A $420 input budget on an acidified 2.5 ha maize field
Daniel farms 2.5 ha of hybrid maize on sandy loam in a sub-humid district. Yield has fallen from 3.2 t/ha to 2.1 t/ha over three seasons despite timely planting. A 0–20 cm soil test reports pH(water) 4.7, exchangeable Al 1.9 cmol/kg, CEC 5.6 cmol/kg, organic matter 1.4%, Mehlich-3 P 6 mg/kg and exchangeable K 0.14 cmol/kg. The laboratory recommends 3 t/ha of agricultural lime plus 250 kg/ha of a 23-21-0+4S basal blend. Last season he applied 200 kg/ha of the same blend and saw no visible improvement. Daniel has US$420 saved. Lime costs US$95/t delivered and spread, and the blend costs US$0.62/kg. The co-op holds only 4 t of lime; the credit union will lend at 18% per year. Planting should start in three weeks. About 30% of the field is on a 6% slope draining toward a communal borehole, and his oxen plough to roughly 15 cm.
1. Allocate the US$420. State exactly what you buy, where it goes, and what you deliberately do not buy. Show the arithmetic and justify the priority order.
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The full-rate plan does not fit. Lime 3 t × $95 = $285. Blend 250 kg/ha × 2.5 ha = 625 kg × $0.62 = $387.50. Total $672.50 against $420 - a gap of $252.50, about 38% of the plan.
Priority rule. At pH 4.7 with 1.9 cmol/kg exchangeable Al, a large fraction of any P applied is fixed by aluminium and iron and never reaches the crop. That is consistent with the observation that 200 kg/ha of blend produced no visible response. Lime has the largest, longest-lasting effect (3–4 seasons residual) and the lowest cost per unit of expected yield gain, so it is funded first. The blend is not abandoned, but it is cut to a banded starter rather than a blanket broadcast rate.
Recommended purchase
- 3 t agricultural lime - $285. Apply 2 t/ha to the 1.25 ha carrying the worst symptoms (pale stunted patches, eroded upper slope) and 0.4 t/ha to the remaining 1.25 ha: 2.5 t + 0.5 t = 3 t.
- 200 kg of the 23-21-0+4S blend - $124. Band at 80 kg/ha in the planting furrow over 2.5 ha (about 18 kg N and 17 kg P2O5/ha), placed near the seed to limit contact with free Al.
- Hold the $11 balance for transport or seed contingency.
Deliberately not bought: the full 250 kg/ha broadcast rate (a further $263), and no 18% loan. At 2.1 t/ha maize is a thin-margin crop; $250 borrowed at 18% costs about $45 per year and needs roughly 0.3–0.4 t/ha of extra yield just to service the debt, which is not assured in the first season after liming.
Caveat: if the 4 t co-op price holds, one extra tonne ($95) is defensible only if spread at 2 t/ha on the highest-Al half-hectare, where the probability of response is greatest. Do not spread the same money thinly over the whole field - a sub-tonne rate leaves subsoil Al largely untouched.
2. Explain how and when the lime should be applied, taking account of the 15 cm plough depth and the sloping section, and state what liming will and will not fix.
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Incorporation is the point. Lime neutralises acidity by contact; broadcast on the surface it moves down only a few centimetres per year. Ox-ploughing to 15 cm means only the top 15 cm will be corrected, and the plough pan and subsoil below stay acidic. That is acceptable if the 0–15 cm layer is brought to about pH 5.3–5.5, because maize roots concentrate there, but Daniel should not expect the soil test at 20–30 cm to change.
Timing. Three weeks before planting is workable but tight; 4–6 weeks is better so the lime reacts before the roots arrive. Spread evenly (two passes at half rate rather than one heavy pass), incorporate immediately, and do not leave heaps or windrows - they create patches of over-liming and bare spots.
Slope and water. The 30% sloping section drains to a communal drinking borehole. Apply on the contour, not up and down the slope; keep at least a 10 m untreated buffer beside any channel or waterway; avoid spreading within 48 hours of forecast heavy rain. Agricultural lime is low-toxicity, but fines washed into the borehole raise turbidity and pH, and the community will notice.
What liming fixes: exchangeable Al toxicity, low pH, and the resulting poor P availability and stunted roots. What it does not fix: the low Mehlich-3 P itself (6 mg/kg is well below the ~15–20 mg/kg sufficiency range), the low K, the 1.4% organic matter, or compaction. Those need fertilizer, manure/residue retention and, in later seasons, a soil test to check whether Mg is also low - if so, a dolomitic lime is the better buy.
3. What will you measure over the next two seasons to judge whether the lime paid off, and what result would make you change course?
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Measurement plan
- Soil re-test at 12–18 months, same depth (0–20 cm), same laboratory, same sampling pattern - compare pH and exchangeable Al, not just 'looks better'. A rise of 0.3–0.5 pH units and a fall in Al below about 0.5 cmol/kg would be a normal first-season response on this soil.
- Yield strips. Leave two unlimed strips (e.g. 5 m × 20 m) in the same part of the field and weigh grain from them separately. A comparison of 2.1 t/ha against the limed area is the only clean evidence of a response; comparing against last year's field average confounds rainfall and variety.
- Root and early vigour scores at 4–6 weeks: root mass and colour in limed versus unlimed strips is fast, cheap evidence.
- Cost per tonne of grain, not yield alone - lime at $285 over a 3–4 season residual is about $25–35/ha/year, which needs only a modest yield gain to pay.
Triggers to change course
- pH barely moves after 12 months: suspect an under-application, coarse lime that has not reacted, or continued heavy acidification from ammonium-based N. Recalculate the rate rather than repeating blindly.
- pH climbs above about 6.0: stop liming; over-liming locks up zinc, boron and manganese and will show as interveinal chlorosis.
- Yield improves but P deficiency symptoms persist: the constraint has shifted to P, so the next dollar belongs in the blend at a full rate and possibly in banded placement rather than more lime.
- A strong response on the limed half but none on the light-rate half tells you the marginal dollar belongs in lime, not fertilizer, and justifies a loan for the remaining 4.5 t next season.