Sodic Soil with a Hardpan: Reading the Report Before Buying Gypsum
A farmer in Sangli district cultivates 1.2 ha of black cotton soil under cotton. A soil test reports pH 8.9, EC 0.31 dS/m, organic carbon 0.38%, free calcium carbonate 12%, CEC 42 cmol(+)/kg and exchangeable sodium percentage 18%. A hard murum layer lies at 45 cm, and the field stays waterlogged for 3–4 days after heavy rain. In patches the cotton turns yellow, with white flecks on the upper leaves, and growth stops. The input dealer advises 10 t/ha gypsum, twelve heavy irrigations to 'wash out the salt', and one season of paddy. The farmer shares a well that can give only four irrigations of 5 cm each this season, has no drainage channel, and cannot grow paddy. He must choose between cotton, soybean, sorghum and berseem for the next two seasons and has asked for a plan he can actually follow.
1. Read the soil test. Is the main constraint salinity, sodicity, lime-induced chlorosis or drainage? Support your answer with the reported values and explain the yellow patches with white flecks.
Read the worked answer
Reading the report
- EC 0.31 dS/m is well below the 1 dS/m threshold for saline soil. Salinity is not the problem, and the four available irrigations are chemically safe.
- ESP 18% is above the usual 15% threshold, with pH 8.9: the exchange complex carries excess sodium. This is a sodic (alkali) soil, not a saline one.
- Free CaCO3 12% keeps pH high and locks up iron and zinc. White flecks on the upper, younger leaves with patchy yellowing are typical of lime-induced Zn/Fe deficiency, not of a pest or virus.
- Murum at 45 cm plus 3–4 days of ponding means a perched water table. Waterlogged roots suffer oxygen shortage, which produces the same yellowing and stunting, and explains why the patches are irregular and worst in the lower part of the field.
So this is a combined problem: sodicity, high-lime high-pH chemistry, and impeded drainage. Heavy irrigation aimed only at 'salt' addresses none of the three, and without an outlet it would raise the perched water table further.
The two deficiencies are easy to confirm before spending money: spray 0.5% ZnSO4 plus 0.5% FeSO4 on a few yellow patches. Greening within 8–10 days confirms the diagnosis; no response means the drainage problem is dominant.
2. Critically evaluate the dealer's advice and give a same-season plan the farmer can afford.
Read the worked answer
Where the advice fails
- Twelve irrigations are impossible: the well gives only 4 × 5 cm = 20 cm, and there is no drainage outlet. Leaching without drainage only worsens the perched water table on the murum.
- Paddy is not feasible here (water, land, income) and is not necessary; gypsum plus drainage does the same job.
- The 10 t/ha figure is not derived from this report. The dose must come from ΔESP × CEC, not from the lime content.
Gypsum dose for this soil
Sodium to be replaced = (18 − 10)/100 × 42 ≈ 3.4 cmol(+)/kg, which is roughly 2.9 g gypsum per kg soil. For the top 15 cm (about 2.24 million kg soil/ha at bulk density 1.5) that is about 6.5 t/ha, i.e. roughly 0.8 t/ha per ESP unit. The full dose for 1.2 ha would be about 7.8 t; with the available cash and water, apply 4 t/ha (≈4.8 t total) this year, broadcast and incorporated in the top 10 cm before the monsoon, and repeat next year after retesting. Target ESP 10, not 5, because water is limited.
Same-season plan
- Open a drainage outlet and lay out broad beds with furrows along the slope, or chisel the murum layer.
- Use the monsoon itself as leaching water; do not buy twelve irrigations.
- Add 10 t/ha FYM or a dhaincha green manure for structure and organic carbon.
- Apply 25 kg/ha zinc sulphate to the soil and foliar-spray the yellow patches to confirm the deficiency.
- Do not grow cotton on the worst patches this year.
3. Which crop and sequence would you choose for the next two seasons, and what will you monitor to know the plan is working?
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Crop and sequence
- Kharif 1: dhaincha (Sesbania) green manure, not cotton. Dhaincha germinates and grows on sodic soil, its roots and the 10–15 t/ha of biomass improve infiltration, and incorporation raises organic carbon from the limiting 0.38%.
- Rabi 1: berseem or oats on 0.4 ha only, next to the well, using the scarce irrigation for fodder and keeping a legume in the rotation. Do not spread the limited water over all 1.2 ha.
- Kharif 2: sorghum (grain or fodder) on the main block, on broad beds. Sorghum tolerates sodicity better than soybean.
- Save cotton - higher value but more sensitive - for season 3, once ESP is below 12 and drainage works.
- Soybean is the trap: it is more sensitive to waterlogging and high pH than sorghum, and this field still ponds.
Monitoring that tells you it is working
- Retest pH, ESP and organic carbon every year at the same spots; a fall of 1–2 ESP units per year is realistic and enough.
- Measure infiltration: a 5 cm depth of water in a ring should disappear visibly faster than before.
- Record how many days the field ponds after a 50 mm rain; the target is under one day.
- Watch new leaves: if white flecks return after the zinc sprays, the high-lime problem is still active and the next foliar spray is due.
- Keep the drainage outlet open through the monsoon; a blocked outlet undoes a season of gypsum.