Lateritic groundnut plot: lime, DAP and a 4% slope
A farmer in the red-lateritic belt of Bankura owns 1.2 ha of sandy loam with pH 5.3 (1:2.5 soil:water), organic carbon 0.38%, low available nitrogen, 18 kg/ha available P2O5 and CEC 6.2 cmol(p+)/kg. The field slopes about 4% towards a seasonal stream and shows fresh rills after every heavy shower. He plans groundnut in pre-kharif and has 500 kg agricultural lime, 150 kg DAP, 100 kg urea and 150 kg MOP in stock. Because labour is available only once, he wants to broadcast all the lime and mix the DAP into the same operation two days before sowing. Sowing is fixed for the third week of June, ahead of the monsoon. A neighbour who limed at 2 t/ha last year and grew groundnut harvested 2.3 t/ha against the farmer's usual 1.4 t/ha. The farmer wants a decision before he books the tractor and buys more lime.
1. Which constraint should be treated first on this field - acidity or phosphorus - and how much weight can you place on the neighbour's 2.3 t/ha result?
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Acidity first.
- At pH 5.3 on a soil with CEC only 6.2 cmol(p+)/kg, exchangeable Al and Mn dominate the exchange complex; groundnut rhizobia nodulate poorly below pH 5.5, Mo uptake falls, and P applied as DAP is rapidly fixed by Fe/Al oxides.
- Bray-1 P of 18 kg/ha is medium, not critically low, so P is secondary to the pH/calcium problem.
- Exchangeable Ca is low, which matters twice for groundnut: for peg penetration and for pod filling, where Ca must be absorbed directly by the pod.
Evidence weight: the neighbour's figures are a single unreplicated plot in a different season, so they are a signal, not proof. The 64% jump is too large to be pure lime effect; cultivar, gypsum use and rainfall may also differ. Treat it as justification for a lime trial strip, not as a guaranteed response.
Priority action: correct pH towards 6.0–6.5 before or at sowing, and plan gypsum at pegging (about 400 kg/ha) as the direct Ca source for pod filling, since lime applied to soil does not reliably reach the pegging zone.
2. Explain why broadcasting lime and mixing DAP in the same operation is agronomically wrong, and give a corrected plan using the materials he has.
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Why the plan fails
- Lime needs 3–4 weeks and adequate moisture to neutralise acidity; applied 2 days before sowing it does almost nothing for this crop.
- Mixing lime with DAP puts a pH spike and free Ca directly around the P granule; water-soluble P is converted to less available Ca/Fe phosphates, and ammonia loss from DAP rises at high pH.
- The rate is far too small: 500 kg over 1.2 ha = 417 kg/ha, only about one-fifth of the 2 t/ha his neighbour used.
Corrected plan
- Either buy about 1.9 t more lime to treat the whole 1.2 ha at 2 t/ha, or keep the 500 kg and treat a 0.25 ha block at the full rate, so at least one block gives a readable response.
- Broadcast lime 3–4 weeks before sowing and incorporate to 10 cm; do not mix with fertiliser.
- At sowing, place DAP (150 kg = 27 kg N + 69 kg P2O5, i.e. ~57 kg P2O5/ha) in bands 3–5 cm below and beside the seed.
- Split urea; apply MOP basal and again at pegging; add gypsum at flowering.
- Re-test pH after harvest and watch for Zn/B deficiency if pH overshoots 6.5.
3. Recommend one in-field soil conservation measure for the 4% slope, and state how you would judge within one season whether it has worked.
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Recommendation: contour bunding at a vertical interval of about 1 m (roughly 25 m horizontal spacing at 4%), with a grassed waterway to carry the safe discharge to the stream, and live vetiver or lemon-grass barriers on the bunds. This is preferred over bench terracing, which is uneconomic and unnecessary at 4% on a 1.2 ha plot, and over simple vegetative barriers alone, which will not stop rill initiation in the first storms.
Season-level check
- Compare rill depth and number in the treated field against the untreated lower strip after each major storm (a 30 cm graduated rod is enough).
- Measure sediment trapped behind the bunds and in the waterway outlet.
- Sink two simple runoff stakes or install small collection drums to compare soil loss upslope and downslope.
- Record topsoil depth in the deposition zone and check whether the lower part of the field stops sanding over.
A measure that fails shows deepening rills, breaching at the bund ends, or sediment fanning into the waterway within the first monsoon.