STCR: the science behind targeted fertiliser dosing
Indian farmers historically followed "general fertiliser recommendations" — say 120-60-40 N-P-K kg/ha for wheat in Uttar Pradesh, or 100-50-50 for paddy in Andhra Pradesh — published in state Package of Practices booklets. These are blanket recommendations calibrated to the average state soil; they ignore the wide variation in N, P, K availability between a sandy loam in Meerut and a clay loam in Pratapgarh, both in UP. Blanket dosing leads to systemic overuse of N (low-cost, subsidised urea is the convenient knob), under-use of K (expensive MOP), and a gradual soil-degradation spiral measured by ICAR-IISS at -0.02% organic carbon per year across the Indo-Gangetic plains.
The Soil Test Crop Response (STCR) approach, pioneered by Indian Council of Agricultural Research — Indian Institute of Soil Science (ICAR-IISS, Bhopal) under the All India Coordinated Research Project on STCR, replaces the blanket with a site-specific equation:
FN = (NR × T − Cs × SN) ÷ Cf
where FN is the fertiliser N dose (kg/ha), T is the yield target (q/ha), SN is the soil-test available N (kg/ha), NR is the crop's nutrient requirement per quintal of grain, Cs is the soil-contribution coefficient and Cf is the fertiliser-recovery coefficient. The same form applies to P₂O₅ (FP) and K₂O (FK). Forty-one AICRP-STCR sub-stations across India have published calibrated coefficient sets for the major crop-soil combinations.
The four coefficients explained
For irrigated wheat on Indo-Gangetic alluvium, the calibrated coefficients (ICAR-IISS 2020) are NRN = 2.86, CsN = 0.51, CfN = 0.70. Reading them: every quintal of wheat extracts 2.86 kg of N from the system; the soil's own available-N supply contributes a useful fraction of 0.51 (meaning 51% of the soil-test N is actually plant-available); and fertiliser N is recovered by the crop at 70% efficiency. The remaining 30% of applied N is lost to volatilisation (urea broadcast on warm soil), denitrification (waterlogged paddy), leaching (light sandy soils), and immobilisation in microbial biomass. Improving Cf is the single biggest lever in Indian agronomy: split-dose urea (33% basal + 33% tiller + 33% boot), deep-placement urea briquettes, neem-coated urea, and band-application all push Cf from 0.70 to 0.85+.
For phosphorus the coefficients are NRP = 0.94, CsP = 1.66, CfP = 0.31. Cs above 1 means soil-test phosphorus over-predicts availability — Olsen-P is a strong extractant; band-applied DAP at sowing gets only ~31% recovered in the first season but residual P is taken up by the next crop. For potassium, NRK = 2.50, CsK = 0.18, CfK = 0.99 — high recovery (potassium is mobile, plants luxuriate K when available), but low soil contribution because exchangeable-K extractant is a poor predictor in low-K illite-vermiculite Indo-Gangetic clays.
Worked example — irrigated wheat in Meerut
Soil Health Card values: N = 210 kg/ha (low), P = 8 kg/ha (low), K = 200 kg/ha (medium). Target yield 45 q/ha (DBW 187 variety, irrigated). Plug into the calculator:
- FN = (2.86 × 45 − 0.51 × 210) ÷ 0.70 = (128.7 − 107.1) ÷ 0.70 = 31 kg/ha N shortfall — supply via urea.
- FP = (0.94 × 45 − 1.66 × 8) ÷ 0.31 = (42.3 − 13.3) ÷ 0.31 = 94 kg/ha P₂O₅ — supply via DAP/SSP.
- FK = (2.50 × 45 − 0.18 × 200) ÷ 0.99 = (112.5 − 36) ÷ 0.99 = 77 kg/ha K₂O — supply via MOP.
Notice the rebalancing vs blanket UP recommendation 120-60-40: STCR is reducing N by 76% and increasing P and K substantially. This is the typical correction for a low-P-low-N Meerut soil and explains why blanket dosing often leaves Indian wheat farmers nutrient-imbalanced.
From dose to bags — and to ₹
Once you have the kg/ha N-P-K, the NPK Bag Converter (linked below) tells you how many 50-kg bags of urea + DAP + MOP that translates to. For 31-94-77 kg/ha: solve P first with DAP (94 ÷ 0.46 / 50 ≈ 4.1 bags), which adds ~17 kg N (DAP is 18% N) and 0 K. The residual N (31 − 17 = 14 kg) needs ~0.6 bag of urea. K needs 77 ÷ 0.60 / 50 = 2.6 bags of MOP. Total subsidised-retail cost (April 2026 prices: urea ₹266, DAP ₹1,350, MOP ₹1,700 per 50 kg) is roughly ₹160 + ₹5,535 + ₹4,420 = ₹10,115 per hectare — much cheaper than the blanket dose for the same yield target because we're not over-buying urea.
Why STCR matters under PM-PRANAM
The PM-PRANAM scheme (Programme for Restoration, Awareness, Nourishment and Amelioration of Mother Earth) launched in 2023 rewards states for reducing chemical-fertiliser consumption while maintaining yields. The reward is 50% of the saved subsidy paid to the state, and 50% of that is recycled to incentivise alternate fertilisers (organic, bio, nano-urea). STCR-based dosing is the policy's preferred tool because it scientifically lowers urea overuse without yield loss. ICAR-IISS pilots in Andhra Pradesh, Karnataka and Madhya Pradesh in 2023-24 documented average urea reductions of 22 kg/ha (~18% saving on N) with no statistically significant yield change.
Equally, the Soil Health Card scheme (now in its 3.0 iteration) requires every Indian farmer to receive a soil test result every 3 years. The card prints N, P, K, OC, pH, EC plus secondary nutrients (S, Zn, B, Fe). Without the STCR equation, that data is decorative. With it, the SHC becomes the single most valuable document a farmer holds.
Adjustments and caveats
- Crop variety effect: NR varies by variety (KRL-210 wheat extracts less N than HD 3226). Use the calibrated coefficient set from your state KVK if available.
- Stubble retention / residue: when paddy straw is incorporated rather than burnt, the next wheat crop sees +15-25 kg N/ha from mineralisation — subtract from the dose.
- FYM / vermicompost: 1 tonne FYM ≈ 5 kg N + 2.5 kg P + 5 kg K. Use the FYM calculator to convert organic inputs into nutrient equivalents.
- Acid soils: at pH < 5.5, P-availability drops by 50%. Apply lime first (lime calculator) before relying on STCR.
- Sulphur / Zn / B: STCR equations published here cover N-P-K only. Sulphur 20 kg/ha for oilseeds, Zn 5 kg ZnSO₄/ha basal for paddy in low-Zn alluvium are recommended in addition.
Sources
ICAR-IISS Bhopal AICRP-STCR Handbook 2020 (Singh, Wanjari, Subba Rao); Department of Agriculture & Farmers Welfare Soil Health Card Operational Guidelines 3.0 (2024); PM-PRANAM Cabinet note 28 June 2023; ICAR-IISS Annual Report 2023-24; Department of Fertilisers Nutrient-Based Subsidy rates April 2026.