liming question, not a urea question.
- Organic matter 1.8% and total N 0.09% are low to medium: the soil cannot supply much N, so splitting is essential and FYM should continue.
- Available P2O5 of 12 kg/ha is below the critical level used for rice (roughly 20 kg/ha in most Nepali recommendations). This needs build-up plus maintenance, so raise the P dose by about 50%: 45 kg P2O5/ha.
- Exchangeable K2O of 95 kg/ha is medium. Keep it at the recommended level but raise slightly to 40 kg K2O/ha because straw is removed and the pH is low.
- Keep N at 100 kg/ha but split it; more total N without correcting P and pH is wasted money.
Arithmetic
- DAP (18-46-0): 45 ÷ 0.46 = 98 kg/ha, which also supplies 98 × 0.18 = 17.6 kg N.
- Basal N target 30 kg: 30 − 17.6 = 12.4 kg N from urea = 12.4 ÷ 0.46 = 27 kg urea.
- Remaining 70 kg N in two splits: 35 kg N = 76 kg urea at active tillering (25–30 DAT) and 76 kg urea at panicle initiation (55–60 DAT).
- MOP (60% K2O): 40 ÷ 0.60 = 67 kg/ha.
Schedule: all DAP, all MOP and the 27 kg basal urea incorporated at final puddling; first top-dress 76 kg urea at tillering; second 76 kg urea at panicle initiation; 5 t/ha FYM before puddling. Total per hectare: about 179 kg urea, 98 kg DAP, 67 kg MOP.
2. Is the uniform yellowing at 25 DAT simply nitrogen deficiency? Set out a differential diagnosis and state the decision rule you would use before recommending any top-dressing.
Read the worked answer
Not automatically. Uniform, whole-field yellowing at 25 DAT is consistent with N deficiency, but three other possibilities fit the same field conditions.
Differential diagnosis
- N deficiency – older leaves yellow first, plants thin, pale from the base upward; the field responds to N.
- Iron toxicity – very likely at pH 5.4 with stagnant water. Look for rusty brown spots on older leaves, blackened roots with a rotten-egg smell (also H2S toxicity). Extra urea will not fix this and can worsen it.
- Sulphur deficiency – pale young leaves, more common on low organic matter and with DAP instead of single superphosphate. Distinguish by leaf age.
- Zinc deficiency – brown streaks and stunting on middle leaves.
- Waterlogging, cold irrigation water, herbicide carry-over, or a basal fertilizer application error – all produce patchy-to-uniform pale crops.
Diagnostic steps before spending money
- Walk the field: uniform, or patchy with low spots? Uniform points to soil chemistry or a blanket application error; low spots point to Fe toxicity or waterlogging.
- Dig five plants, wash roots, smell them, check leaf age of first symptoms and look for brown spots.
- Check the basal application record: was DAP actually applied and incorporated?
- Lay a 1 m² response strip with 20 kg N/ha and compare after 7 days.
- Send a tissue sample if the answer is still unclear.
Decision rule: top-dress only when N deficiency is confirmed, the field is at tillering, and water is controlled. If it is Fe toxicity, drain and dry the field for a few days, stop fresh organic matter, and keep P and K adequate. In any case, the first scheduled N split of 76 kg urea is already due at 25–30 DAT: any extra 30 kg N should replace part of that split, not be added on top of it.
3. Karki asks whether to buy lime. What information is missing, what rate and timing would you propose, and how would you justify it economically?
Read the worked answer
Missing information
- A lime requirement test (buffer pH or exchangeable acidity and Al saturation). pH alone tells you lime is likely to help, not how much is needed.
- Lime quality: calcium carbonate equivalent, fineness, moisture, and therefore the effective neutralizing value.
- Price delivered to the farm and the expected yield gain in the following wheat crop.
Proposal
- Target pH 6.0–6.5 for a rice–wheat rotation. At pH 5.4 on a silt loam, agricultural lime at roughly 1–2 t/ha is the normal starting range, but finalize the rate only after the lime requirement test.
- Broadcast and incorporate into the dry soil 2–3 weeks before transplanting, then puddle. Do not apply lime and urea or DAP together in the same operation; ammonia loss and P re-fixation reduce efficiency. Apply lime first, then basal fertilizer at puddling.
- Re-test after 2–3 years. Lime moves slowly in soil, so this is a capital investment, not a seasonal input.
- Keep applying FYM; organic matter and lime work in the same direction on this soil.
Economic justification Compare the delivered cost per tonne of lime with the expected yield gain. At the current yield of 3.9 t/ha against 5.2 t/ha on the neighbouring field, the gap is 1.3 t/ha of paddy. If liming plus corrected P recovers even half of that gap, the return is large compared with a one-off cost of 1–2 t/ha of lime spread over three years. A simple check: (expected extra yield in kg × farm-gate price) must exceed (lime cost ÷ 3 years). If the buffer pH test shows no acidity problem, skip the lime and put the money into P build-up and drainage instead.