चुना गरज हिशेब

India's 36 million hectares of acid soil

ICAR-NBSS&LUP Nagpur mapped India's acid soils in 2022: 36 million hectares — about 25% of net sown area — have pH below 5.5. Strongly acid soils (pH < 4.5) cover 12 million ha and span Kerala, Karnataka coastal districts, Goa, the Konkan, the Western Ghats, Assam, Meghalaya, Manipur, Mizoram, Nagaland, Arunachal Pradesh, the Dooars belt of West Bengal, eastern Bihar, Jharkhand, Chhattisgarh, parts of Odisha and Andhra Pradesh's Visakhapatnam hills. The dominant causes are leaching of bases in high-rainfall regimes (> 1,500 mm/year), continuous N fertilisation that lowers pH through nitrification, and laterisation in tropical-humid landforms.

Acid soils suppress crop yield by three mechanisms: aluminium toxicity (Al³⁺ at pH < 5 damages root tips), manganese toxicity (Mn²⁺ above 100 ppm chlorosis), and P-immobilisation (Fe-Al phosphates fix the P your DAP supplies). Wheat yield drops 20% below pH 5.5; soybean drops 30%; rajmah (kidney bean) is nearly unviable below pH 5.0 without correction.

The lime-requirement formula and what it captures

The classical lime-requirement (LR) equation used by ICAR-CSSRI Karnal and adopted by state KVKs is:

LR (t/ha) = Exchangeable Acidity (cmol/kg) × 1.5 × Bulk Density × Depth × 0.1

The factor 1.5 is an empirical buffering coefficient — soil acidity reservoir is larger than active acidity because Al-hydroxy interlayered minerals release H⁺ over time. Depth is taken as 15 cm for shallow incorporation; bulk density 1.3-1.5 g/cc for normal mineral soils. Multiply by 0.1 because 1 cmol(+) = 0.5 g CaCO₃, scaled to t/ha. The output is in pure calcium-carbonate equivalent. Real lime sources are not pure CaCO₃ — agricultural lime (basic slag, dolomitic limestone) has Calcium Carbonate Equivalent (CCE) 80-95%, so the actual application has to be scaled up by CCE.

Worked example: Meghalaya hill paddy

A Ri-Bhoi terraced paddy field tests pH 4.6, exchangeable acidity 5 cmol(+)/kg, bulk density 1.2 g/cc. Calculator returns LR = 5 × 1.5 × 1.2 × 15 × 0.1 = 13.5 t/ha pure CaCO₃. With locally available dolomitic limestone at 92% CCE, the actual application is 13.5 ÷ 0.92 = 14.7 t/ha. That is a very heavy dose: the calculator advises split-twice strategy — half in pre-monsoon May, half in October post-paddy. Each application is broadcast, incorporated with the rotavator, and given one heavy rain to dissolve before sowing.

Cost: dolomite at ₹3-4/kg gives ₹44,100-58,800/ha lime cost — a real expense. But ICAR-CSSRI long-term trials at Umiam show wheat-paddy gross income +₹35,000/ha and +₹28,000/ha in the first and second seasons respectively after liming a pH-4.7 plot, achieving full payback in year 2.

Choosing your lime source

• Calcitic limestone (CaCO₃ ≥ 95% CCE) — quarried in Rajasthan, Madhya Pradesh, Andhra Pradesh; price ₹2-3/kg bulk; cheapest per kg of CCE.
• Dolomitic limestone [CaMg(CO₃)₂, ~95% CCE] — high Mg, useful where soil Mg is also low; common in Meghalaya quarries.
• Quicklime (CaO, CCE 178%) — caustic, dangerous to handle; rarely used on-farm; ICAR doesn't recommend for direct application.
• Hydrated lime [Ca(OH)₂, CCE 135%] — fast-acting but expensive; used for orchards in Coorg coffee belt.
• Basic slag / Mussoorie rock phosphate — by-products carrying 30-40% CCE; cheap or free where steel plants are nearby (Bokaro, Bhilai).
• Paper-mill lime sludge — waste stream from kraft paper mills, 60-80% CCE; the Karnataka Forest Industries Corporation distributes free in Shivamogga district to acid-soil farmers as a circular-economy initiative.

How fast does pH change after liming?

Coarse agricultural lime (1-2 mm) takes 6-9 months to fully react; fine ground lime (<0.25 mm, "60-mesh") reacts in 2-3 months. Liming raises pH from 4.5 to 5.5-6.0 in one season when applied at calculated rate; raising further to 6.5+ may need a second application after re-testing. Lime does NOT need to be reapplied annually — CSSRI trials show pH stays corrected for 3-5 years if you also stop ammonium-sulphate and ammonium-chloride fertilisers and switch to nitrogenous sources with lower acidifying potential (urea is slightly acidifying; calcium nitrate is neutral).

Integrated approach: lime + organic + cropping

Liming alone is incomplete on highly leached red and lateritic soils. ICAR-CRIDA's integrated package combines: (1) lime at calculated dose, (2) FYM 10 t/ha to feed the microbial community and supply micronutrients, (3) Rhizobium / PSB / KSB inoculants for biological N fixation and P solubilisation, (4) rotation that includes legumes (cowpea, horsegram) to add biological N, (5) avoiding ammonium-sulphate which re-acidifies. The bundle delivers durable productivity improvement; the Eastern India Rainfed Farming Project under NICRA documented +35% yield in millets and pulses over a 4-year window.

Reading the calculator output

The widget shows three numbers: pure CaCO₃ equivalent (the formula result), actual lime tonnage at your CCE, and a split strategy (single dose if < 5 t/ha, two splits for higher doses). It also outputs "none" if your reported pH is already ≥ 6.5, avoiding wasteful application. Cross-reference your soil test for exchangeable acidity — many SHC reports omit it and report only pH; insist on EA from your KVK laboratory if planning a liming programme.

Sources

ICAR-CSSRI Karnal Handbook of Soil Salinity and Acidity Management 2023; ICAR-NBSS&LUP Nagpur Soil Atlas of India 3rd ed. 2022; ICAR-IISWC Dehradun Lime requirement trials in Himalayan watersheds 2024; NICRA Annual Report 2023-24; ICAR-CRIDA Rainfed Farming Bulletin 2024.