酸性土壤施石灰：施用量、施用时间与肥料混配
A farmer in Chhindwara has 1.2 ha of light-textured soil with pH 5.2 (1:2.5 soil–water), organic carbon 0.9%, exchangeable aluminium 2.4 cmol(p+)/kg and low available phosphorus. The rotation is soybean followed by wheat; both crops grow poorly and the soybean roots are stubby with few root hairs. A buffer-method test gives a lime requirement of 3.2 t/ha of pure calcium carbonate equivalent (CCE) to raise pH to 6.5. The local agricultural lime analyses 75% CCE on a dry basis and contains 20% moisture. The farmer intends to spread this lime and, in the same operation, band 150 kg DAP/ha and 100 kg single super phosphate/ha into the seed furrows, arguing that all three are powders and can therefore be mixed. He also plans a zinc sulphate spray for the interveinal chlorosis visible on the younger leaves.
1. Calculate the amount of lime product needed per hectare and for the whole field, after correcting for CCE and moisture content.
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The buffer test gives the requirement in terms of pure CCE, so the product must be corrected twice - for purity and for the water it carries.
Effective CCE per tonne of product = 0.75 (dry-basis purity) × 0.80 (dry matter, since moisture is 20%) = 0.60 t CCE per tonne of product.
Product per hectare = 3.2 ÷ 0.60 = 5.33 t/ha.
For 1.2 ha = 5.33 × 1.2 = 6.4 t of lime product.
A quick cross-check: 6.4 t of product × 0.60 = 3.84 t CCE for 1.2 ha, which is 3.2 t CCE/ha as demanded. If the farmer buys only 3.2 t/ha (the buffer figure) he will apply barely 60% of the required neutralising power and the pH will rise only to about 5.8, leaving aluminium still toxic. The lime should be from a freshly analysed lot, because CCE and moisture vary between batches.
2. Explain why mixing lime with DAP and SSP in the seed furrow is agronomically wrong, and give the correct timing and placement for each material.
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Three separate problems arise from the proposed mix.
- Loss of soluble phosphorus. Lime raises pH sharply in the immediate microsite; the calcium released and the phosphate from DAP/SSP react to form sparingly soluble calcium phosphates, so the P that was meant for the seedling is fixed before the root reaches it. SSP also contains free acid and gypsum, which react with lime and make the mixture lumpy and uneven.
- Ammonia loss. Both urea-based DAP and soil-applied nitrogen hydrolyse to ammonium; in the alkaline microsite created by lime, ammonium converts to ammonia gas and escapes. Nitrogen is lost before the crop can use it.
- Seed and seedling injury. Lime in the seed furrow creates a strongly alkaline, high-calcium, high-salt zone right against the germinating seed, reducing germination and damaging the radicle - exactly the opposite of what an acid-soil field needs.
Correct practice: broadcast the full 6.4 t of lime evenly and incorporate it to about 10 cm depth, at least 2–3 weeks before sowing, ideally after the previous harvest and before the pre-sowing irrigation. Band DAP and SSP in the seed furrow only at sowing, separate from the lime. The zinc sulphate spray is compatible with this plan and can go on as a foliar spray after the pH has begun to rise.
3. Predict how the availability of P, Mo, Zn, Mn and B changes as pH rises from 5.2 to 6.5, and explain why over-liming to pH 7.8 would be worse than staying at 6.5.
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From pH 5.2 to 6.5:
- Phosphorus - availability rises sharply. At pH 5.2 P is fixed by free aluminium and iron; as pH rises and exchangeable Al is neutralised, less fixation occurs and P solubility peaks around pH 6.5.
- Molybdenum - availability increases, because Mo is adsorbed more strongly as acidity increases; this matters for the nitrogenase of the soybean crop.
- Zinc, manganese and boron - availability falls, but at pH 6.5 these nutrients generally remain adequate. Manganese and zinc were probably near toxic at pH 5.2, so the fall is beneficial rather than harmful.
- Aluminium - exchangeable Al falls from 2.4 cmol(p+)/kg towards zero, which removes the stubbing and root-hair loss seen in soybean.
At pH 7.8: the balance reverses. Zinc, iron, manganese and boron become deficient, so the interveinal chlorosis the farmer already sees would worsen rather than improve. Phosphorus availability falls again as insoluble calcium phosphates form, and surface-applied urea loses ammonia because the soil surface is alkaline. Lime is therefore a corrective input with an optimum, not a tonic - apply the calculated dose to reach 6.5, verify with a fresh soil test after one season and stop there.