Reading a soil test report with high pH
Balwinder Singh has a soil test report from the district soil testing laboratory for his 1.6 hectare field: pH 8.8, electrical conductivity 0.35 dS/m, organic carbon 0.30 percent, available nitrogen low (105 kg/ha), available phosphorus medium (16 kg/ha Olsen P) and available potassium high (320 kg/ha). He grows a paddy-wheat rotation and irrigates from a tube well. For the coming wheat crop he has booked, per acre, two bags of DAP, three bags of urea and one bag of MOP, all to be applied at sowing in one go. A neighbour has offered 40 kg of gypsum free of cost, saying it will fix the soil, but Balwinder fears gypsum will burn the wheat. He keeps no cattle and has no farmyard manure. His main complaint is that yields have stopped rising even though he applies more fertilizer every year.
1. Explain what each figure in the report means for the field, and answer his complaint about yields not rising.
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pH 8.8 with EC only 0.35 dS/m: this is a sodic (alkaline) soil, not a saline one. Excess sodium, not salt, is the problem; the soil tends to crust, disperse and take water slowly.
Organic carbon 0.30 percent: low (below about 0.5 percent is poor). This is the real constraint - a soil with this little organic matter holds little water and nutrients.
Nitrogen low (105 kg/ha): genuine deficiency.
Phosphorus medium (16 kg/ha): the crop responds to phosphorus, but only to the recommended amount.
Potassium high (320 kg/ha): no potash is needed at all for wheat.
His complaint: yields plateau because he is not short of fertilizer - he is short of organic matter and he has pH locking up zinc and other micronutrients. Adding more P and K to a soil already medium in P and high in K does not raise yield. At high pH, zinc deficiency is common, so get zinc tested or use zinc sulphate on the recommendation.
2. Recalculate his per-acre fertilizer plan for irrigated wheat and show what he wastes under his present booking.
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Wheat in Punjab needs roughly 50 kg N, 25 kg P2O5 and 12 kg K2O per acre.
Potash: soil K is high, so the MOP bag is entirely wasted - drop it.
Phosphorus: 25 kg P2O5 per acre is right for medium P. One 50 kg bag of DAP (18-46-0) supplies about 9 kg N and 23 kg P2O5 - almost the full P requirement. His two bags give about 46 kg P2O5, nearly double, and the excess buys no yield. Reduce DAP to one bag.
Nitrogen: the one bag of DAP gives about 9 kg N; the remaining 41 kg N comes from about 91 kg urea, that is two bags, not three.
So: per acre, one bag DAP plus two bags urea, and no MOP.
Timing: apply DAP and one-third of the urea at sowing, one-third with the first irrigation and one-third with the second. A single blanket dose at sowing loses nitrogen and feeds weeds.
Savings: roughly one bag of DAP and one bag of MOP per acre released, which can be spent on zinc sulphate or green manure instead.
3. Should Balwinder accept the free 40 kg of gypsum, and what should he do about the sodicity in the long run?
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On the free gypsum: gypsum is the correct amendment for a sodic soil, but 40 kg spread over 1.6 hectares will change nothing measurable. The dose must come from the laboratory's gypsum requirement, calculated from the exchangeable sodium percentage - usually quintals per hectare, not kilograms. So the gift is harmless but pointless; do not draw conclusions from it.
Will it burn wheat? No, if it is spread, incorporated and watered well before sowing; the fear is misplaced, the real risk is applying too little.
The bigger lever: organic carbon at 0.30 percent is what limits yield. Grow a green-manure crop such as dhaincha or sunhemp in the summer window and incorporate it before paddy, retain paddy straw rather than burning it, and add any compost available. This costs him nothing but time.
Also: get the tube-well water tested for residual sodium carbonate. If the water itself is sodic, every irrigation adds to the problem and no amount of fertilizer will fix it. Apply zinc sulphate where the soil test shows zinc deficiency.
Sequence: correct the sodicity with the full gypsum dose, build organic matter every year, and stop over-buying P and K.