Acid soil, a switch to banana, and a farmer in a hurry
A one-hectare field in Palakkad has grown paddy for twenty years. The farmer now wants to switch to Nendran banana. Soil test: pH 4.9, organic carbon 0.35%, exchangeable aluminium 2.8 cmol(+)/kg soil, available phosphorus 8 kg/ha (low), exchangeable potassium medium. The field is level, receives about 2500 mm of rain, and has assured canal irrigation; the subsoil is clay-rich and drains slowly. The laboratory recommends applying 3 tonnes of lime per hectare to raise the pH towards 6.0. The farmer proposes to broadcast the lime, urea and rock phosphate together, plough them in, and plant tissue-culture banana suckers five days later, on the reasoning that 'everything goes into the soil at once, so the plant gets everything at once.' He is also willing to keep one-third of the field under paddy to spread his risk.
1. Which single constraint in this soil test would you correct first, and why would extra urea and rock phosphate alone give a poor return?
Read the worked answer
Fix the acidity first; everything else is secondary.
At pH 4.9 with 2.8 cmol(+)/kg of exchangeable aluminium, a large share of the cation exchange sites is occupied by Al³⁺, and soluble Al³⁺ is directly toxic to root tips: roots become stubby and brown, and water and nutrient uptake collapse. Banana is reasonably tolerant of acidity but not of active aluminium.
The phosphorus figure is misleading if read alone. Available P is 8 kg/ha, but in a strongly acid soil most applied phosphate - especially rock phosphate - is fixed as iron and aluminium phosphates that the plant cannot use. Adding P without liming is money buried in the soil. The same applies to applied ammonium: without lime, nitrification is slow and the response to nitrogen is poor.
The exchangeable Al is the rate-limiting constraint, so lime is the first input, not the last. Liming to pH 5.8–6.0 precipitates the Al, raises base saturation and unlocks both native and applied phosphorus. Organic carbon at 0.35% is also very low, so organic matter or green manure should go in along with the lime to improve structure and water-holding in a clay subsoil.
2. Critique the farmer's plan of applying lime, urea and rock phosphate together and planting five days later, and give a corrected sequence with timings.
Read the worked answer
Three separate errors.
- Five days is too short for lime to act. Lime must be incorporated into moist soil and needs roughly two to four weeks to react and neutralise acidity. Planting five days later means the suckers meet a soil that is still acid and still high in exchangeable Al - the very condition being treated.
- Lime must never be mixed with urea or ammonium fertilisers. The reaction releases ammonia gas, so nitrogen is lost to the atmosphere and the liming reaction is wasted. The two belong in different operations at different times.
- Rock phosphate plus lime is contradictory. Rock phosphate dissolves only in acid; if the soil is being limed, its dissolution slows and it acts as a very slow-release P source at best. It is more sensible to apply it before liming and incorporate it, then use a soluble P source such as single superphosphate for the planted crop.
Corrected sequence
- Week 0: broadcast and incorporate the full 3 t/ha of lime to 15–20 cm; improve drainage with raised beds and open drains, because the clay subsoil is the second constraint.
- Week 2–4: allow the lime to react; keep the soil moist but not waterlogged.
- At planting: plant suckers; apply well-rotted organic manure in the pit.
- After establishment: apply nitrogen and potassium in four to six splits; keep nitrogen and any remaining lime at least a week apart; place phosphorus near the root zone, not broadcast on the surface.
3. Would you advise him to convert the whole hectare, or keep part under paddy? Justify using the soil chemistry and the drainage.
Read the worked answer
Keeping one-third under paddy is good judgement, but for reasons worth stating precisely.
Why paddy suits this field. In a flooded paddy soil the redox potential falls. Ferric iron is reduced to ferrous iron and sulphate to sulphide, and the reduction reactions consume hydrogen ions, so the pH of an acid soil drifts up towards neutrality when submerged. Aluminium toxicity, the principal problem in this field, largely disappears under standing water. Paddy is therefore the one crop that tolerates the pre-liming condition, and the 2500 mm rainfall plus canal supply matches it.
Why banana is risky on part of the field. Banana has a shallow, fleshy root system and is intolerant of waterlogging; waterlogged clay subsoil causes root rot and toppling. After liming to pH 5.8, the pH is suitable, but drainage is not, so banana should go only on the raised, better-drained portion, in beds with open drains.
Practical split. Lime the banana block (two-thirds) and raise beds there; leave the lowest, wettest third under paddy, which needs no lime and benefits from the flooding. If any part of the paddy third is later drained, coconut is more tolerant of residual acidity than banana and can be planted on the wettest margins. Re-test the soil after the first harvest rather than assuming the lime recommendation is a fixed quantity.