Reading a Soil Test Report on a Sloping Laterite Field
A 1.2 ha upland plot in the Eastern Ghats, at about 900 m elevation, has grown upland paddy for eleven years. The land slopes 7–9% towards a seasonal stream. This summer's soil test reports pH 5.2, EC 0.04 dS/m, organic carbon 0.34%, available N 148 kg/ha, available P 8.5 kg/ha, available K 165 kg/ha and exchangeable aluminium 2.4 cmol(p+)/kg. The farmer applies 100 kg urea and 50 kg DAP per hectare, burns the crop residue every April, and has never limed. Last monsoon a single 65 mm storm in 45 minutes cut a 30 cm deep rill across the lower 0.3 ha and, by his own estimate, carried off 4 tonnes of topsoil. A fertiliser trader has since sold him two bags of gypsum, saying it "cures acidity and stops erosion". The farmer now plans to broadcast the gypsum with extra urea a week before sowing.
1. Evaluate the farmer's plan to apply gypsum plus extra urea. State what should be applied instead and show how the lime rate is arrived at.
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Gypsum is calcium sulphate. It supplies Ca and S and can reduce subsoil aluminium in some profiles, but as a sulphate it has no carbonate to neutralise H+ ions, so it will not raise pH 5.2. It also does nothing for infiltration on this slope. The plan therefore treats a symptom the farmer does not have while ignoring the binding constraint, exchangeable Al at 2.4 cmol(p+)/kg.
Lime is the correct amendment. For mineral soils a working estimate is lime requirement ≈ 1.5 × exchangeable Al (cmol/kg) ≈ 3.6 t/ha of agricultural lime, i.e. about 4.3 t for 1.2 ha, aiming at pH 6.0–6.2. This figure should be confirmed by a buffer-pH (SMP) test and adjusted for texture and target crop; split application over two seasons is acceptable if cash is short. Apply 3–4 weeks before sowing and incorporate. Do not broadcast urea on the same day as lime; incorporate and separate the two by at least a week.
Extra urea without liming is largely wasted: nitrification slows at pH 5.2, and the DAP supplies only about 23 kg P2O5/ha into a soil with 8.5 kg/ha available P where Fe and Al oxides fix phosphate. Raise pH first, then band P.
2. Identify the erosion process and justify the order in which conservation measures should be taken on this 1.2 ha.
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A 30 cm incision in one storm is rill erosion already deepening towards a gully; once the sides slump it becomes permanent and the 0.3 ha is effectively lost. Sheet and splash loss on the bare April–May surface, after the residue burn, are feeding it.
Order of work follows ridge-to-valley logic, not the farmer's instinct to patch the visible rill. First, stop the residue burn and retain at least 30% ground cover; that alone cuts splash detachment. Second, lay contour or graded bunds across the upper 0.9 ha to shorten slope length and cut the velocity of water arriving at the lower strip: slope length, not area, drives the loss. Third, put a vegetative barrier of vetiver or grass on the contour every 15–20 m. Fourth, plug the rill with small brushwood or earthen checks seeded with grass, and lead the overflow into a grassed waterway. Treating the rill alone without the upslope works simply repeats the damage next June.
3. The farmer argues that the rill appeared in one freak storm, so the problem is the rain and not his field. Respond to this argument.
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One intense storm can trigger the visible failure, but it only succeeds where the field has been made vulnerable. 65 mm in 45 minutes is a high-intensity event, yet the runoff it generated was concentrated because slope length had never been broken, ground cover was burnt off, and no bund existed above the rill.
Conservation structures should not be designed for a single observed storm, nor dismissed because the storm looks unusual. Design storms use a chosen return period - commonly a 10-year, 24-hour rainfall for field bunds in this zone - and the block's rainfall records should supply that figure. If a 65 mm burst closes the gap, a 10-year design will be larger still, not smaller.
The correct response is to explain that the event exposed a weakness that was already there, and that the same field without cover and without bunds will fail again in an ordinary season, only more slowly. Show him the depth of the rill against the depth of topsoil: 4 tonnes of lost soil from 0.3 ha is the nutrient-rich layer, not subsoil.