Liming acid soil for rabi maize: material choice, rate and placement
A young farmer in Goalpara plans to grow rabi maize on 0.8 ha of newly cleared upland. Soil test results are pH 4.8; organic carbon 0.48%; exchangeable Al 1.8 cmolc/kg; exchangeable Ca 1.1 and Mg 0.5 cmolc/kg; available P 7 kg/ha (Bray-1); available K 95 kg/ha. The district soil testing laboratory uses a lime requirement rule for maize of 1.2 t agricultural lime per cmolc exchangeable Al per hectare. He can buy dolomitic lime with a neutralizing value of 85% relative to pure calcium carbonate and 12% Mg at Rs 6/kg, or calcitic lime with 100% neutralizing value at Rs 5/kg. He also has di-ammonium phosphate (18-46-0) and muriate of potash. He wants to apply lime and DAP on the same day in the seed furrow to save labour, and asks whether this is acceptable.
1. Calculate the lime requirement for 0.8 ha in tonnes of each material. Recommend one material, considering both cost and the soil Mg status, and show your reasoning.
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Base requirement. Lime requirement = 1.2 t per cmolc Al × 1.8 cmolc/kg = 2.16 t/ha of agricultural lime equivalent. For 0.8 ha: 2.16 × 0.8 = 1.728 t.
Calcitic lime. At 100% neutralizing value, he needs 1.728 t. Cost = 1,728 kg × Rs 5/kg = Rs 8,640.
Dolomitic lime. At 85% neutralizing value, the field rate must be increased: 1.728 ÷ 0.85 = 2.033 t. Cost = 2,033 kg × Rs 6/kg = Rs 12,198. The extra cost is about Rs 3,558 for the 0.8 ha.
Recommendation. The exchangeable Mg is only 0.5 cmolc/kg, which is low, and maize is sensitive to Mg deficiency on strongly acid soils. Dolomitic lime corrects acidity and supplies Mg in one operation; at 12% Mg, 2.033 t supplies about 244 kg Mg over the field, which is more than the crop needs, but it removes the need for a separate Mg fertilizer and improves the Ca:Mg balance. If cash is very tight, calcitic lime plus a separate magnesium source is defensible, but the dolomitic option is the better agronomic choice here. The decision should not be based on cost per tonne alone, because the materials have different neutralizing values.
2. Explain why lime should not be placed in the seed furrow with DAP on the same day. Describe a correct application sequence and placement for lime, DAP and muriate of potash before sowing.
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Why not mix them. Lime raises pH sharply in a small volume of soil. If it is mixed with DAP in the seed furrow, the localized high pH and high calcium concentration can precipitate phosphate as calcium phosphates, reducing P availability to seedlings. The alkaline microsite can also increase ammonia volatilisation from the ammonium in DAP and create a salt-affected zone around the germinating seed. The result can be poor germination, root injury, and less efficient use of an expensive fertilizer.
Correct sequence. Apply lime first, broadcast uniformly, and incorporate it into the top 10-15 cm of soil at least 3-4 weeks before sowing. This allows time for the lime to react and for exchangeable Al to decrease. Sow only after the lime has been mixed through the root-zone soil.
Fertilizer placement. At sowing, place DAP in a band about 5 cm below and 5 cm to the side of the seed, not in direct contact with the seed. Apply muriate of potash as a band or broadcast and incorporate it before sowing; do not concentrate it with the seed. If DAP and lime must be applied in the same season, keep them separated in space and time. This preserves the P response and avoids seedling injury.
3. What would happen to phosphorus availability if the farmer skipped liming? Explain using aluminium chemistry at pH 4.8.
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At pH 4.8, aluminium is solubilised as Al3+ and hydroxy-Al species. These ions react strongly with phosphate, forming insoluble aluminium phosphates and adsorbed P complexes that are not readily available to maize roots. The measured Bray-1 P of 7 kg/ha is already low, so a large fraction of any DAP applied would be fixed in the acid soil rather than taken up. In addition, Al3+ itself damages root tips, reducing the root system's ability to explore the soil and acquire the little available P. Liming raises pH and precipitates Al as Al(OH)3, lowering exchangeable Al and releasing some fixed P. That is why the lime recommendation is based on exchangeable Al: the target is to remove Al toxicity, not merely to raise pH to a textbook value. Skipping lime would therefore waste part of the DAP and increase the risk of poor early maize growth. The lime should be applied well before sowing and incorporated, as described, rather than placed with the seed.