Liming a Two-Block Field When the Money Will Not Stretch
Mrs Kgosi farms 1.6 ha in two blocks. Block A (0.9 ha) is a sandy loam on a gentle slope; Block B (0.7 ha) is a flat clay loam that stands under water for two to three days after heavy rain and shows grey mottles 30 cm down. A soil test of Block A reports pH 5.2 (water), exchangeable aluminium 1.8 cmol/kg, calcium 1.2 cmol/kg, magnesium 0.5 cmol/kg, potassium 0.30 cmol/kg, available phosphorus 7 mg/kg and organic matter 1.0%. Maize on Block A yielded 1.2 t/ha against a district average of 3.5 t/ha, with yellow, stunted plants. Groundnuts on Block B had yellow leaves and many rotten, empty pods. The extension table states that 2.5 t/ha of lime at 100% calcium carbonate equivalent is needed to raise this soil from pH 5.2 to 6.0. She has 5 t of dolomitic lime (78% CCE, 18% Mg) at $60 per tonne and $400 to spend.
1. Identify the two soil constraints limiting production and decide which one her first $400 should address. Justify the priority you choose.
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Constraint 1 - acidity on Block A. A pH of 5.2 with 1.8 cmol/kg exchangeable aluminium means aluminium toxicity is pruning the roots, so the plant cannot exploit subsoil water or nutrients. Low pH also locks up the 7 mg/kg available phosphorus, and the low calcium (1.2) and magnesium (0.5) show the exchange complex is dominated by hydrogen and aluminium. Yellow, stunted maize at one third of district yield fits this exactly.
Constraint 2 - waterlogging on Block B. Grey mottles 30 cm down are a gleying sign: the profile saturates and turns anaerobic for days at a time. Roots run out of oxygen, nitrogen is lost by denitrification, manganese and iron become toxic, and groundnut pods rot in the wet zone, which explains the empty pods.
Priority judgement. Spend first on Block A, because liming there is cheap, quick and produces a return in the coming season. On Block B, lime applied to a waterlogged soil fixes very little: the soil stays reduced and any surface-applied lime is unevenly effective. However, do not ignore B completely - spend labour, not cash, on a shallow cut-off drain across the top of the flat and two open drains down the slope. Lime Block B only after drainage works and after a dry-season survey.
2. Calculate the lime needed for Block A using her dolomitic material and state the cost. Does the 5 t supply cover the whole farm, and how should the balance be used?
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Step 1 - correct the rate for purity. The table rate is expressed at 100% CCE, her material is 78% CCE, so the field rate must be increased: 3.2 t/ha ≈ 2.5 ÷ 0.78 = 3.21 t/ha.
Step 2 - scale to Block A. 3.21 × 0.9 ha = 2.89 t, so about 2.9 t.
Step 3 - cost. 2.9 × $60 = $174, leaving roughly $226 of the $400.
Step 4 - the balance. She has 5 t, so 2.1 t remain. At 3.21 t/ha that is only 0.65 ha, slightly less than Block B's 0.7 ha. Applying a reduced rate of about 3.0 t/ha over 0.7 ha would need 2.1 t, which is exactly what she has but only if the whole block is drained first. The dolomitic material is also a bonus: at 18% Mg it will lift the very low magnesium (0.5 cmol/kg) that Block A shares with B, so no separate magnesium fertiliser is needed.
Judgement. Do not spread the remaining 2.1 t thinly over the whole farm, which would treat neither block properly. Store it under cover, drain Block B, and apply it there next dry season once the drains are working and the pH response can be measured.
3. She plans to broadcast the lime and place 200 kg/ha of urea in the same planting furrow one week later. Advise her on this plan.
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Three problems must be separated.
1. Lime needs time. Lime reacts only when it is in contact with moist soil, and the pH shift that neutralises aluminium takes weeks to months. Broadcasting and incorporating it into the top 15 cm three to four weeks before planting is far better than a week. If she has already limed late, she should accept a smaller first-season response rather than compensate by adding more.
2. Urea and lime in the same furrow is a loss. Lime raises pH at the point where the urea granule dissolves. In an alkaline micro-site, ammonium converts to ammonia gas and escapes, so part of the nitrogen is lost before the plant can use it. High pH plus a concentrated salt band also burns germinating seed and young roots.
3. Placement. Urea should be banded 5 to 10 cm to the side of the seed and slightly below it, never in direct contact and never mixed with lime.
Revised plan: incorporate the 2.9 t of lime on Block A now; plant without urea in the furrow; band one third of the nitrogen at planting and the rest in two splits at six leaves and tasselling. Keep lime and all ammonium or urea fertiliser at least a few weeks and a few centimetres apart.