Liming an acid Ultisol on a limited budget and a short planting window
A farmer in the wet zone has 1 ha of red-yellow Ultisol (clay 28%, organic C 1.2%) that has grown cassava for six years. Analysis of the 0–20 cm layer gives pH (1:2.5 water) 4.6; exchangeable Al 2.8, Ca 0.8, Mg 0.3 and K 0.15 cmol/kg; CEC 6.5 cmol/kg; Olsen P 4 mg/kg. The 20–40 cm layer still holds 1.9 cmol/kg exchangeable Al. The farmer wants to plant 0.5 ha of capsicum, a crop rated as sensitive to aluminium, in four weeks. There are 2 t of dolomitic lime in the shed; extra lime can only be delivered after eight weeks, which would miss the planting window. Local guidance is that aluminium saturation should be below 10% for sensitive crops and that the lime factor for this soil depth is 1.5–2.0 t of agricultural lime per cmol/kg of exchangeable Al.
1. Calculate the aluminium saturation, the lime requirement to reach the 10% threshold, and the saturation expected if the 2 t of lime are used. Decide whether capsicum can be planted in four weeks, showing your reasoning.
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Current aluminium saturation = 2.8 ÷ 6.5 × 100 = 43%, which is severely toxic for a sensitive crop.
Target. For 10% saturation, exchangeable Al must fall to about 0.65 cmol/kg (10% of the CEC; liming raises CEC slightly, so this is a conservative target).
Aluminium to be neutralised = 2.8 − 0.65 = 2.15 cmol/kg.
Lime requirement using the local factor: 2.15 × 1.5 to 2.15 × 2.0 = 3.2 to 4.3 t/ha.
The important judgement is which area the lime is spread over. The 2 t are not a whole-farm rate of 2 t/ha; they are 2 t on 0.5 ha = 4 t/ha on the capsicum block. That falls inside the 3.2–4.3 t/ha requirement. The trap is to divide by 1 ha and conclude the farmer is short of lime, and then either abandon the crop or buy late lime and miss the season.
Decision: plant the capsicum on 0.5 ha, not 1 ha. Concentrate the entire 2 t on that block, incorporate it thoroughly to 15–20 cm with two crossed harrowings, and apply it immediately rather than in four weeks' time, because lime needs moisture and about three to four weeks to react. The remaining 0.5 ha should go to an acid-tolerant crop this season (cowpea, groundnut or a tolerant cereal) and be limed for the following season. Delivery after eight weeks is not a reason to delay; it is a reason to phase the programme.
Two caveats to state: dolomitic lime dissolves more slowly than calcitic lime, so use the finest available fraction and ensure the soil is moist; and at pH 4.6 with Olsen P of 4 mg/kg, banded phosphate near the roots is essential, because P will remain fixed by aluminium even after partial liming.
2. Set out the lime application plan and explain the risks of over-liming this soil. What else must be corrected in the same season?
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Application plan
- Spread all 2 t evenly on the 0.5 ha block (4 t/ha), then incorporate to 15–20 cm immediately. Surface broadcasting without incorporation is close to useless on a high-Al soil because lime moves only 1–2 cm a year.
- Do this as early as possible; 3–4 weeks of moist soil before planting is the minimum.
- Sample again after harvest; if Al saturation is still above 10%, apply the balance (0–1 t/ha) before the next crop.
- Do not exceed a target pH of about 6.0–6.5 in the topsoil.
Risks of over-liming
- Above pH 6.5, manganese, zinc and boron become less available; on a sandy, low-organic-matter soil this produces induced micronutrient deficiency that looks much like the original problem.
- Lime-induced zinc and boron deficiency is difficult and expensive to correct in-season.
- Over-application wastes money and can disperse or, on some soils, harden structure.
- Lime does not fix phosphorus supply: with Olsen P of 4 mg/kg, band 60–80 kg P2O5/ha as TSP or rock phosphate in the planting rows rather than broadcasting.
Other corrections
- Magnesium: 0.3 cmol/kg is low. Dolomitic lime will help, but it acts slowly, so include 20–30 kg Mg/ha as kieserite or foliar magnesium in the first season.
- Potassium: 0.15 cmol/kg is low; apply 100–120 kg K2O/ha in splits.
- Organic matter: 1.2% organic C is low; add compost or a green manure to improve CEC, moisture retention and the efficiency of the lime already applied. This is cheaper per unit of benefit than buying more lime.
- Molybdenum: acidity strongly reduces Mo availability; a small seed or foliar treatment helps legumes in the rotation.
3. The 20–40 cm layer has 1.9 cmol/kg exchangeable Al. Explain why the lime plan will not correct this and what you would do about it over the next few seasons.
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Why surface lime will not fix the subsoil
Lime is only sparingly soluble and moves downwards extremely slowly, typically 1–2 cm a year in these soils, so a single application incorporated to 20 cm has essentially no effect at 20–40 cm within the life of the crop. The subsoil therefore stays at about 1.9 cmol/kg exchangeable Al, which is still enough to stop root elongation. The practical consequence is a shallow root system: the crop will grow adequately in a wet spell and collapse in the first dry spell, because it cannot reach subsoil moisture. That is exactly the failure that gets misread as drought or poor variety.
What to do over several seasons
- Gypsum as a subsoil amendment. Gypsum supplies calcium and the sulphate ion pairs with Al, allowing calcium to move deeper than lime can. It does not raise pH, so it complements the lime rather than replacing it. Apply it with the first rains so the calcium is carried down.
- Biological drilling with deep-rooted species. A deep-rooted, aluminium-tolerant green manure or pasture crop in the rotation creates root channels that carry lime and gypsum downwards and add organic matter at depth. Use a species known to tolerate acidity rather than one that will fail.
- Keep the surface corrected. Maintain topsoil Al saturation below 10% so that roots are not blocked at the surface before they can reach the subsoil.
- Avoid deep compaction by restricting tillage to the depth that has been limed; a plough pan defeats any chemical correction.
- Match the land use to the soil for now. While the subsoil remains acidic, deep-rooted, acid-tolerant crops are lower-risk than capsicum. Save sensitive, high-value crops for the block whose profile has been corrected.
The judgement to state clearly is that the four-week lime decision is a short-term fix for a topsoil constraint; the subsoil constraint is a two- to three-season programme, and the rotation should be planned on that basis rather than on one lime application.