Reading a Single Soil Test on a 1.2 ha Acid Sandy Field
A farmer in a semi-arid area has 1.2 ha of gently sloping (6%) sandy loam that has grown maize for twelve consecutive seasons. She applies two 25 kg bags of 2:3:2 at planting and top-dresses once with urea, but yields have stayed near 1.2 t/ha. A composite sample taken to 20 cm gives: pH (water) 4.7; available P (Bray-1) 4 mg/kg; exchangeable K 0.15 cmol/kg; exchangeable Ca 1.2 cmol/kg; exchangeable Mg 0.4 cmol/kg; CEC 4.5 cmol/kg; organic carbon 0.6%; sand 62%, silt 20%, clay 18%. The laboratory's buffer-pH test indicates 2.5 t/ha of agricultural lime to raise pH to 5.8. Lime costs $0.09/kg delivered; she has $180 and cannot borrow. She also has 3 t of well-rotted cattle manure and wants to plant within eight weeks.
1. Interpret the soil test and rank the constraints. State whether lime is justified this season, calculate how much she can buy with $180 and what rate that gives over 1.2 ha, and recommend a rate and placement.
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Interpretation. pH 4.7 in water is strongly acid; aluminium and manganese become soluble at this pH and restrict root growth and phosphorus uptake, so the field will not respond fully to fertiliser until acidity is corrected. Available P of 4 mg/kg is very low for maize. K of 0.15 cmol/kg is low. A CEC of 4.5 cmol/kg with 62% sand means poor nutrient and water retention and rapid leaching. Organic carbon at 0.6% is low, and the 6% slope adds erosion risk.
Ranking. (1) Acidity, because it suppresses the response to everything else; (2) phosphorus; (3) potassium and organic matter; (4) erosion control.
Lime arithmetic. Full requirement: 2.5 t/ha × 1.2 ha = 3.0 t = 3 000 kg × $0.09 = $270. Her $180 buys 2 000 kg, which is 1.67 t/ha over the whole field.
Judgement. Apply 1.67 t/ha across the whole field rather than 2.5 t/ha on 0.8 ha, provided it is broadcast and incorporated into the top 15 cm at least six weeks before planting. Aluminium toxicity affects every part of the field, and the low CEC means a moderate rate still shifts pH usefully. Two checks before buying: the lime's neutralising value and fineness (a coarse or weak lime needs a higher rate), and accurate measurement of the area - liming part of the field leaves the rest limited.
2. Design the first-season amendment and management plan with quantities, timing and placement. Explain why applying everything at once would be a mistake.
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First-season plan, per hectare.
- Lime: 1.67 t/ha broadcast and incorporated 6–8 weeks before planting.
- Phosphorus: about 60 kg P2O5/ha as single superphosphate, banded beside the seed at planting rather than broadcast, because acid sandy soil fixes broadcast P quickly; single superphosphate also supplies calcium and sulphur.
- Potassium: 30 kg K2O/ha as KCl, split, because the low CEC will not hold a large single dose.
- Nitrogen: 80–100 kg N/ha split three ways (planting, four weeks, seven weeks) to limit leaching.
- Organic matter: 2–3 t/ha of the rotted manure, spread before planting; retain some for the next season.
- Erosion: tied or contour ridges across the slope, with residue or mulch between rows.
- Monitoring: leaf symptoms and colour during the season, and a repeat soil test after two seasons.
Why not everything at once. On a 4.5 cmol/kg CEC soil a single massive application leaches before the crop can use it, wasting money she cannot replace. Lime needs weeks to react and cannot substitute for phosphorus. Over-liming can induce zinc and boron deficiency. Loading manure plus a full mineral programme in one season risks lodging and salt injury and leaves no reserve for the following season. Sequencing spreads cost and risk and lets the soil test guide the next step.