Allocating a Limited Gypsum Stock in a Sodic Canal-Command Field
A farmer in the Indira Gandhi Canal command holds 3 ha of sandy loam. Soil test: pH 9.4, ECe 1.9 dS/m, ESP 32, CEC 16 cmol/kg, organic carbon 0.21%, infiltration 0.4 cm/h. The laboratory's gypsum requirement is 9 t/ha for the 0–15 cm layer. He has 10 t of gypsum and three hectares. Canal water has RSC 4.2 meq/L; the tube well water has EC 4.8 dS/m. His plan is to spread the entire 10 t uniformly over 3 ha before kharif, grow rice in basins, then wheat. The field is uneven, has no field drains, and a 0.4 ha patch shows pH 10.1 with a white salt crust. He asks whether to buy 17 t more gypsum or to use the 10 t differently. His cash limit is ₹25,000, and gypsum delivered costs ₹1,100/t.
1. Allocate the 10 t of gypsum. Show the calculation and explain why uniform application is or is not defensible.
Read the worked answer
Uniform spreading gives 10 t ÷ 3 ha = 3.3 t/ha, against a laboratory requirement of 9 t/ha. That is only about one-third of the dose; it will lower ESP slightly but will not create a clearly reclaimed block, and the farmer will have spent his entire cash limit with little visible response.
Allocate 9 t to 1 ha and keep 1 t in reserve: 9 t/ha × 1 ha = 9 t. Choose the hectare that is most responsive and where drainage can be arranged, not simply the worst 0.4 ha patch. The worst patch has pH 10.1 and a salt crust; without drainage, gypsum plus ponding there could raise the water table and concentrate salts.
Do not buy 17 t more now. First level and bund the selected hectare, incorporate the gypsum into the top 15 cm, and irrigate. Buy the next lot only after the first block shows a fall in ESP and a better crop. This is a cash-flow and risk decision as much as an agronomic one.
2. Set out the sequence of operations and crop choices for the first two years. Include the role of rice, dhaincha and canal versus tube-well water.
Read the worked answer
Sequence: (1) level and bund the selected 1 ha and cut a drainage outlet; (2) apply 9 t gypsum evenly, incorporate it in the top 15 cm, and irrigate to dissolve it; (3) pond with canal water and grow rice in basins; (4) after rice, grow barley or wheat with canal water; (5) grow dhaincha in the summer and incorporate it as green manure.
Rice is useful because it tolerates standing water and the ponding helps displace sodium, but it is not a cure by itself. It must be followed by a tolerant crop and organic matter. Dhaincha adds organic matter, improves infiltration and, if nodulating, adds nitrogen. Use canal water for leaching; the tube well at EC 4.8 dS/m is too salty for the leaching step and should be used only as a supplemental irrigation after the ESP has fallen. Test ESP after the first season. Expand the treated area only when the first block shows a response.
3. What will you do about the 0.4 ha patch with pH 10.1 and white crust? Explain the risks of treating it like the rest of the field.
Read the worked answer
Treat the 0.4 ha patch separately. The white crust indicates salts at the surface, and pH 10.1 indicates strong sodicity; the patch may also have free lime. Take a separate sample for pH, ECe, ESP and lime. First provide shallow drains and leach with canal water; otherwise gypsum will release sodium that has nowhere to go. Then apply gypsum at the full 9 t/ha rate to that patch, incorporate it and grow a tolerant crop such as barley or cotton, not a sensitive crop.
Do not include this patch in the first 1 ha allocation unless the drainage is in place. It is the highest-risk part of the field and the most likely to fail. Treating it like the rest of the field would waste gypsum and could worsen the salt crust.