Liming an acid sandy loam: rate, timing and the phosphate clash
Ramesh farms 2.0 ha of a sandy loam (clay 12%, organic carbon 0.38%) in a semi-arid block. A composite 0-15 cm sample analysed last month gave pH 5.2 (1:2.5 in water), EC 0.11 dS/m, CEC 9.6 cmol(p+)/kg, exchangeable Al 2.8 cmol(p+)/kg, base saturation 42%, available N 180 kg/ha, Olsen P 9 kg/ha and available K 145 kg/ha. Groundnut in kharif nodulates poorly (3-5 small, white nodules per plant) and yields about 1.1 t/ha; wheat after it yields 2.4 t/ha. He has arranged 4 t of agricultural limestone (95% CaCO3 equivalent, 60% passing 0.25 mm), 130 kg DAP and 275 kg urea, and intends to broadcast the lime and DAP together on 5 June, incorporate both with a single harrow pass, and sow groundnut on 8 June. Monsoon onset is expected around 20 June. The neighbouring field was limed to pH 7.4 three years ago and now shows interveinal white striping on young wheat leaves.
1. Is 4 t/ha of this limestone a sound rate for this field, and should the whole amount go on in one application? Show your reasoning.
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Work from exchangeable Al, not pH alone.
- Al-based requirement. For a 15 cm plough layer (about 2.2 x 10^6 kg soil/ha), 1 cmol(p+)/kg of exchangeable Al needs roughly 1 t/ha of pure CaCO3. With a 1.5 safety factor: 2.8 x 1.5 = 4.2 cmol/kg, i.e. about 4.2 t/ha of pure CaCO3.
- Material correction. 4.2/0.95 = about 4.4 t/ha of this limestone; and since only 60% passes 0.25 mm, perhaps 65-70% reacts in the first season - the rest is a reserve.
- Buffer check. CEC 9.6 cmol/kg and 12% clay mean the soil has little buffering. A single 4 t/ha dose can push patches to pH 7+, which will lock up Mn, Zn and B and reduce P availability.
- Crop target. Groundnut performs best at pH 6.0-6.5; there is no benefit above 6.5.
Verdict: the total requirement of about 4-4.5 t/ha is defensible, but the dose must be split. Apply 2 t/ha now, incorporate 10-15 cm, use the pre-monsoon showers, re-sample before the next season and apply the balance (about 2-2.5 t/ha) before wheat if pH is still below 6.3.
2. He proposes to broadcast the lime and the DAP together and incorporate both at once. What is chemically wrong with that, and what sequence would you prescribe?
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Three separate problems, all avoidable:
- Phosphate precipitation. As limestone dissolves it raises pH and Ca2+ in the same soil volume as the DAP granule. H2PO4- released by DAP is converted to dicalcium phosphate and ultimately to carbonate-apatite; the water-soluble P fraction - the part that actually feeds the crop - can collapse. Simultaneously, the lime's neutralising power is consumed by the acidity of DAP, so the limestone does half its intended job.
- Ammonia injury. Above pH 7.5, NH4+ from DAP shifts to NH3. Groundnut germinating in a band of DAP plus fresh lime is very sensitive to ammonia and salt burn; germ and seedling loss is likely.
- Timing. Limestone needs 3-4 weeks and adequate moisture to react with soil acidity. Eight days with no rain is not enough.
Prescription: broadcast the 2 t/ha of lime now, incorporate to 10-15 cm and let the pre-monsoon showers react it. At sowing, drill DAP in bands 5 cm below and 3-5 cm to the side of the seed, never mixed with lime and not in direct seed contact. Sow when the monsoon is established, not into dry soil.
3. How should the neighbour's striping at pH 7.4 change the lime recommendation for Ramesh?
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Interveinal white/yellow striping on the youngest wheat leaves after liming is the classic signature of lime-induced Mn deficiency, sometimes with Zn or B. The chemistry is straightforward: each unit rise in pH lowers Mn2+ activity roughly a hundredfold as Mn2+ oxidises to insoluble MnO2; Fe, Zn, B and Cu behave similarly. The neighbour's field is a demonstration plot of over-liming, not a coincidence.
Implications for Ramesh:
- Cap the target at pH 6.3-6.5, not 7.0+. Groundnut and wheat both perform well there, and micronutrients stay available.
- Test before the second dose. Get DTPA-Mn, DTPA-Zn and DTPA-Fe along with pH; on this sandy, low-organic-carbon soil the micronutrient reserve is small.
- Keep a rescue option. If Mn deficiency appears in wheat, foliar MnSO4 at 0.5% (with 0.1% citric acid to acidify the spray water) gives quick correction.
- Never lime and band phosphate together, as above.
The neighbour's failure is the strongest argument for split liming with a mid-course soil test rather than a single 4 t/ha dose.