One Tonne of Lime, Two Very Different Plots
Mrs Moyo farms two adjacent 0.4 ha plots on the same sandy loam soil. Plot A has grown continuous maize for eight seasons: pH 4.9 (water), exchangeable aluminium 1.8 cmol(+)/kg, available phosphorus 4 mg/kg. Plot B carried a legume fallow for the last two seasons: pH 5.9, aluminium 0.2 cmol(+)/kg, phosphorus 11 mg/kg. Last season, with the same hybrid seed and 150 kg/ha of 23:21:0+4S, Plot A yielded 1.1 t/ha and Plot B 2.9 t/ha. The soil test report recommends 2 t/ha of agricultural lime to raise pH by 0.5 units and warns that over-liming sandy soils can induce micronutrient deficiency. Lime costs $60/t at the depot plus $25/t transport. Mrs Moyo already has one tonne paid for and delivered, and no money for more. She intends to spread it thinly over both plots 'to be fair' and plant in three weeks, with rains forecast in four weeks.
1. Where should the tonne of lime go? Show the calculation and justify the decision, including application method and timing.
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Placement: all of it on Plot A, none on Plot B.
Calculation: the recommendation is 2 t/ha. Plot A is 0.4 ha, so it needs 2 × 0.4 = 0.8 t. The single tonne therefore fully liming Plot A still leaves 0.2 t.
Why not share it: 0.2 t spread over Plot B's 0.4 ha is only 0.5 t/ha, one-quarter of the rate needed to shift pH meaningfully. Splitting the lime gives 2.5 t/ha on A (slightly above target) and a useless sprinkle on B. Plot B at pH 5.9 is already close to the 5.8–6.5 range that suits maize and has low aluminium, so it is not the limiting plot.
Why A responds: at pH 4.9 with 1.8 cmol(+)/kg exchangeable aluminium, root growth is restricted and phosphorus is fixed. Correcting aluminium toxicity is the first step before fertiliser efficiency can improve.
Method and timing: spread evenly and incorporate into the top 10–15 cm, because lime moves very slowly down the profile. Do not band it with fertiliser, since mixing lime and ammonium fertiliser releases ammonia. Three weeks before planting is tight but acceptable if the soil is moist and incorporation is thorough; expect most benefit in the following season rather than a full recovery now.
2. Plot B still out-yields Plot A even after pH is considered. Explain what else is likely to be happening, and what this means for Mrs Moyo's expectations after liming.
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pH and aluminium explain part of the gap, but three other differences matter.
Nitrogen from the legume fallow. A two-season legume added nitrogen through biological fixation and mineralisation of residues. Continuous maize on Plot A has had no such input, so A is likely nitrogen-limited as well as aluminium-limited.
Organic matter and structure. Eight seasons of maize with residue removal have probably reduced organic matter, worsening water-holding capacity, aggregate stability and nutrient retention on a sandy soil. Plot B's fallow will have improved infiltration and reduced crusting.
Pest and disease build-up. Continuous maize favours stalk borers, root diseases and, where present, Striga. A fallow break interrupts these cycles, so Plot B may simply carry less pest pressure.
Implication: lime removes one constraint but does not replace nitrogen, organic matter or rotation. Mrs Moyo should not expect Plot A to reach 2.9 t/ha in one season from lime alone. She should combine lime with manure or compost, retain residues, and rotate. A realistic target is a partial yield recovery, with further gains over two to three seasons.
3. Design a simple, low-cost comparison she could run next season to check whether the lime recommendation is working, given she has no replicated trial design and only two plots.
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A defensible approach is a strip comparison within Plot A rather than a whole-plot comparison.
Design: divide Plot A into four strips of 0.1 ha each running across the slope, alternating limed and unlimed. Apply the full 0.8 t to the two limed strips (4 t/ha equivalent on those strips) and leave the two alternating strips unlimed. This gives two pairs, which is weak replication but far better than one treated and one untreated plot.
Controls: use the same maize hybrid, planting date, fertiliser rate and spacing on every strip, and keep Plot B out of the comparison because its history differs.
Measurements: record lime applied per strip, plant stand, height at tasselling, and grain weight from a measured harvest area (for example 10 m² per strip), converted to t/ha. Note any visible aluminium symptoms such as stubby roots.
Interpretation: compare limed and unlimed strips within pairs. If limed strips are consistently better, the recommendation is supported. A single season is still weak evidence, so repeat once. She should also record soil pH after harvest to confirm the lime actually raised it.