Sodic Field with Poor-Quality Tube Well Water
A farmer in Jaipur district has 0.8 ha of sodic land. Soil test (0-15 cm): pH 9.4 (1:2.5), EC 0.32 dS/m, organic carbon 0.29%, CEC 15 cmol(p+)/kg, ESP 28, Olsen P 8.5 kg/ha, available K 340 kg/ha, DTPA-Zn 0.34 mg/kg. The tube well water has EC 1.4 dS/m and RSC 3.6 meq/L. Last wheat crop yielded 1.4 t/ha with patchy emergence; plants were stunted, with white bands on middle leaves and poor tillering. He plans wheat again with 150 kg DAP and 100 kg urea, and has 1.5 t gypsum in the store. He asks whether to spread the gypsum on the whole field before sowing, sell it, or use it some other way, and whether the tube well can be used for irrigation. The sowing window closes in 12 days; 2 t FYM and green-manure seed are also available. His neighbour's field with similar soil but annual dhaincha incorporation yields about 3.0 t/ha.
1. Diagnose the field's constraints and explain the white bands and patchy emergence. Rank the constraints in the order they should be addressed, and state why DAP alone will not solve the problem.
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Diagnosis: pH 9.4 with ESP 28 and low EC is a sodic soil, not a saline one. Sodium has dispersed the clay, so the surface crusts, emergence is patchy and infiltration is poor. The tube well makes it worse: RSC 3.6 meq/L adds sodium carbonate/bicarbonate with every irrigation.
White bands on middle leaves are typical zinc deficiency induced by high pH, not a disease. DTPA-Zn 0.34 mg/kg is below the critical level of about 0.6 mg/kg. Olsen P 8.5 kg/ha is low; K 340 kg/ha is high, so no K is needed. Organic carbon 0.29% is very low.
Ranking: (1) Sodicity and irrigation-water RSC, because they control emergence, root growth and nutrient availability; (2) zinc deficiency; (3) low P; (4) low organic matter.
Why DAP alone fails: at pH 9.4, applied P is fixed as calcium/magnesium phosphates and its movement is negligible; dispersal also restricts root exploration, so the crop cannot reach the P. Until sodium is reduced and Zn corrected, extra DAP gives little response and wastes money.
2. Using GR (t/ha) = (ESP_initial - ESP_target)/100 x CEC x 3.9 for 0-15 cm soil, calculate the gypsum requirement to bring ESP to 10. Assume the stored gypsum is 70% pure. Compare this with the 1.5 t available and recommend how the material should be used, including amendment of the irrigation water.
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Gypsum requirement: (28 - 10)/100 x 15 x 3.9 = 10.5 t/ha. For 0.8 ha: 8.4 t of pure gypsum. At 70% purity: 12.0 t of store material. He has 1.5 t, only about 12.5% of the requirement.
Water amendment: 3 cm over 1 ha = 300,000 L; at 3.6 meq/L this is 1.08 million meq = 1,080 equivalents. At 86 g gypsum per equivalent, neutralising one 3 cm irrigation needs about 93 kg gypsum/ha, about 74 kg for 0.8 ha. Four such irrigations need roughly 300 kg.
Recommendation: Do not spread 1.5 t thinly over 0.8 ha and expect reclamation (that is only 1.9 t/ha). Instead:
- Divert about 300 kg to the irrigation water/field channel, applied with each irrigation, to counter RSC.
- Apply the remaining 1.2 t plus 2 t FYM on the worst-affected patch and incorporate it in the top 10 cm before sowing.
- Include dhaincha in the kharif and incorporate it at flowering to improve structure and infiltration.
- Use the tube well with gypsum amendment, not raw; avoid leaching with this water alone. Arrange canal or rain water for any leaching attempt and ensure drainage.
3. Given the limited amendment and 12 days to sowing, advise the crop and agronomic package. Should he grow wheat, and how should DAP, zinc and FYM be handled?
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With only 12.5% of the gypsum requirement, the crop must be one that tolerates sodicity, and inputs must be placed where roots can reach them.
Crop: Barley is more sodicity-tolerant than wheat and matures earlier. If wheat is preferred for food, choose a sodic-tolerant variety and accept a moderate yield. Do not expect 3 t/ha in the first year; the neighbour's 3.0 t/ha reflects years of dhaincha, not one dose of gypsum.
Package:
- Apply the available gypsum and 2 t FYM to the worst patch before sowing and mix into the top 10 cm; do not leave gypsum on the surface.
- Place 100-120 kg DAP/ha in the seed furrow rather than broadcasting, because P is immobile at pH 9.4.
- Apply 25 kg ZnSO4/ha at sowing and a 0.5% zinc sulphate foliar spray at tillering, since the white bands show established deficiency.
- Split N into two doses; skip K because soil K is high.
- Sow on the flat with a slight ridge-furrow system to reduce crusting, keep a higher seed rate, and give a light post-sowing irrigation.
- Plan dhaincha in kharif and repeat gypsum annually; reclamation is a multi-year programme, not a single operation.