Onion Fertigation: Double Phosphorus, Half Potassium
An onion grower in Nashik has 1 ha of drip-irrigated rabi onion on a medium-black soil. The soil test shows low available nitrogen, medium phosphorus and high potassium; the recommendation for his situation is 125 kg N, 50 kg P2O5 and 75 kg K2O per hectare. He incorporated 15 t/ha of well-rotted FYM (0.5% N, 0.2% P2O5, 0.5% K2O) before planting. At planting he broadcast 200 kg DAP and 200 kg urea. From week 3 he has been sending 5 kg of 19:19:19 per week through the drip and plans to continue for 10 weeks. Bulbs are now forming, tips are lush and dark green, and one corner of the field shows leaf-tip burn. He intends to add another 50 kg urea in the next fertigation and to mix calcium nitrate in the same tank to 'strengthen the bulbs', then stop all fertilizer 10 days before harvest.
1. Calculate the N, P2O5 and K2O already planned (basal plus ten weekly fertigations), compare with the recommendation and the FYM credit, and identify what is wrong.
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What the plan actually supplies (per ha)
| Source | N (kg) | P2O5 (kg) | K2O (kg) |
|---|---|---|---|
| 200 kg DAP (18-46-0) | 36 | 92 | 0 |
| 200 kg urea (46-0-0) | 92 | 0 | 0 |
| 50 kg 19:19:19 (5 kg × 10 weeks) | 9.5 | 9.5 | 9.5 |
| Fertilizer total | 137.5 | 101.5 | 9.5 |
| 15 t FYM (0.5-0.2-0.5) | 75 total, ~22 available in year 1 | 30 total, ~8 available | 75 total, ~37 available |
| Roughly plant-available total | ~160 | ~110 | ~47 |
Against 125 : 50 : 75 this is P at about double the dose and K at roughly half.
Diagnosis
- The soil test already showed high potassium, yet almost no K is being applied - the 19:19:19 is a maintenance fertilizer, not a bulb-filling one.
- Excess P is fixed in this soil, adds cost and can induce zinc deficiency; it also has no route to the bulb.
- The 19:19:19 is simply the wrong product at bulb formation, where the crop needs K and sulphur.
- The leaf-tip burn in one corner is more likely salt accumulation or uneven dripper discharge than a nutrient shortage - check emitter output there before adding anything.
2. Design a corrected feeding plan for the remaining six weeks and state the tank-mixing rule for calcium nitrate.
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Corrected plan for the last six weeks (per ha)
- Stop DAP and 19:19:19 immediately. The phosphorus requirement is already met several times over.
- Potassium: about 60–65 kg K2O still needed. Apply 20 kg MOP (60% K2O) per week for five weeks, or, if storage quality and chloride are a concern, sulphate of potash at 24 kg/week (50% K2O), which also supplies sulphur.
- Nitrogen: only about 20–25 kg remains useful - 8 kg urea per week for three weeks, then stop. Late N softens bulbs and thickens necks.
- Micronutrients: one foliar spray of 0.5% ZnSO4 plus 0.25% borax at bulb initiation.
- Stop all fertilizer 12–15 days before harvest and stop irrigation 10–12 days before harvest.
Tank-mixing rule
Calcium nitrate must never share a tank with DAP, MAP, MOP, SOP or any phosphate or sulphate fertilizer. Calcium with phosphate forms insoluble calcium phosphate; calcium with sulphate forms gypsum. Both precipitate, block the drippers and make the nutrients unavailable. If calcium is needed, inject it alone in a separate fertigation cycle after flushing the lines.
Operational point: clean the filters, acid-flush the laterals and check the uneven corner. Uniform discharge will improve bulb size this season more than any extra bag of fertilizer, and it costs nothing but a morning's work.
3. The farmer wants to add 50 kg urea to increase bulb size. Explain why this is risky and how you would judge whether extra nitrogen is actually needed.
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Why the extra urea is a poor decision
- At bulb formation the plant switches from leaf to bulb. Extra N keeps it vegetative: thick necks, delayed maturity, more splitting and much poorer keeping quality follow, and the crop becomes more susceptible to thrips and purple blotch.
- The 50 kg urea supplies 23 kg N. Nitrogen is already covered by the basal urea, the DAP and the FYM; the crop is dark green, so there is no visible shortage.
- Economics: the urea is cheap, but if late N and late water cause even a 5% loss in store, on a 400 q/ha crop at ₹1,500/q that is ₹30,000 lost - far more than the cost of the urea.
- Potassium, not nitrogen, is the nutrient actually short here (see the earlier calculation).
How to decide properly
- Leaf tissue test at bulb initiation: 3–4.5% N in the youngest fully opened leaf is adequate; below 2.5% would justify a small top-up of 10 kg N, not 23 kg.
- Soil and history: with FYM plus 137 kg N already applied, N deficiency is unlikely.
- Judge by the crop: uniform dark green with good leaf area means stop; pale lower leaves with slow growth would justify a small correction.
- Spend the money on K and on drip maintenance instead - that is where the yield response is.