Liming and Fertilizer Decisions on an Acidic Nagaon Plot
A farmer in Nagaon district, Assam, cultivates 0.4 ha and has a soil test report from the district soil testing laboratory: pH 4.9, organic carbon 0.48%, available N 210 kg/ha (low), available P2O5 9 kg/ha (low), available K2O 110 kg/ha (medium), CEC 8.0 cmol(p+)/kg and exchangeable Al 1.8 cmol(p+)/kg. The farmer plans boro rice followed immediately by greengram on the same land and has already purchased 150 kg urea, 100 kg SSP and 30 kg MOP. A local dealer has 200 kg of dolomitic lime in stock and advises applying it all with the basal fertilizer at rice transplanting. The farmer cannot afford a full lime requirement test and asks whether the available lime will solve the acidity problem, when and how to apply it, and how to adjust the purchased fertilizers. There is no facility for soil incubation and the greengram sowing window closes about 20 days after rice harvest.
1. Interpret the soil test and advise how the 200 kg of lime should be allocated between boro rice and greengram. Justify your advice using soil chemistry and crop sensitivity.
Read the worked answer
The pH of 4.9 is strongly acidic, and exchangeable Al of 1.8 cmol(p+)/kg is high. Approximate Al saturation is 1.8/8.0 × 100 = 22.5%, which is above the level at which many crops suffer root damage. Available P is low partly because Al and Fe fix phosphate at this pH. Rice is relatively tolerant of acidity when flooded, whereas greengram is an upland, Al-sensitive legume whose rhizobia and molybdenum uptake are also depressed in acid soil.
The 200 kg stock spread over 0.4 ha is only 500 kg/ha, well below the 1–2 t/ha typically needed to correct such acidity. It cannot 'solve' the problem, so it should be used where the return is greatest: on the greengram. Apply it after rice harvest on residual moisture, incorporate it into the top 10 cm about two weeks before sowing, and do not mix it with SSP on the same day. For the rice, rely on tolerant varieties, organic matter and careful water management rather than a token lime dose.
2. Construct a fertilizer plan for the two crops using the purchased urea, SSP and MOP, and state whether extra P or S is needed.
Read the worked answer
Use recommended per-hectare doses scaled to 0.4 ha.
Boro rice, assuming 60:20:20 kg N:P2O5:K2O per ha:
- N = 24 kg → urea 24/0.46 = 52 kg
- P2O5 = 8 kg → SSP 8/0.16 = 50 kg
- K2O = 8 kg → MOP 8/0.60 = 13 kg
Greengram, assuming 20:40:20 kg per ha:
- N = 8 kg → urea about 17 kg (reduce if nodules are active)
- P2O5 = 16 kg → SSP 100 kg
- K2O = 8 kg → MOP 13 kg
The purchased 100 kg SSP is just enough for the rice, so another 100 kg SSP must be bought for the greengram. SSP is preferable to DAP here because it supplies sulphur (about 11 kg S per 100 kg) and does not add unnecessary N. Do not apply lime and SSP together; separate them by at least 10–14 days. Keep the remaining urea for top-dressing rice, and apply the greengram P in the seed furrow rather than broadcasting.
3. Explain the risk if the farmer follows the dealer's advice of applying all the lime with basal fertilizer at rice transplanting.
Read the worked answer
Three problems arise. First, mixing lime with SSP in the same basal application can precipitate water-soluble phosphate as calcium phosphate, reducing the very P the soil already lacks. Second, lime incorporated into puddled, flooded rice soil is much less effective than in an upland soil because reduced conditions and waterlogging buffer pH changes; the farmer would spend the material where it does least good. Third, if lime and urea are applied together, the local pH rise around the fertilizer granule can increase ammonia volatilization, especially with surface application.
The greengram would then receive no benefit at all, although it is the more acid-sensitive crop. A further risk is false confidence: 500 kg/ha is a partial correction, not a cure, so the farmer may stop using organic matter, tolerant varieties or furrow placement of P. The correct advice is to keep lime separate from P fertilizer, apply it to the upland greengram before sowing, and treat the 200 kg as a starter amendment rather than a complete reclamation.