A sodic, saline-irrigated mustard field that wilts while wet
A farmer near Hisar, Haryana, grows Indian mustard on 4 hectares irrigated only from a tube well of EC 7.8 dS/m with high residual sodium carbonate. Soil pH is 9.1 and exchangeable sodium percentage is 28%. In December the crop is stunted and uneven. Older leaves show marginal scorch; young leaves are interveinally pale. The field looks drought-stressed at midday even though the top 15 cm is at field capacity after a canal irrigation; EC of the 0–30 cm layer is currently only 2 dS/m because of that fresh irrigation. Soil tests show low available N, Olsen P 9 kg/ha, and DTPA-extractable Fe 2.1 ppm against a critical level of 4.5. The farmer proposes broadcasting 25 kg/ha extra urea and spraying a chelated micronutrient mixture. His neighbour's field, 800 m away on the same soil series but gypsum-treated three years ago, is dark green and about 30 cm taller.
1. The soil is at field capacity, yet the plants behave as if droughted. Separate the osmotic, ionic and nutritional components of the problem and tie each to a specific symptom.
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Three distinct mechanisms are operating, and the symptoms localise them.
Osmotic (whole plant, midday). An EC of 7.8 dS/m in the irrigation water corresponds to a solute potential of roughly −0.28 MPa (EC × ≈0.036). Roots must lower their water potential below this to absorb water; when transpirational demand peaks at midday, turgor cannot be maintained and stomata close. Photosynthesis falls, growth is stunted, and the plant recovers overnight - the classic "wet soil, drought appearance" signature. The temporarily low EC of the topsoil after canal irrigation explains why the plants look better for a few days after each canal turn.
Ionic (older leaves). Sodium is phloem-immobile, so it accumulates in transpiring older leaves and causes marginal scorch and necrosis. An ESP of 28% also disperses clay, sealing the surface and restricting root penetration and aeration.
Nutritional (young leaves). At pH 9.1 and with bicarbonate-rich water, Fe, Zn and Mn are insoluble; iron is not retranslocated, so young leaves show interveinal chlorosis. The DTPA-Fe of 2.1 ppm confirms deficiency. N and P are separately low.
Distinguishing old-leaf marginal burn from young-leaf interveinal pallor is the fastest field-level diagnostic step.
2. Evaluate the plan of extra urea plus a foliar micronutrient spray. What would you do instead, and in what order?
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Extra urea is the wrong move. Urea hydrolysis releases ammonium and locally raises pH, worsening ammonia volatilisation and iron availability; the added salt increases osmotic load. Nitrogen is limiting, but the form, placement and timing matter more than the amount. Broadcast urea on a sodic surface is the least efficient option.
Foliar chelate is a rescue, not a cure. It will green up the young leaves within a week and can recover some yield, but it does nothing about sodium, the osmotic barrier or the structure.
Priority order
- Gypsum at a rate calculated from ESP (of the order of 2.5–5 t/ha for ESP 28%, based on soil CEC and the 15 cm plough layer), incorporated, followed by leaching with low-EC canal water and assured drainage. This is the only measure that addresses sodium at the exchange complex.
- Reduce the salt load: blend or alternate water sources, irrigate in greater depths less often to push salt below the root zone, and use raised beds or ridge planting so the seed zone is less saline.
- Band a modest N and P dose rather than broadcasting; consider a nitrification-friendly, acidifying source such as ammonium sulphate where sulphur is also needed.
- Foliar Fe-EDDHA (and Zn) as a short-term correction.
- Monitor ECe, ESP, leaf Na and tissue Fe; if improvement is slow, shift to a more tolerant crop in the rotation rather than chasing yield with fertilizer.
3. Explain, in chemical terms, why soil-applied FeSO4 fails at pH 9 while Fe-EDDHA works.
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Why FeSO4 fails. Ferrous sulphate oxidises rapidly to Fe(III), which at pH 9 precipitates as sparingly soluble oxyhydroxides; ferric iron solubility falls by roughly three orders of magnitude for each unit rise in pH above neutral. The applied iron therefore immobilises within a centimetre or two of the granule and never reaches the root surface. Bicarbonate in the irrigation water compounds this by inactivating iron in the leaf apoplast after uptake.
Why EDDHA works. The ortho-ortho isomer of Fe-EDDHA forms a hexadentate chelate with Fe(III) that remains stable and soluble up to about pH 9–10, keeping iron in a form roots can take up. EDTA and citrate chelates, by contrast, lose iron above pH 7–8 and are ineffective here.
Caveats. EDDHA is expensive and mobile in soil water, so it is best banded rather than broadcast; foliar Fe-EDDHA bypasses the soil entirely for rapid correction of chlorosis. Neither product fixes the underlying problem: acidification (elemental sulphur), gypsum and organic matter are needed to lower pH and displace sodium before iron chemistry can normalise.