Site-Specific & NPK From Your Yield Gap

A normal calculator hands you a blanket recommended dose for the crop regardless of the field. This one is site-specific: it estimates how much yield your field already supports on its own indigenous nutrient supply, then calculates only the extra fertiliser needed to climb from that level to your attainable-yield target. Two fields with the same crop can get very different rates — which is the whole point of SSNM.

It follows the IRRI/IPNI Nutrient-Expert and QUEFTS balance: fertiliser nutrient = (target yield × nutrient uptake per tonne of grain − indigenous nutrient supply) ÷ the in-season recovery fraction. P and K are then converted to P2O5 and K2O (× 2.291 and × 1.205) and split into urea, DAP and MOP product weights. Every figure is an explicit nutrient balance, not a lookup table.

Indigenous supply is the amount of each nutrient (kg/ha) your soil delivers to the crop with no fertiliser of that nutrient applied. The gold-standard measurement is a nutrient-omission plot — a small area left unfertilised for one nutrient — where the yield tells you the supply. A soil test gives a reasonable proxy. Enter elemental P and K, not the oxide forms.

Attainable yield is the realistic ceiling for your field and season — set by your variety, water supply and climate, not by nutrients. It is usually 70–80% of the experimental yield potential. The tool aims fertiliser at a target between your indigenous-supply yield and this ceiling; you choose how much of that gap to close.

Because the last slice of yield is the most expensive in fertiliser per extra tonne. Closing 70–80% of the gap is often the most profitable target, leaving the final stretch to other factors. The yield-gap slider lets you see how the NPK rate and the expected gain change as you raise or lower the target, so you can pick an economically sensible level.

The tool checks which nutrient your indigenous supply runs out of first — that nutrient caps the yield your field reaches unfertilised. If potassium limits the field, adding more nitrogen alone will not lift yield much; you must correct the limiting nutrient first. This is Liebig's law of the minimum applied to a real field, and it is why balanced SSNM beats blanket urea.

At balanced nutrition, irrigated lowland rice takes up about 14.7 kg N, 2.6 kg P and 14.5 kg K per tonne of grain (Witt et al. 1999, the QUEFTS reciprocal internal efficiencies). The tool multiplies these by your target yield to get total uptake, subtracts indigenous supply, and divides by the recovery fraction (≈ 0.46 for N) to get the fertiliser rate.

Yes — both the attainable yield and the indigenous N, P and K supply are editable. The crop selection loads sensible defaults, but the calculator is built to take your omission-plot or soil-test figures. The more accurate your indigenous-supply numbers, the sharper the recommendation.

It depends entirely on your field. If your soil already supplies enough N to grow 4 t/ha and you are targeting 7 t/ha, the balance may call for around 90–95 kg fertiliser N/ha — but a poorer soil targeting the same yield needs more, and a rich soil needs less. That field-by-field answer, rather than a single 'recommended' number, is exactly what this tool gives.

Split N into about three applications — a basal dose, an early-vegetative dose, and a dose at the reproductive (panicle-initiation or jointing) stage — so it matches crop demand and is not lost to leaching or volatilisation. Apply the phosphorus as basal DAP at sowing and band the potassium near the row. Splitting is what turns a calculated rate into recovered nutrient.

It sizes P and K to meet the crop's uptake for the target yield above indigenous supply, which keeps the field in balance season to season. On very deficient soils you may want to add a build-up increment over several years; on rich soils the rate can fall to zero, which the tool will show when indigenous supply already exceeds uptake.

The tool carries Nutrient-Expert/QUEFTS parameters for irrigated and rainfed rice, irrigated and rainfed wheat, and temperate and tropical maize — 6 crop settings in all, each with its own nutrient-uptake-per-tonne and recovery fractions from IRRI, IPNI and the Witt and Pampolino literature.