Saline-Sodic Paddy Land and a Farmer's Plan to Grow Groundnut
A farmer in a coastal village of Cuddalore district has 0.8 ha of paddy land. Average yield fell from 5.2 t/ha to 3.1 t/ha over five seasons. Soil testing shows pH 8.7, ECe 5.2 dS/m, ESP 24%, cation exchange capacity 22 cmol/kg, organic carbon 0.45%, and a water table at 1.2 m. Canal water is available only from November to January. The farmer applied 25 t/ha of farmyard manure and extra urea, but the field still crusts after irrigation and seedlings show patchy yellowing. He now wants to grow groundnut in the rabi season because it is a short-duration cash crop. He has heard that gypsum is used for groundnut and plans to apply 400 kg/ha just before sowing.
1. Classify the soil constraint and explain why extra urea and farmyard manure did not restore yield. State whether groundnut is a suitable immediate choice.
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The soil is saline-sodic: ECe 5.2 dS/m is above the 4 dS/m salinity threshold, and ESP 24% is above the 15% sodicity threshold, with pH 8.7. The combination explains the crusting, poor infiltration and patchy yellowing.
Extra urea did not help because nitrogen was not the limiting factor. Urea adds salt and can temporarily raise osmotic stress; it does not replace sodium on the exchange complex. Farmyard manure at 25 t/ha is useful for structure and biological activity, but it cannot supply enough calcium to displace sodium at ESP 24%. Without drainage and leaching, the sodium remains.
Groundnut is not a good immediate choice. It is moderately salt-sensitive, with a threshold near ECe 3.2 dS/m; at 5.2 dS/m it can lose more than half its potential yield. Sodic soil also crusts, restricts pegging and pod filling, and has poor aeration. The 400 kg/ha gypsum is a calcium nutrition dose for groundnut, not a reclamation dose. Groundnut can follow reclamation, but the first crop should be rice, which tolerates standing water and helps leach salts.
2. Using the gypsum requirement formula, estimate the gypsum needed to lower ESP to about 10%, and outline a safe reclamation sequence for the 0.8 ha. Address the water constraint.
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Use: GR (t/ha) = (ESP initial - ESP target)/100 × CEC × 1.72.
GR = (24 - 10)/100 × 22 × 1.72 = 0.14 × 22 × 1.72 = 5.3 t/ha of pure gypsum. Field-grade gypsum is less efficient; at 80% efficiency the dose is about 6.6 t/ha. For 0.8 ha that is about 5.3 t at field efficiency.
Sequence:
- Level the field and open shallow field drains, because gypsum will not work without somewhere for the sodium salts to go.
- Apply gypsum in two splits, incorporate into the top 10 cm, and avoid direct contact with seed.
- Pond with good-quality canal water and leach. A clay soil may need several pondings; even 0.15 m per ponding over 0.8 ha is 1,200 m³, so five pondings would need about 6,000 m³.
- Retest ECe and ESP before sowing.
- Grow rice first, then groundnut with a normal 400 kg/ha gypsum application in the furrow.
The water constraint is decisive. Canal water is available only November-January, so the farmer should reclaim 0.25 ha first, near the channel, rather than spread 5 t of gypsum over 0.8 ha without enough leaching water.
3. The farmer wants to reclaim only 0.25 ha this year because gypsum is costly. Evaluate this partial approach.
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Partial reclamation is technically and economically defensible, provided it is done properly. The gypsum need for 0.25 ha at 6.6 t/ha is about 1.65 t, roughly one-third of the full-field requirement, and the water and drainage effort also falls to one-third. That concentrates limited water on a block where leaching can actually succeed.
But the farmer must understand the limits. The untreated 0.55 ha will continue to crust and yield poorly, and salts from the untreated area can move back into the reclaimed block if drainage is poor. The reclaimed strip should therefore be separated by a small bund and drain, not just treated in isolation.
Economically, the yield gain on 0.25 ha may not repay the gypsum in one season. If paddy rises from 3.1 to 5.0 t/ha on 0.25 ha, the extra 0.475 t is worth roughly ₹1,100 at ₹2,300/t, while 1.65 t of gypsum may cost ₹4,000-5,000. This is a multi-season investment, so the farmer should judge it over three to four seasons and use rice, not groundnut, in the first year.
The partial approach is better than applying a thin dose over the whole field, which would neither reclaim nor pay.